docs-rst: convert lsm from DocBook to ReST
This file is outdated. Still, as it is the only one left at DocBook dir, convert it, and store it, with a .txt extension, under Documentation/lsm.txt. This way, we can get rid of DocBook from the building system, without needing to wait for someone to take care of it. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>zero-colors
Mauro Carvalho Chehab
parent
bffac837f3
commit
415008af32
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@ -264,6 +264,8 @@ logo.gif
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- full colour GIF image of Linux logo (penguin - Tux).
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logo.txt
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- info on creator of above logo & site to get additional images from.
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lsm.txt
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- Linux Security Modules: General Security Hooks for Linux
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lzo.txt
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- kernel LZO decompressor input formats
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m68k/
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@ -1,276 +1 @@
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###
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# This makefile is used to generate the kernel documentation,
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# primarily based on in-line comments in various source files.
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# See Documentation/kernel-doc-nano-HOWTO.txt for instruction in how
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# to document the SRC - and how to read it.
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# To add a new book the only step required is to add the book to the
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# list of DOCBOOKS.
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DOCBOOKS := lsm.xml
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ifeq ($(DOCBOOKS),)
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# Skip DocBook build if the user explicitly requested no DOCBOOKS.
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.DEFAULT:
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@echo " SKIP DocBook $@ target (DOCBOOKS=\"\" specified)."
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else
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ifneq ($(SPHINXDIRS),)
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# Skip DocBook build if the user explicitly requested a sphinx dir
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.DEFAULT:
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@echo " SKIP DocBook $@ target (SPHINXDIRS specified)."
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else
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###
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# The build process is as follows (targets):
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# (xmldocs) [by docproc]
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# file.tmpl --> file.xml +--> file.ps (psdocs) [by db2ps or xmlto]
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# +--> file.pdf (pdfdocs) [by db2pdf or xmlto]
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# +--> DIR=file (htmldocs) [by xmlto]
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# +--> man/ (mandocs) [by xmlto]
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# for PDF and PS output you can choose between xmlto and docbook-utils tools
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PDF_METHOD = $(prefer-db2x)
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PS_METHOD = $(prefer-db2x)
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targets += $(DOCBOOKS)
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BOOKS := $(addprefix $(obj)/,$(DOCBOOKS))
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xmldocs: $(BOOKS)
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sgmldocs: xmldocs
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PS := $(patsubst %.xml, %.ps, $(BOOKS))
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psdocs: $(PS)
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PDF := $(patsubst %.xml, %.pdf, $(BOOKS))
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pdfdocs: $(PDF)
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HTML := $(sort $(patsubst %.xml, %.html, $(BOOKS)))
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htmldocs: $(HTML)
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$(call cmd,build_main_index)
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MAN := $(patsubst %.xml, %.9, $(BOOKS))
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mandocs: $(MAN)
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find $(obj)/man -name '*.9' | xargs gzip -nf
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# Default location for installed man pages
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export INSTALL_MAN_PATH = $(objtree)/usr
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installmandocs: mandocs
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mkdir -p $(INSTALL_MAN_PATH)/man/man9/
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find $(obj)/man -name '*.9.gz' -printf '%h %f\n' | \
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sort -k 2 -k 1 | uniq -f 1 | sed -e 's: :/:' | \
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xargs install -m 644 -t $(INSTALL_MAN_PATH)/man/man9/
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# no-op for the DocBook toolchain
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epubdocs:
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latexdocs:
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linkcheckdocs:
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###
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#External programs used
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KERNELDOCXMLREF = $(srctree)/scripts/kernel-doc-xml-ref
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KERNELDOC = $(srctree)/scripts/kernel-doc
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DOCPROC = $(objtree)/scripts/docproc
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CHECK_LC_CTYPE = $(objtree)/scripts/check-lc_ctype
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# Use a fixed encoding - UTF-8 if the C library has support built-in
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# or ASCII if not
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LC_CTYPE := $(call try-run, LC_CTYPE=C.UTF-8 $(CHECK_LC_CTYPE),C.UTF-8,C)
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export LC_CTYPE
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XMLTOFLAGS = -m $(srctree)/$(src)/stylesheet.xsl
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XMLTOFLAGS += --skip-validation
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###
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# DOCPROC is used for two purposes:
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# 1) To generate a dependency list for a .tmpl file
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# 2) To preprocess a .tmpl file and call kernel-doc with
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# appropriate parameters.
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# The following rules are used to generate the .xml documentation
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# required to generate the final targets. (ps, pdf, html).
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quiet_cmd_docproc = DOCPROC $@
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cmd_docproc = SRCTREE=$(srctree)/ $(DOCPROC) doc $< >$@
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define rule_docproc
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set -e; \
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$(if $($(quiet)cmd_$(1)),echo ' $($(quiet)cmd_$(1))';) \
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$(cmd_$(1)); \
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( \
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echo 'cmd_$@ := $(cmd_$(1))'; \
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echo $@: `SRCTREE=$(srctree) $(DOCPROC) depend $<`; \
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) > $(dir $@).$(notdir $@).cmd
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endef
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%.xml: %.tmpl $(KERNELDOC) $(DOCPROC) $(KERNELDOCXMLREF) FORCE
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$(call if_changed_rule,docproc)
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# Tell kbuild to always build the programs
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always := $(hostprogs-y)
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notfoundtemplate = echo "*** You have to install docbook-utils or xmlto ***"; \
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exit 1
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db2xtemplate = db2TYPE -o $(dir $@) $<
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xmltotemplate = xmlto TYPE $(XMLTOFLAGS) -o $(dir $@) $<
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# determine which methods are available
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ifeq ($(shell which db2ps >/dev/null 2>&1 && echo found),found)
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use-db2x = db2x
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prefer-db2x = db2x
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else
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use-db2x = notfound
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prefer-db2x = $(use-xmlto)
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endif
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ifeq ($(shell which xmlto >/dev/null 2>&1 && echo found),found)
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use-xmlto = xmlto
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prefer-xmlto = xmlto
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else
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use-xmlto = notfound
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prefer-xmlto = $(use-db2x)
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endif
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# the commands, generated from the chosen template
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quiet_cmd_db2ps = PS $@
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cmd_db2ps = $(subst TYPE,ps, $($(PS_METHOD)template))
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%.ps : %.xml
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$(call cmd,db2ps)
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quiet_cmd_db2pdf = PDF $@
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cmd_db2pdf = $(subst TYPE,pdf, $($(PDF_METHOD)template))
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%.pdf : %.xml
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$(call cmd,db2pdf)
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index = index.html
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main_idx = $(obj)/$(index)
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quiet_cmd_build_main_index = HTML $(main_idx)
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cmd_build_main_index = rm -rf $(main_idx); \
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echo '<h1>Linux Kernel HTML Documentation</h1>' >> $(main_idx) && \
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echo '<h2>Kernel Version: $(KERNELVERSION)</h2>' >> $(main_idx) && \
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cat $(HTML) >> $(main_idx)
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quiet_cmd_db2html = HTML $@
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cmd_db2html = xmlto html $(XMLTOFLAGS) -o $(patsubst %.html,%,$@) $< && \
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echo '<a HREF="$(patsubst %.html,%,$(notdir $@))/index.html"> \
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$(patsubst %.html,%,$(notdir $@))</a><p>' > $@
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###
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# Rules to create an aux XML and .db, and use them to re-process the DocBook XML
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# to fill internal hyperlinks
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gen_aux_xml = :
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quiet_gen_aux_xml = echo ' XMLREF $@'
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silent_gen_aux_xml = :
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%.aux.xml: %.xml
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@$($(quiet)gen_aux_xml)
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@rm -rf $@
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@(cat $< | egrep "^<refentry id" | egrep -o "\".*\"" | cut -f 2 -d \" > $<.db)
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@$(KERNELDOCXMLREF) -db $<.db $< > $@
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.PRECIOUS: %.aux.xml
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%.html: %.aux.xml
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@(which xmlto > /dev/null 2>&1) || \
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(echo "*** You need to install xmlto ***"; \
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exit 1)
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@rm -rf $@ $(patsubst %.html,%,$@)
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$(call cmd,db2html)
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@if [ ! -z "$(PNG-$(basename $(notdir $@)))" ]; then \
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cp $(PNG-$(basename $(notdir $@))) $(patsubst %.html,%,$@); fi
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quiet_cmd_db2man = MAN $@
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cmd_db2man = if grep -q refentry $<; then xmlto man $(XMLTOFLAGS) -o $(obj)/man/$(*F) $< ; fi
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%.9 : %.xml
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@(which xmlto > /dev/null 2>&1) || \
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(echo "*** You need to install xmlto ***"; \
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exit 1)
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$(Q)mkdir -p $(obj)/man/$(*F)
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$(call cmd,db2man)
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@touch $@
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###
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# Rules to generate postscripts and PNG images from .fig format files
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quiet_cmd_fig2eps = FIG2EPS $@
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cmd_fig2eps = fig2dev -Leps $< $@
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%.eps: %.fig
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@(which fig2dev > /dev/null 2>&1) || \
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(echo "*** You need to install transfig ***"; \
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exit 1)
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$(call cmd,fig2eps)
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quiet_cmd_fig2png = FIG2PNG $@
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cmd_fig2png = fig2dev -Lpng $< $@
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%.png: %.fig
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@(which fig2dev > /dev/null 2>&1) || \
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(echo "*** You need to install transfig ***"; \
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exit 1)
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$(call cmd,fig2png)
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###
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# Rule to convert a .c file to inline XML documentation
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gen_xml = :
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quiet_gen_xml = echo ' GEN $@'
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silent_gen_xml = :
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%.xml: %.c
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@$($(quiet)gen_xml)
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@( \
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echo "<programlisting>"; \
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expand --tabs=8 < $< | \
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sed -e "s/&/\\&/g" \
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-e "s/</\\</g" \
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-e "s/>/\\>/g"; \
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echo "</programlisting>") > $@
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endif # DOCBOOKS=""
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endif # SPHINDIR=...
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###
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# Help targets as used by the top-level makefile
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dochelp:
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@echo ' Linux kernel internal documentation in different formats (DocBook):'
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@echo ' htmldocs - HTML'
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@echo ' pdfdocs - PDF'
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@echo ' psdocs - Postscript'
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@echo ' xmldocs - XML DocBook'
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@echo ' mandocs - man pages'
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@echo ' installmandocs - install man pages generated by mandocs to INSTALL_MAN_PATH'; \
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echo ' (default: $(INSTALL_MAN_PATH))'; \
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echo ''
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@echo ' cleandocs - clean all generated DocBook files'
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@echo
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@echo ' make DOCBOOKS="s1.xml s2.xml" [target] Generate only docs s1.xml s2.xml'
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@echo ' valid values for DOCBOOKS are: $(DOCBOOKS)'
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@echo
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@echo " make DOCBOOKS=\"\" [target] Don't generate docs from Docbook"
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@echo ' This is useful to generate only the ReST docs (Sphinx)'
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###
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# Temporary files left by various tools
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clean-files := $(DOCBOOKS) \
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$(patsubst %.xml, %.dvi, $(DOCBOOKS)) \
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$(patsubst %.xml, %.aux, $(DOCBOOKS)) \
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$(patsubst %.xml, %.tex, $(DOCBOOKS)) \
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$(patsubst %.xml, %.log, $(DOCBOOKS)) \
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$(patsubst %.xml, %.out, $(DOCBOOKS)) \
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$(patsubst %.xml, %.ps, $(DOCBOOKS)) \
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$(patsubst %.xml, %.pdf, $(DOCBOOKS)) \
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$(patsubst %.xml, %.html, $(DOCBOOKS)) \
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$(patsubst %.xml, %.9, $(DOCBOOKS)) \
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$(patsubst %.xml, %.aux.xml, $(DOCBOOKS)) \
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$(patsubst %.xml, %.xml.db, $(DOCBOOKS)) \
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$(patsubst %.xml, %.xml, $(DOCBOOKS)) \
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$(patsubst %.xml, .%.xml.cmd, $(DOCBOOKS)) \
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$(index)
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clean-dirs := $(patsubst %.xml,%,$(DOCBOOKS)) man
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cleandocs:
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$(Q)rm -f $(call objectify, $(clean-files))
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$(Q)rm -rf $(call objectify, $(clean-dirs))
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# Declare the contents of the .PHONY variable as phony. We keep that
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# information in a variable so we can use it in if_changed and friends.
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.PHONY: $(PHONY)
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@ -1,265 +0,0 @@
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
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<article class="whitepaper" id="LinuxSecurityModule" lang="en">
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<articleinfo>
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<title>Linux Security Modules: General Security Hooks for Linux</title>
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<authorgroup>
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<author>
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<firstname>Stephen</firstname>
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<surname>Smalley</surname>
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<affiliation>
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<orgname>NAI Labs</orgname>
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<address><email>ssmalley@nai.com</email></address>
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</affiliation>
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</author>
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<author>
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<firstname>Timothy</firstname>
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<surname>Fraser</surname>
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<affiliation>
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<orgname>NAI Labs</orgname>
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<address><email>tfraser@nai.com</email></address>
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</affiliation>
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</author>
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<author>
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<firstname>Chris</firstname>
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<surname>Vance</surname>
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<affiliation>
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<orgname>NAI Labs</orgname>
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<address><email>cvance@nai.com</email></address>
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</affiliation>
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</author>
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</authorgroup>
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</articleinfo>
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<sect1 id="Introduction"><title>Introduction</title>
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<para>
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In March 2001, the National Security Agency (NSA) gave a presentation
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about Security-Enhanced Linux (SELinux) at the 2.5 Linux Kernel
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Summit. SELinux is an implementation of flexible and fine-grained
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nondiscretionary access controls in the Linux kernel, originally
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implemented as its own particular kernel patch. Several other
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security projects (e.g. RSBAC, Medusa) have also developed flexible
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access control architectures for the Linux kernel, and various
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projects have developed particular access control models for Linux
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(e.g. LIDS, DTE, SubDomain). Each project has developed and
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maintained its own kernel patch to support its security needs.
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</para>
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<para>
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In response to the NSA presentation, Linus Torvalds made a set of
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remarks that described a security framework he would be willing to
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consider for inclusion in the mainstream Linux kernel. He described a
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general framework that would provide a set of security hooks to
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control operations on kernel objects and a set of opaque security
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fields in kernel data structures for maintaining security attributes.
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This framework could then be used by loadable kernel modules to
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implement any desired model of security. Linus also suggested the
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possibility of migrating the Linux capabilities code into such a
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module.
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</para>
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<para>
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The Linux Security Modules (LSM) project was started by WireX to
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develop such a framework. LSM is a joint development effort by
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several security projects, including Immunix, SELinux, SGI and Janus,
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and several individuals, including Greg Kroah-Hartman and James
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Morris, to develop a Linux kernel patch that implements this
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framework. The patch is currently tracking the 2.4 series and is
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targeted for integration into the 2.5 development series. This
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technical report provides an overview of the framework and the example
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capabilities security module provided by the LSM kernel patch.
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</para>
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</sect1>
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<sect1 id="framework"><title>LSM Framework</title>
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<para>
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The LSM kernel patch provides a general kernel framework to support
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security modules. In particular, the LSM framework is primarily
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focused on supporting access control modules, although future
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development is likely to address other security needs such as
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auditing. By itself, the framework does not provide any additional
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security; it merely provides the infrastructure to support security
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modules. The LSM kernel patch also moves most of the capabilities
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logic into an optional security module, with the system defaulting
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to the traditional superuser logic. This capabilities module
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is discussed further in <xref linkend="cap"/>.
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</para>
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<para>
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The LSM kernel patch adds security fields to kernel data structures
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and inserts calls to hook functions at critical points in the kernel
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code to manage the security fields and to perform access control. It
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also adds functions for registering and unregistering security
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modules, and adds a general <function>security</function> system call
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to support new system calls for security-aware applications.
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</para>
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<para>
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The LSM security fields are simply <type>void*</type> pointers. For
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process and program execution security information, security fields
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were added to <structname>struct task_struct</structname> and
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<structname>struct linux_binprm</structname>. For filesystem security
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information, a security field was added to
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<structname>struct super_block</structname>. For pipe, file, and socket
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security information, security fields were added to
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<structname>struct inode</structname> and
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<structname>struct file</structname>. For packet and network device security
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information, security fields were added to
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<structname>struct sk_buff</structname> and
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<structname>struct net_device</structname>. For System V IPC security
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information, security fields were added to
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<structname>struct kern_ipc_perm</structname> and
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<structname>struct msg_msg</structname>; additionally, the definitions
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for <structname>struct msg_msg</structname>, <structname>struct
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msg_queue</structname>, and <structname>struct
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shmid_kernel</structname> were moved to header files
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(<filename>include/linux/msg.h</filename> and
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<filename>include/linux/shm.h</filename> as appropriate) to allow
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the security modules to use these definitions.
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</para>
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<para>
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Each LSM hook is a function pointer in a global table,
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security_ops. This table is a
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<structname>security_operations</structname> structure as defined by
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<filename>include/linux/security.h</filename>. Detailed documentation
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for each hook is included in this header file. At present, this
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structure consists of a collection of substructures that group related
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hooks based on the kernel object (e.g. task, inode, file, sk_buff,
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etc) as well as some top-level hook function pointers for system
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operations. This structure is likely to be flattened in the future
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for performance. The placement of the hook calls in the kernel code
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is described by the "called:" lines in the per-hook documentation in
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the header file. The hook calls can also be easily found in the
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kernel code by looking for the string "security_ops->".
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</para>
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||||
|
||||
<para>
|
||||
Linus mentioned per-process security hooks in his original remarks as a
|
||||
possible alternative to global security hooks. However, if LSM were
|
||||
to start from the perspective of per-process hooks, then the base
|
||||
framework would have to deal with how to handle operations that
|
||||
involve multiple processes (e.g. kill), since each process might have
|
||||
its own hook for controlling the operation. This would require a
|
||||
general mechanism for composing hooks in the base framework.
|
||||
Additionally, LSM would still need global hooks for operations that
|
||||
have no process context (e.g. network input operations).
|
||||
Consequently, LSM provides global security hooks, but a security
|
||||
module is free to implement per-process hooks (where that makes sense)
|
||||
by storing a security_ops table in each process' security field and
|
||||
then invoking these per-process hooks from the global hooks.
|
||||
The problem of composition is thus deferred to the module.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
The global security_ops table is initialized to a set of hook
|
||||
functions provided by a dummy security module that provides
|
||||
traditional superuser logic. A <function>register_security</function>
|
||||
function (in <filename>security/security.c</filename>) is provided to
|
||||
allow a security module to set security_ops to refer to its own hook
|
||||
functions, and an <function>unregister_security</function> function is
|
||||
provided to revert security_ops to the dummy module hooks. This
|
||||
mechanism is used to set the primary security module, which is
|
||||
responsible for making the final decision for each hook.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
LSM also provides a simple mechanism for stacking additional security
|
||||
modules with the primary security module. It defines
|
||||
<function>register_security</function> and
|
||||
<function>unregister_security</function> hooks in the
|
||||
<structname>security_operations</structname> structure and provides
|
||||
<function>mod_reg_security</function> and
|
||||
<function>mod_unreg_security</function> functions that invoke these
|
||||
hooks after performing some sanity checking. A security module can
|
||||
call these functions in order to stack with other modules. However,
|
||||
the actual details of how this stacking is handled are deferred to the
|
||||
module, which can implement these hooks in any way it wishes
|
||||
(including always returning an error if it does not wish to support
|
||||
stacking). In this manner, LSM again defers the problem of
|
||||
composition to the module.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
Although the LSM hooks are organized into substructures based on
|
||||
kernel object, all of the hooks can be viewed as falling into two
|
||||
major categories: hooks that are used to manage the security fields
|
||||
and hooks that are used to perform access control. Examples of the
|
||||
first category of hooks include the
|
||||
<function>alloc_security</function> and
|
||||
<function>free_security</function> hooks defined for each kernel data
|
||||
structure that has a security field. These hooks are used to allocate
|
||||
and free security structures for kernel objects. The first category
|
||||
of hooks also includes hooks that set information in the security
|
||||
field after allocation, such as the <function>post_lookup</function>
|
||||
hook in <structname>struct inode_security_ops</structname>. This hook
|
||||
is used to set security information for inodes after successful lookup
|
||||
operations. An example of the second category of hooks is the
|
||||
<function>permission</function> hook in
|
||||
<structname>struct inode_security_ops</structname>. This hook checks
|
||||
permission when accessing an inode.
|
||||
</para>
|
||||
|
||||
</sect1>
|
||||
|
||||
<sect1 id="cap"><title>LSM Capabilities Module</title>
|
||||
|
||||
<para>
|
||||
The LSM kernel patch moves most of the existing POSIX.1e capabilities
|
||||
logic into an optional security module stored in the file
|
||||
<filename>security/capability.c</filename>. This change allows
|
||||
users who do not want to use capabilities to omit this code entirely
|
||||
from their kernel, instead using the dummy module for traditional
|
||||
superuser logic or any other module that they desire. This change
|
||||
also allows the developers of the capabilities logic to maintain and
|
||||
enhance their code more freely, without needing to integrate patches
|
||||
back into the base kernel.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
In addition to moving the capabilities logic, the LSM kernel patch
|
||||
could move the capability-related fields from the kernel data
|
||||
structures into the new security fields managed by the security
|
||||
modules. However, at present, the LSM kernel patch leaves the
|
||||
capability fields in the kernel data structures. In his original
|
||||
remarks, Linus suggested that this might be preferable so that other
|
||||
security modules can be easily stacked with the capabilities module
|
||||
without needing to chain multiple security structures on the security field.
|
||||
It also avoids imposing extra overhead on the capabilities module
|
||||
to manage the security fields. However, the LSM framework could
|
||||
certainly support such a move if it is determined to be desirable,
|
||||
with only a few additional changes described below.
|
||||
</para>
|
||||
|
||||
<para>
|
||||
At present, the capabilities logic for computing process capabilities
|
||||
on <function>execve</function> and <function>set*uid</function>,
|
||||
checking capabilities for a particular process, saving and checking
|
||||
capabilities for netlink messages, and handling the
|
||||
<function>capget</function> and <function>capset</function> system
|
||||
calls have been moved into the capabilities module. There are still a
|
||||
few locations in the base kernel where capability-related fields are
|
||||
directly examined or modified, but the current version of the LSM
|
||||
patch does allow a security module to completely replace the
|
||||
assignment and testing of capabilities. These few locations would
|
||||
need to be changed if the capability-related fields were moved into
|
||||
the security field. The following is a list of known locations that
|
||||
still perform such direct examination or modification of
|
||||
capability-related fields:
|
||||
<itemizedlist>
|
||||
<listitem><para><filename>fs/open.c</filename>:<function>sys_access</function></para></listitem>
|
||||
<listitem><para><filename>fs/lockd/host.c</filename>:<function>nlm_bind_host</function></para></listitem>
|
||||
<listitem><para><filename>fs/nfsd/auth.c</filename>:<function>nfsd_setuser</function></para></listitem>
|
||||
<listitem><para><filename>fs/proc/array.c</filename>:<function>task_cap</function></para></listitem>
|
||||
</itemizedlist>
|
||||
</para>
|
||||
|
||||
</sect1>
|
||||
|
||||
</article>
|
||||
|
|
@ -0,0 +1,201 @@
|
|||
========================================================
|
||||
Linux Security Modules: General Security Hooks for Linux
|
||||
========================================================
|
||||
|
||||
:Author: Stephen Smalley
|
||||
:Author: Timothy Fraser
|
||||
:Author: Chris Vance
|
||||
|
||||
.. note::
|
||||
|
||||
The APIs described in this book are outdated.
|
||||
|
||||
Introduction
|
||||
============
|
||||
|
||||
In March 2001, the National Security Agency (NSA) gave a presentation
|
||||
about Security-Enhanced Linux (SELinux) at the 2.5 Linux Kernel Summit.
|
||||
SELinux is an implementation of flexible and fine-grained
|
||||
nondiscretionary access controls in the Linux kernel, originally
|
||||
implemented as its own particular kernel patch. Several other security
|
||||
projects (e.g. RSBAC, Medusa) have also developed flexible access
|
||||
control architectures for the Linux kernel, and various projects have
|
||||
developed particular access control models for Linux (e.g. LIDS, DTE,
|
||||
SubDomain). Each project has developed and maintained its own kernel
|
||||
patch to support its security needs.
|
||||
|
||||
In response to the NSA presentation, Linus Torvalds made a set of
|
||||
remarks that described a security framework he would be willing to
|
||||
consider for inclusion in the mainstream Linux kernel. He described a
|
||||
general framework that would provide a set of security hooks to control
|
||||
operations on kernel objects and a set of opaque security fields in
|
||||
kernel data structures for maintaining security attributes. This
|
||||
framework could then be used by loadable kernel modules to implement any
|
||||
desired model of security. Linus also suggested the possibility of
|
||||
migrating the Linux capabilities code into such a module.
|
||||
|
||||
The Linux Security Modules (LSM) project was started by WireX to develop
|
||||
such a framework. LSM is a joint development effort by several security
|
||||
projects, including Immunix, SELinux, SGI and Janus, and several
|
||||
individuals, including Greg Kroah-Hartman and James Morris, to develop a
|
||||
Linux kernel patch that implements this framework. The patch is
|
||||
currently tracking the 2.4 series and is targeted for integration into
|
||||
the 2.5 development series. This technical report provides an overview
|
||||
of the framework and the example capabilities security module provided
|
||||
by the LSM kernel patch.
|
||||
|
||||
LSM Framework
|
||||
=============
|
||||
|
||||
The LSM kernel patch provides a general kernel framework to support
|
||||
security modules. In particular, the LSM framework is primarily focused
|
||||
on supporting access control modules, although future development is
|
||||
likely to address other security needs such as auditing. By itself, the
|
||||
framework does not provide any additional security; it merely provides
|
||||
the infrastructure to support security modules. The LSM kernel patch
|
||||
also moves most of the capabilities logic into an optional security
|
||||
module, with the system defaulting to the traditional superuser logic.
|
||||
This capabilities module is discussed further in
|
||||
`LSM Capabilities Module <#cap>`__.
|
||||
|
||||
The LSM kernel patch adds security fields to kernel data structures and
|
||||
inserts calls to hook functions at critical points in the kernel code to
|
||||
manage the security fields and to perform access control. It also adds
|
||||
functions for registering and unregistering security modules, and adds a
|
||||
general :c:func:`security()` system call to support new system calls
|
||||
for security-aware applications.
|
||||
|
||||
The LSM security fields are simply ``void*`` pointers. For process and
|
||||
program execution security information, security fields were added to
|
||||
:c:type:`struct task_struct <task_struct>` and
|
||||
:c:type:`struct linux_binprm <linux_binprm>`. For filesystem
|
||||
security information, a security field was added to :c:type:`struct
|
||||
super_block <super_block>`. For pipe, file, and socket security
|
||||
information, security fields were added to :c:type:`struct inode
|
||||
<inode>` and :c:type:`struct file <file>`. For packet and
|
||||
network device security information, security fields were added to
|
||||
:c:type:`struct sk_buff <sk_buff>` and :c:type:`struct
|
||||
net_device <net_device>`. For System V IPC security information,
|
||||
security fields were added to :c:type:`struct kern_ipc_perm
|
||||
<kern_ipc_perm>` and :c:type:`struct msg_msg
|
||||
<msg_msg>`; additionally, the definitions for :c:type:`struct
|
||||
msg_msg <msg_msg>`, struct msg_queue, and struct shmid_kernel
|
||||
were moved to header files (``include/linux/msg.h`` and
|
||||
``include/linux/shm.h`` as appropriate) to allow the security modules to
|
||||
use these definitions.
|
||||
|
||||
Each LSM hook is a function pointer in a global table, security_ops.
|
||||
This table is a :c:type:`struct security_operations
|
||||
<security_operations>` structure as defined by
|
||||
``include/linux/security.h``. Detailed documentation for each hook is
|
||||
included in this header file. At present, this structure consists of a
|
||||
collection of substructures that group related hooks based on the kernel
|
||||
object (e.g. task, inode, file, sk_buff, etc) as well as some top-level
|
||||
hook function pointers for system operations. This structure is likely
|
||||
to be flattened in the future for performance. The placement of the hook
|
||||
calls in the kernel code is described by the "called:" lines in the
|
||||
per-hook documentation in the header file. The hook calls can also be
|
||||
easily found in the kernel code by looking for the string
|
||||
"security_ops->".
|
||||
|
||||
Linus mentioned per-process security hooks in his original remarks as a
|
||||
possible alternative to global security hooks. However, if LSM were to
|
||||
start from the perspective of per-process hooks, then the base framework
|
||||
would have to deal with how to handle operations that involve multiple
|
||||
processes (e.g. kill), since each process might have its own hook for
|
||||
controlling the operation. This would require a general mechanism for
|
||||
composing hooks in the base framework. Additionally, LSM would still
|
||||
need global hooks for operations that have no process context (e.g.
|
||||
network input operations). Consequently, LSM provides global security
|
||||
hooks, but a security module is free to implement per-process hooks
|
||||
(where that makes sense) by storing a security_ops table in each
|
||||
process' security field and then invoking these per-process hooks from
|
||||
the global hooks. The problem of composition is thus deferred to the
|
||||
module.
|
||||
|
||||
The global security_ops table is initialized to a set of hook functions
|
||||
provided by a dummy security module that provides traditional superuser
|
||||
logic. A :c:func:`register_security()` function (in
|
||||
``security/security.c``) is provided to allow a security module to set
|
||||
security_ops to refer to its own hook functions, and an
|
||||
:c:func:`unregister_security()` function is provided to revert
|
||||
security_ops to the dummy module hooks. This mechanism is used to set
|
||||
the primary security module, which is responsible for making the final
|
||||
decision for each hook.
|
||||
|
||||
LSM also provides a simple mechanism for stacking additional security
|
||||
modules with the primary security module. It defines
|
||||
:c:func:`register_security()` and
|
||||
:c:func:`unregister_security()` hooks in the :c:type:`struct
|
||||
security_operations <security_operations>` structure and
|
||||
provides :c:func:`mod_reg_security()` and
|
||||
:c:func:`mod_unreg_security()` functions that invoke these hooks
|
||||
after performing some sanity checking. A security module can call these
|
||||
functions in order to stack with other modules. However, the actual
|
||||
details of how this stacking is handled are deferred to the module,
|
||||
which can implement these hooks in any way it wishes (including always
|
||||
returning an error if it does not wish to support stacking). In this
|
||||
manner, LSM again defers the problem of composition to the module.
|
||||
|
||||
Although the LSM hooks are organized into substructures based on kernel
|
||||
object, all of the hooks can be viewed as falling into two major
|
||||
categories: hooks that are used to manage the security fields and hooks
|
||||
that are used to perform access control. Examples of the first category
|
||||
of hooks include the :c:func:`alloc_security()` and
|
||||
:c:func:`free_security()` hooks defined for each kernel data
|
||||
structure that has a security field. These hooks are used to allocate
|
||||
and free security structures for kernel objects. The first category of
|
||||
hooks also includes hooks that set information in the security field
|
||||
after allocation, such as the :c:func:`post_lookup()` hook in
|
||||
:c:type:`struct inode_security_ops <inode_security_ops>`.
|
||||
This hook is used to set security information for inodes after
|
||||
successful lookup operations. An example of the second category of hooks
|
||||
is the :c:func:`permission()` hook in :c:type:`struct
|
||||
inode_security_ops <inode_security_ops>`. This hook checks
|
||||
permission when accessing an inode.
|
||||
|
||||
LSM Capabilities Module
|
||||
=======================
|
||||
|
||||
The LSM kernel patch moves most of the existing POSIX.1e capabilities
|
||||
logic into an optional security module stored in the file
|
||||
``security/capability.c``. This change allows users who do not want to
|
||||
use capabilities to omit this code entirely from their kernel, instead
|
||||
using the dummy module for traditional superuser logic or any other
|
||||
module that they desire. This change also allows the developers of the
|
||||
capabilities logic to maintain and enhance their code more freely,
|
||||
without needing to integrate patches back into the base kernel.
|
||||
|
||||
In addition to moving the capabilities logic, the LSM kernel patch could
|
||||
move the capability-related fields from the kernel data structures into
|
||||
the new security fields managed by the security modules. However, at
|
||||
present, the LSM kernel patch leaves the capability fields in the kernel
|
||||
data structures. In his original remarks, Linus suggested that this
|
||||
might be preferable so that other security modules can be easily stacked
|
||||
with the capabilities module without needing to chain multiple security
|
||||
structures on the security field. It also avoids imposing extra overhead
|
||||
on the capabilities module to manage the security fields. However, the
|
||||
LSM framework could certainly support such a move if it is determined to
|
||||
be desirable, with only a few additional changes described below.
|
||||
|
||||
At present, the capabilities logic for computing process capabilities on
|
||||
:c:func:`execve()` and :c:func:`set\*uid()`, checking
|
||||
capabilities for a particular process, saving and checking capabilities
|
||||
for netlink messages, and handling the :c:func:`capget()` and
|
||||
:c:func:`capset()` system calls have been moved into the
|
||||
capabilities module. There are still a few locations in the base kernel
|
||||
where capability-related fields are directly examined or modified, but
|
||||
the current version of the LSM patch does allow a security module to
|
||||
completely replace the assignment and testing of capabilities. These few
|
||||
locations would need to be changed if the capability-related fields were
|
||||
moved into the security field. The following is a list of known
|
||||
locations that still perform such direct examination or modification of
|
||||
capability-related fields:
|
||||
|
||||
- ``fs/open.c``::c:func:`sys_access()`
|
||||
|
||||
- ``fs/lockd/host.c``::c:func:`nlm_bind_host()`
|
||||
|
||||
- ``fs/nfsd/auth.c``::c:func:`nfsd_setuser()`
|
||||
|
||||
- ``fs/proc/array.c``::c:func:`task_cap()`
|
||||
Loading…
Reference in New Issue