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Petr Mladek 2ac5a3bf70 vsprintf: Do not break early boot with probing addresses 
The commit 3e5903eb9c ("vsprintf: Prevent crash when dereferencing
invalid pointers") broke boot on several architectures. The common
pattern is that probe_kernel_read() is not working during early
boot because userspace access framework is not ready.

It is a generic problem. We have to avoid any complex external
functions in vsprintf() code, especially in the common path.
They might break printk() easily and are hard to debug.

Replace probe_kernel_read() with some simple checks for obvious
problems.

Details:

1. Report on Power:

Kernel crashes very early during boot with with CONFIG_PPC_KUAP and
CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG

The problem is the combination of some new code called via printk(),
check_pointer() which calls probe_kernel_read(). That then calls
allow_user_access() (PPC_KUAP) and that uses mmu_has_feature() too early
(before we've patched features). With the JUMP_LABEL debug enabled that
causes us to call printk() & dump_stack() and we end up recursing and
overflowing the stack.

Because it happens so early you don't get any output, just an apparently
dead system.

The stack trace (which you don't see) is something like:

  ...
  dump_stack+0xdc
  probe_kernel_read+0x1a4
  check_pointer+0x58
  string+0x3c
  vsnprintf+0x1bc
  vscnprintf+0x20
  printk_safe_log_store+0x7c
  printk+0x40
  dump_stack_print_info+0xbc
  dump_stack+0x8
  probe_kernel_read+0x1a4
  probe_kernel_read+0x19c
  check_pointer+0x58
  string+0x3c
  vsnprintf+0x1bc
  vscnprintf+0x20
  vprintk_store+0x6c
  vprintk_emit+0xec
  vprintk_func+0xd4
  printk+0x40
  cpufeatures_process_feature+0xc8
  scan_cpufeatures_subnodes+0x380
  of_scan_flat_dt_subnodes+0xb4
  dt_cpu_ftrs_scan_callback+0x158
  of_scan_flat_dt+0xf0
  dt_cpu_ftrs_scan+0x3c
  early_init_devtree+0x360
  early_setup+0x9c

2. Report on s390:

vsnprintf invocations, are broken on s390. For example, the early boot
output now looks like this where the first (efault) should be
the linux_banner:

[    0.099985] (efault)
[    0.099985] setup: Linux is running as a z/VM guest operating system in 64-bit mode
[    0.100066] setup: The maximum memory size is 8192MB
[    0.100070] cma: Reserved 4 MiB at (efault)
[    0.100100] numa: NUMA mode: (efault)

The reason for this, is that the code assumes that
probe_kernel_address() works very early. This however is not true on
at least s390. Uaccess on KERNEL_DS works only after page tables have
been setup on s390, which happens with setup_arch()->paging_init().

Any probe_kernel_address() invocation before that will return -EFAULT.

Fixes: 3e5903eb9c ("vsprintf: Prevent crash when dereferencing invalid pointers")
Link: http://lkml.kernel.org/r/20190510084213.22149-1-pmladek@suse.com
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: "Tobin C . Harding" <me@tobin.cc>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Russell Currey <ruscur@russell.cc>
Cc: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Stephen Rothwell <sfr@ozlabs.org>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: linux-arch@vger.kernel.org
Cc: linux-s390@vger.kernel.org
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Petr Mladek <pmladek@suse.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
2019-05-10 16:17:26 +02:00
arch Printk changes for 5.2 2019-05-07 09:18:12 -07:00
block bfq: update internal depth state when queue depth changes 2019-04-13 19:08:22 -06:00
certs kexec, KEYS: Make use of platform keyring for signature verify 2019-02-04 17:34:07 -05:00
crypto Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2019-05-06 20:15:06 -07:00
Documentation Printk changes for 5.2 2019-05-07 09:18:12 -07:00
drivers Printk changes for 5.2 2019-05-07 09:18:12 -07:00
fs Printk changes for 5.2 2019-05-07 09:18:12 -07:00
include Printk changes for 5.2 2019-05-07 09:18:12 -07:00
init Printk changes for 5.2 2019-05-07 09:18:12 -07:00
ipc Merge branch 'work.mount' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2019-03-12 14:08:19 -07:00
kernel Printk changes for 5.2 2019-05-07 09:18:12 -07:00
lib vsprintf: Do not break early boot with probing addresses 2019-05-10 16:17:26 +02:00
LICENSES LICENSES: Add GCC runtime library exception text 2019-01-16 14:54:15 -07:00
mm Printk changes for 5.2 2019-05-07 09:18:12 -07:00
net Printk changes for 5.2 2019-05-07 09:18:12 -07:00
samples Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2019-03-11 08:54:01 -07:00
scripts Merge branch 'core-objtool-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-05-06 11:39:17 -07:00
security Merge branch 'next-general' of git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security 2019-05-07 08:39:54 -07:00
sound spi: Updates for v5.2 2019-05-07 07:44:33 -07:00
tools Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/livepatching/livepatching 2019-05-07 08:56:04 -07:00
usr user/Makefile: Fix typo and capitalization in comment section 2018-12-11 00:18:03 +09:00
virt arm64 updates for 5.2 2019-05-06 17:54:22 -07:00
.clang-format clang-format: Update with the latest for_each macro list 2019-04-12 12:49:54 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore kbuild: Add support for DT binding schema checks 2018-12-13 09:41:32 -06:00
.mailmap Update Nicolas Pitre's email address 2019-04-02 18:12:44 -10:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS Char/Misc driver patches for 5.1-rc1 2019-03-06 14:18:59 -08:00
Kbuild Kbuild updates for v5.1 2019-03-10 17:48:21 -07:00
Kconfig kconfig: move the "Executable file formats" menu to fs/Kconfig.binfmt 2018-08-02 08:06:55 +09:00
MAINTAINERS hwmon updates for v5.2 2019-05-06 19:56:51 -07:00
Makefile Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/livepatching/livepatching 2019-05-07 08:56:04 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.