![Dave Hansen](/assets/img/avatar_default.png)
Global pages are bad for hardening because they potentially let an exploit read the kernel image via a Meltdown-style attack which makes it easier to find gadgets. But, global pages are good for performance because they reduce TLB misses when making user/kernel transitions, especially when PCIDs are not available, such as on older hardware, or where a hypervisor has disabled them for some reason. This patch implements a basic, sane policy: If you have PCIDs, you only map a minimal amount of kernel text global. If you do not have PCIDs, you map all kernel text global. This policy effectively makes PCIDs something that not only adds performance but a little bit of hardening as well. I ran a simple "lseek" microbenchmark[1] to test the benefit on a modern Atom microserver. Most of the benefit comes from applying the series before this patch ("entry only"), but there is still a signifiant benefit from this patch. No Global Lines (baseline ): 6077741 lseeks/sec 88 Global Lines (entry only): 7528609 lseeks/sec (+23.9%) 94 Global Lines (this patch): 8433111 lseeks/sec (+38.8%) [1.] https://github.com/antonblanchard/will-it-scale/blob/master/tests/lseek1.c Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Arjan van de Ven <arjan@linux.intel.com> Cc: Borislav Petkov <bp@alien8.de> Cc: Dan Williams <dan.j.williams@intel.com> Cc: David Woodhouse <dwmw2@infradead.org> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Hugh Dickins <hughd@google.com> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Juergen Gross <jgross@suse.com> Cc: Kees Cook <keescook@google.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Nadav Amit <namit@vmware.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-mm@kvack.org Link: http://lkml.kernel.org/r/20180406205518.E3D989EB@viggo.jf.intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
485 lines
13 KiB
C
485 lines
13 KiB
C
/*
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* Copyright(c) 2017 Intel Corporation. All rights reserved.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of version 2 of the GNU General Public License as
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* published by the Free Software Foundation.
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*
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* This program is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* General Public License for more details.
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*
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* This code is based in part on work published here:
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*
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* https://github.com/IAIK/KAISER
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*
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* The original work was written by and and signed off by for the Linux
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* kernel by:
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*
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* Signed-off-by: Richard Fellner <richard.fellner@student.tugraz.at>
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* Signed-off-by: Moritz Lipp <moritz.lipp@iaik.tugraz.at>
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* Signed-off-by: Daniel Gruss <daniel.gruss@iaik.tugraz.at>
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* Signed-off-by: Michael Schwarz <michael.schwarz@iaik.tugraz.at>
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*
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* Major changes to the original code by: Dave Hansen <dave.hansen@intel.com>
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* Mostly rewritten by Thomas Gleixner <tglx@linutronix.de> and
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* Andy Lutomirsky <luto@amacapital.net>
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*/
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#include <linux/kernel.h>
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#include <linux/errno.h>
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#include <linux/string.h>
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#include <linux/types.h>
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#include <linux/bug.h>
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#include <linux/init.h>
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#include <linux/spinlock.h>
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#include <linux/mm.h>
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#include <linux/uaccess.h>
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#include <asm/cpufeature.h>
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#include <asm/hypervisor.h>
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#include <asm/vsyscall.h>
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#include <asm/cmdline.h>
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#include <asm/pti.h>
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#include <asm/pgtable.h>
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#include <asm/pgalloc.h>
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#include <asm/tlbflush.h>
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#include <asm/desc.h>
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#undef pr_fmt
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#define pr_fmt(fmt) "Kernel/User page tables isolation: " fmt
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/* Backporting helper */
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#ifndef __GFP_NOTRACK
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#define __GFP_NOTRACK 0
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#endif
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static void __init pti_print_if_insecure(const char *reason)
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{
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if (boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
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pr_info("%s\n", reason);
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}
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static void __init pti_print_if_secure(const char *reason)
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{
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if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
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pr_info("%s\n", reason);
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}
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enum pti_mode {
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PTI_AUTO = 0,
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PTI_FORCE_OFF,
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PTI_FORCE_ON
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} pti_mode;
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void __init pti_check_boottime_disable(void)
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{
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char arg[5];
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int ret;
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/* Assume mode is auto unless overridden. */
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pti_mode = PTI_AUTO;
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if (hypervisor_is_type(X86_HYPER_XEN_PV)) {
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pti_mode = PTI_FORCE_OFF;
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pti_print_if_insecure("disabled on XEN PV.");
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return;
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}
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ret = cmdline_find_option(boot_command_line, "pti", arg, sizeof(arg));
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if (ret > 0) {
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if (ret == 3 && !strncmp(arg, "off", 3)) {
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pti_mode = PTI_FORCE_OFF;
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pti_print_if_insecure("disabled on command line.");
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return;
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}
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if (ret == 2 && !strncmp(arg, "on", 2)) {
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pti_mode = PTI_FORCE_ON;
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pti_print_if_secure("force enabled on command line.");
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goto enable;
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}
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if (ret == 4 && !strncmp(arg, "auto", 4)) {
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pti_mode = PTI_AUTO;
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goto autosel;
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}
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}
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if (cmdline_find_option_bool(boot_command_line, "nopti")) {
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pti_mode = PTI_FORCE_OFF;
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pti_print_if_insecure("disabled on command line.");
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return;
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}
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autosel:
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if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN))
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return;
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enable:
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setup_force_cpu_cap(X86_FEATURE_PTI);
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}
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pgd_t __pti_set_user_pgd(pgd_t *pgdp, pgd_t pgd)
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{
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/*
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* Changes to the high (kernel) portion of the kernelmode page
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* tables are not automatically propagated to the usermode tables.
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*
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* Users should keep in mind that, unlike the kernelmode tables,
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* there is no vmalloc_fault equivalent for the usermode tables.
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* Top-level entries added to init_mm's usermode pgd after boot
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* will not be automatically propagated to other mms.
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*/
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if (!pgdp_maps_userspace(pgdp))
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return pgd;
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/*
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* The user page tables get the full PGD, accessible from
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* userspace:
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*/
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kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd;
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/*
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* If this is normal user memory, make it NX in the kernel
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* pagetables so that, if we somehow screw up and return to
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* usermode with the kernel CR3 loaded, we'll get a page fault
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* instead of allowing user code to execute with the wrong CR3.
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*
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* As exceptions, we don't set NX if:
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* - _PAGE_USER is not set. This could be an executable
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* EFI runtime mapping or something similar, and the kernel
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* may execute from it
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* - we don't have NX support
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* - we're clearing the PGD (i.e. the new pgd is not present).
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*/
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if ((pgd.pgd & (_PAGE_USER|_PAGE_PRESENT)) == (_PAGE_USER|_PAGE_PRESENT) &&
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(__supported_pte_mask & _PAGE_NX))
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pgd.pgd |= _PAGE_NX;
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/* return the copy of the PGD we want the kernel to use: */
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return pgd;
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}
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/*
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* Walk the user copy of the page tables (optionally) trying to allocate
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* page table pages on the way down.
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*
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* Returns a pointer to a P4D on success, or NULL on failure.
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*/
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static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
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{
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pgd_t *pgd = kernel_to_user_pgdp(pgd_offset_k(address));
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gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
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if (address < PAGE_OFFSET) {
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WARN_ONCE(1, "attempt to walk user address\n");
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return NULL;
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}
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if (pgd_none(*pgd)) {
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unsigned long new_p4d_page = __get_free_page(gfp);
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if (!new_p4d_page)
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return NULL;
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set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page)));
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}
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BUILD_BUG_ON(pgd_large(*pgd) != 0);
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return p4d_offset(pgd, address);
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}
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/*
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* Walk the user copy of the page tables (optionally) trying to allocate
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* page table pages on the way down.
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*
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* Returns a pointer to a PMD on success, or NULL on failure.
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*/
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static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
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{
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gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
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p4d_t *p4d = pti_user_pagetable_walk_p4d(address);
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pud_t *pud;
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BUILD_BUG_ON(p4d_large(*p4d) != 0);
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if (p4d_none(*p4d)) {
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unsigned long new_pud_page = __get_free_page(gfp);
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if (!new_pud_page)
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return NULL;
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set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
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}
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pud = pud_offset(p4d, address);
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/* The user page tables do not use large mappings: */
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if (pud_large(*pud)) {
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WARN_ON(1);
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return NULL;
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}
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if (pud_none(*pud)) {
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unsigned long new_pmd_page = __get_free_page(gfp);
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if (!new_pmd_page)
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return NULL;
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set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page)));
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}
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return pmd_offset(pud, address);
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}
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#ifdef CONFIG_X86_VSYSCALL_EMULATION
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/*
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* Walk the shadow copy of the page tables (optionally) trying to allocate
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* page table pages on the way down. Does not support large pages.
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*
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* Note: this is only used when mapping *new* kernel data into the
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* user/shadow page tables. It is never used for userspace data.
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*
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* Returns a pointer to a PTE on success, or NULL on failure.
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*/
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static __init pte_t *pti_user_pagetable_walk_pte(unsigned long address)
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{
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gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
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pmd_t *pmd = pti_user_pagetable_walk_pmd(address);
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pte_t *pte;
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/* We can't do anything sensible if we hit a large mapping. */
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if (pmd_large(*pmd)) {
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WARN_ON(1);
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return NULL;
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}
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if (pmd_none(*pmd)) {
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unsigned long new_pte_page = __get_free_page(gfp);
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if (!new_pte_page)
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return NULL;
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set_pmd(pmd, __pmd(_KERNPG_TABLE | __pa(new_pte_page)));
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}
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pte = pte_offset_kernel(pmd, address);
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if (pte_flags(*pte) & _PAGE_USER) {
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WARN_ONCE(1, "attempt to walk to user pte\n");
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return NULL;
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}
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return pte;
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}
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static void __init pti_setup_vsyscall(void)
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{
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pte_t *pte, *target_pte;
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unsigned int level;
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pte = lookup_address(VSYSCALL_ADDR, &level);
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if (!pte || WARN_ON(level != PG_LEVEL_4K) || pte_none(*pte))
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return;
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target_pte = pti_user_pagetable_walk_pte(VSYSCALL_ADDR);
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if (WARN_ON(!target_pte))
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return;
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*target_pte = *pte;
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set_vsyscall_pgtable_user_bits(kernel_to_user_pgdp(swapper_pg_dir));
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}
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#else
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static void __init pti_setup_vsyscall(void) { }
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#endif
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static void
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pti_clone_pmds(unsigned long start, unsigned long end, pmdval_t clear)
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{
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unsigned long addr;
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/*
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* Clone the populated PMDs which cover start to end. These PMD areas
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* can have holes.
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*/
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for (addr = start; addr < end; addr += PMD_SIZE) {
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pmd_t *pmd, *target_pmd;
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pgd_t *pgd;
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p4d_t *p4d;
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pud_t *pud;
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pgd = pgd_offset_k(addr);
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if (WARN_ON(pgd_none(*pgd)))
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return;
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p4d = p4d_offset(pgd, addr);
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if (WARN_ON(p4d_none(*p4d)))
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return;
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pud = pud_offset(p4d, addr);
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if (pud_none(*pud))
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continue;
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pmd = pmd_offset(pud, addr);
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if (pmd_none(*pmd))
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continue;
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target_pmd = pti_user_pagetable_walk_pmd(addr);
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if (WARN_ON(!target_pmd))
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return;
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/*
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* Only clone present PMDs. This ensures only setting
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* _PAGE_GLOBAL on present PMDs. This should only be
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* called on well-known addresses anyway, so a non-
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* present PMD would be a surprise.
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*/
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if (WARN_ON(!(pmd_flags(*pmd) & _PAGE_PRESENT)))
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return;
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/*
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* Setting 'target_pmd' below creates a mapping in both
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* the user and kernel page tables. It is effectively
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* global, so set it as global in both copies. Note:
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* the X86_FEATURE_PGE check is not _required_ because
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* the CPU ignores _PAGE_GLOBAL when PGE is not
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* supported. The check keeps consistentency with
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* code that only set this bit when supported.
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*/
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if (boot_cpu_has(X86_FEATURE_PGE))
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*pmd = pmd_set_flags(*pmd, _PAGE_GLOBAL);
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/*
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* Copy the PMD. That is, the kernelmode and usermode
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* tables will share the last-level page tables of this
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* address range
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*/
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*target_pmd = pmd_clear_flags(*pmd, clear);
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}
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}
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/*
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* Clone a single p4d (i.e. a top-level entry on 4-level systems and a
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* next-level entry on 5-level systems.
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*/
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static void __init pti_clone_p4d(unsigned long addr)
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{
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p4d_t *kernel_p4d, *user_p4d;
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pgd_t *kernel_pgd;
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user_p4d = pti_user_pagetable_walk_p4d(addr);
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kernel_pgd = pgd_offset_k(addr);
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kernel_p4d = p4d_offset(kernel_pgd, addr);
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*user_p4d = *kernel_p4d;
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}
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/*
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* Clone the CPU_ENTRY_AREA into the user space visible page table.
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*/
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static void __init pti_clone_user_shared(void)
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{
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pti_clone_p4d(CPU_ENTRY_AREA_BASE);
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}
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/*
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* Clone the ESPFIX P4D into the user space visible page table
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*/
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static void __init pti_setup_espfix64(void)
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{
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#ifdef CONFIG_X86_ESPFIX64
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pti_clone_p4d(ESPFIX_BASE_ADDR);
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#endif
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}
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/*
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* Clone the populated PMDs of the entry and irqentry text and force it RO.
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*/
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static void __init pti_clone_entry_text(void)
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{
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pti_clone_pmds((unsigned long) __entry_text_start,
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(unsigned long) __irqentry_text_end,
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_PAGE_RW);
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}
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/*
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* Global pages and PCIDs are both ways to make kernel TLB entries
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* live longer, reduce TLB misses and improve kernel performance.
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* But, leaving all kernel text Global makes it potentially accessible
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* to Meltdown-style attacks which make it trivial to find gadgets or
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* defeat KASLR.
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*
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* Only use global pages when it is really worth it.
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*/
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static inline bool pti_kernel_image_global_ok(void)
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{
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/*
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* Systems with PCIDs get litlle benefit from global
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* kernel text and are not worth the downsides.
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*/
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if (cpu_feature_enabled(X86_FEATURE_PCID))
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return false;
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/*
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* Only do global kernel image for pti=auto. Do the most
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* secure thing (not global) if pti=on specified.
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*/
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if (pti_mode != PTI_AUTO)
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return false;
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/*
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* K8 may not tolerate the cleared _PAGE_RW on the userspace
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* global kernel image pages. Do the safe thing (disable
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* global kernel image). This is unlikely to ever be
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* noticed because PTI is disabled by default on AMD CPUs.
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*/
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if (boot_cpu_has(X86_FEATURE_K8))
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return false;
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return true;
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}
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/*
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* For some configurations, map all of kernel text into the user page
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* tables. This reduces TLB misses, especially on non-PCID systems.
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*/
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void pti_clone_kernel_text(void)
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{
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unsigned long start = PFN_ALIGN(_text);
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unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE);
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if (!pti_kernel_image_global_ok())
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return;
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pti_clone_pmds(start, end, _PAGE_RW);
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}
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/*
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* This is the only user for it and it is not arch-generic like
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* the other set_memory.h functions. Just extern it.
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*/
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extern int set_memory_nonglobal(unsigned long addr, int numpages);
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void pti_set_kernel_image_nonglobal(void)
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{
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/*
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* The identity map is created with PMDs, regardless of the
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* actual length of the kernel. We need to clear
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* _PAGE_GLOBAL up to a PMD boundary, not just to the end
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* of the image.
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*/
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unsigned long start = PFN_ALIGN(_text);
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unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE);
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if (pti_kernel_image_global_ok())
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return;
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pr_debug("set kernel image non-global\n");
|
|
|
|
set_memory_nonglobal(start, (end - start) >> PAGE_SHIFT);
|
|
}
|
|
|
|
/*
|
|
* Initialize kernel page table isolation
|
|
*/
|
|
void __init pti_init(void)
|
|
{
|
|
if (!static_cpu_has(X86_FEATURE_PTI))
|
|
return;
|
|
|
|
pr_info("enabled\n");
|
|
|
|
pti_clone_user_shared();
|
|
|
|
/* Undo all global bits from the init pagetables in head_64.S: */
|
|
pti_set_kernel_image_nonglobal();
|
|
/* Replace some of the global bits just for shared entry text: */
|
|
pti_clone_entry_text();
|
|
pti_setup_espfix64();
|
|
pti_setup_vsyscall();
|
|
}
|