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x86/pti: Leave kernel text global for !PCID 

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>
hifive-unleashed-5.1
Dave Hansen 2018-04-06 13:55:18 -07:00 committed by Ingo Molnar
parent 39114b7a74
commit 8c06c7740d
3 changed files with 82 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
arch/x86/include/asm/pti.h
View File

@ -6,8 +6,10 @@
#ifdef CONFIG_PAGE_TABLE_ISOLATION
extern void pti_init(void);
extern void pti_check_boottime_disable(void);
extern void pti_clone_kernel_text(void);
#else
static inline void pti_check_boottime_disable(void) { }
static inline void pti_clone_kernel_text(void) { }
#endif
#endif /* __ASSEMBLY__ */

6
arch/x86/mm/init_64.c
View File

@ -1290,6 +1290,12 @@ void mark_rodata_ro(void)
(unsigned long) __va(__pa_symbol(_sdata)));
debug_checkwx();
/*
* Do this after all of the manipulation of the
* kernel text page tables are complete.
*/
pti_clone_kernel_text();
}
int kern_addr_valid(unsigned long addr)

78
arch/x86/mm/pti.c
View File

@ -66,12 +66,22 @@ static void __init pti_print_if_secure(const char *reason)
pr_info("%s\n", reason);
}
enum pti_mode {
PTI_AUTO = 0,
PTI_FORCE_OFF,
PTI_FORCE_ON
} pti_mode;
void __init pti_check_boottime_disable(void)
{
char arg[5];
int ret;
/* Assume mode is auto unless overridden. */
pti_mode = PTI_AUTO;
if (hypervisor_is_type(X86_HYPER_XEN_PV)) {
pti_mode = PTI_FORCE_OFF;
pti_print_if_insecure("disabled on XEN PV.");
return;
}
@ -79,18 +89,23 @@ void __init pti_check_boottime_disable(void)
ret = cmdline_find_option(boot_command_line, "pti", arg, sizeof(arg));
if (ret > 0) {
if (ret == 3 && !strncmp(arg, "off", 3)) {
pti_mode = PTI_FORCE_OFF;
pti_print_if_insecure("disabled on command line.");
return;
}
if (ret == 2 && !strncmp(arg, "on", 2)) {
pti_mode = PTI_FORCE_ON;
pti_print_if_secure("force enabled on command line.");
goto enable;
}
if (ret == 4 && !strncmp(arg, "auto", 4))
if (ret == 4 && !strncmp(arg, "auto", 4)) {
pti_mode = PTI_AUTO;
goto autosel;
}
}
if (cmdline_find_option_bool(boot_command_line, "nopti")) {
pti_mode = PTI_FORCE_OFF;
pti_print_if_insecure("disabled on command line.");
return;
}
@ -149,7 +164,7 @@ pgd_t __pti_set_user_pgd(pgd_t *pgdp, pgd_t pgd)
*
* Returns a pointer to a P4D on success, or NULL on failure.
*/
static __init p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
{
pgd_t *pgd = kernel_to_user_pgdp(pgd_offset_k(address));
gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
@ -177,7 +192,7 @@ static __init p4d_t *pti_user_pagetable_walk_p4d(unsigned long address)
*
* Returns a pointer to a PMD on success, or NULL on failure.
*/
static __init pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address)
{
gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
p4d_t *p4d = pti_user_pagetable_walk_p4d(address);
@ -267,7 +282,7 @@ static void __init pti_setup_vsyscall(void)
static void __init pti_setup_vsyscall(void) { }
#endif
static void __init
static void
pti_clone_pmds(unsigned long start, unsigned long end, pmdval_t clear)
{
unsigned long addr;
@ -372,6 +387,58 @@ static void __init pti_clone_entry_text(void)
_PAGE_RW);
}
/*
* Global pages and PCIDs are both ways to make kernel TLB entries
* live longer, reduce TLB misses and improve kernel performance.
* But, leaving all kernel text Global makes it potentially accessible
* to Meltdown-style attacks which make it trivial to find gadgets or
* defeat KASLR.
*
* Only use global pages when it is really worth it.
*/
static inline bool pti_kernel_image_global_ok(void)
{
/*
* Systems with PCIDs get litlle benefit from global
* kernel text and are not worth the downsides.
*/
if (cpu_feature_enabled(X86_FEATURE_PCID))
return false;
/*
* Only do global kernel image for pti=auto. Do the most
* secure thing (not global) if pti=on specified.
*/
if (pti_mode != PTI_AUTO)
return false;
/*
* K8 may not tolerate the cleared _PAGE_RW on the userspace
* global kernel image pages. Do the safe thing (disable
* global kernel image). This is unlikely to ever be
* noticed because PTI is disabled by default on AMD CPUs.
*/
if (boot_cpu_has(X86_FEATURE_K8))
return false;
return true;
}
/*
* For some configurations, map all of kernel text into the user page
* tables. This reduces TLB misses, especially on non-PCID systems.
*/
void pti_clone_kernel_text(void)
{
unsigned long start = PFN_ALIGN(_text);
unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE);
if (!pti_kernel_image_global_ok())
return;
pti_clone_pmds(start, end, _PAGE_RW);
}
/*
* This is the only user for it and it is not arch-generic like
* the other set_memory.h functions. Just extern it.
@ -388,6 +455,9 @@ void pti_set_kernel_image_nonglobal(void)
unsigned long start = PFN_ALIGN(_text);
unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE);
if (pti_kernel_image_global_ok())
return;
pr_debug("set kernel image non-global\n");
set_memory_nonglobal(start, (end - start) >> PAGE_SHIFT);