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kasan: disable freed user page poisoning with HW tags

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Poisoning freed pages protects against kernel use-after-free. The
likelihood of such a bug involving kernel pages is significantly higher
than that for user pages. At the same time, poisoning freed pages can
impose a significant performance cost, which cannot always be justified
for user pages given the lower probability of finding a bug. Therefore,
disable freed user page poisoning when using HW tags. We identify
"user" pages via the flag set GFP_HIGHUSER_MOVABLE, which indicates
a strong likelihood of not being directly accessible to the kernel.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Link: https://linux-review.googlesource.com/id/I716846e2de8ef179f44e835770df7e6307be96c9
Link: https://lore.kernel.org/r/20210602235230.3928842-5-pcc@google.com
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Peter Collingbourne 2021-06-02 16:52:30 -07:00 committed by Will Deacon
parent 013bb59dbb
commit c275c5c6d5
5 changed files with 37 additions and 9 deletions
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13
include/linux/gfp.h
View File

@ -54,8 +54,9 @@ struct vm_area_struct;
#define ___GFP_THISNODE 0x200000u
#define ___GFP_ACCOUNT 0x400000u
#define ___GFP_ZEROTAGS 0x800000u
#define ___GFP_SKIP_KASAN_POISON 0x1000000u
#ifdef CONFIG_LOCKDEP
#define ___GFP_NOLOCKDEP 0x1000000u
#define ___GFP_NOLOCKDEP 0x2000000u
#else
#define ___GFP_NOLOCKDEP 0
#endif
@ -233,17 +234,22 @@ struct vm_area_struct;
*
* %__GFP_ZEROTAGS returns a page with zeroed memory tags on success, if
* __GFP_ZERO is set.
*
* %__GFP_SKIP_KASAN_POISON returns a page which does not need to be poisoned
* on deallocation. Typically used for userspace pages. Currently only has an
* effect in HW tags mode.
*/
#define __GFP_NOWARN ((__force gfp_t)___GFP_NOWARN)
#define __GFP_COMP ((__force gfp_t)___GFP_COMP)
#define __GFP_ZERO ((__force gfp_t)___GFP_ZERO)
#define __GFP_ZEROTAGS ((__force gfp_t)___GFP_ZEROTAGS)
#define __GFP_SKIP_KASAN_POISON ((__force gfp_t)___GFP_SKIP_KASAN_POISON)
/* Disable lockdep for GFP context tracking */
#define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
/* Room for N __GFP_FOO bits */
#define __GFP_BITS_SHIFT (24 + IS_ENABLED(CONFIG_LOCKDEP))
#define __GFP_BITS_SHIFT (25 + IS_ENABLED(CONFIG_LOCKDEP))
#define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
/**
@ -324,7 +330,8 @@ struct vm_area_struct;
#define GFP_DMA __GFP_DMA
#define GFP_DMA32 __GFP_DMA32
#define GFP_HIGHUSER (GFP_USER | __GFP_HIGHMEM)
#define GFP_HIGHUSER_MOVABLE (GFP_HIGHUSER | __GFP_MOVABLE)
#define GFP_HIGHUSER_MOVABLE (GFP_HIGHUSER | __GFP_MOVABLE | \
__GFP_SKIP_KASAN_POISON)
#define GFP_TRANSHUGE_LIGHT ((GFP_HIGHUSER_MOVABLE | __GFP_COMP | \
__GFP_NOMEMALLOC | __GFP_NOWARN) & ~__GFP_RECLAIM)
#define GFP_TRANSHUGE (GFP_TRANSHUGE_LIGHT | __GFP_DIRECT_RECLAIM)

9
include/linux/page-flags.h
View File

@ -137,6 +137,9 @@ enum pageflags {
#endif
#ifdef CONFIG_64BIT
PG_arch_2,
#endif
#ifdef CONFIG_KASAN_HW_TAGS
PG_skip_kasan_poison,
#endif
__NR_PAGEFLAGS,
@ -443,6 +446,12 @@ TESTCLEARFLAG(Young, young, PF_ANY)
PAGEFLAG(Idle, idle, PF_ANY)
#endif
#ifdef CONFIG_KASAN_HW_TAGS
PAGEFLAG(SkipKASanPoison, skip_kasan_poison, PF_HEAD)
#else
PAGEFLAG_FALSE(SkipKASanPoison)
#endif
/*
* PageReported() is used to track reported free pages within the Buddy
* allocator. We can use the non-atomic version of the test and set

9
include/trace/events/mmflags.h
View File

@ -85,6 +85,12 @@
#define IF_HAVE_PG_ARCH_2(flag,string)
#endif
#ifdef CONFIG_KASAN_HW_TAGS
#define IF_HAVE_PG_SKIP_KASAN_POISON(flag,string) ,{1UL << flag, string}
#else
#define IF_HAVE_PG_SKIP_KASAN_POISON(flag,string)
#endif
#define __def_pageflag_names \
{1UL << PG_locked, "locked" }, \
{1UL << PG_waiters, "waiters" }, \
@ -112,7 +118,8 @@ IF_HAVE_PG_UNCACHED(PG_uncached, "uncached" ) \
IF_HAVE_PG_HWPOISON(PG_hwpoison, "hwpoison" ) \
IF_HAVE_PG_IDLE(PG_young, "young" ) \
IF_HAVE_PG_IDLE(PG_idle, "idle" ) \
IF_HAVE_PG_ARCH_2(PG_arch_2, "arch_2" )
IF_HAVE_PG_ARCH_2(PG_arch_2, "arch_2" ) \
IF_HAVE_PG_SKIP_KASAN_POISON(PG_skip_kasan_poison, "skip_kasan_poison")
#define show_page_flags(flags) \
(flags) ? __print_flags(flags, "|", \

3
mm/kasan/hw_tags.c
View File

@ -246,6 +246,9 @@ void kasan_alloc_pages(struct page *page, unsigned int order, gfp_t flags)
*/
bool init = !want_init_on_free() && want_init_on_alloc(flags);
if (flags & __GFP_SKIP_KASAN_POISON)
SetPageSkipKASanPoison(page);
if (flags & __GFP_ZEROTAGS) {
int i;

12
mm/page_alloc.c
View File

@ -394,11 +394,12 @@ static DEFINE_STATIC_KEY_TRUE(deferred_pages);
* on-demand allocation and then freed again before the deferred pages
* initialization is done, but this is not likely to happen.
*/
static inline bool should_skip_kasan_poison(fpi_t fpi_flags)
static inline bool should_skip_kasan_poison(struct page *page, fpi_t fpi_flags)
{
return static_branch_unlikely(&deferred_pages) ||
(!IS_ENABLED(CONFIG_KASAN_GENERIC) &&
(fpi_flags & FPI_SKIP_KASAN_POISON));
(fpi_flags & FPI_SKIP_KASAN_POISON)) ||
PageSkipKASanPoison(page);
}
/* Returns true if the struct page for the pfn is uninitialised */
@ -449,10 +450,11 @@ defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
return false;
}
#else
static inline bool should_skip_kasan_poison(fpi_t fpi_flags)
static inline bool should_skip_kasan_poison(struct page *page, fpi_t fpi_flags)
{
return (!IS_ENABLED(CONFIG_KASAN_GENERIC) &&
(fpi_flags & FPI_SKIP_KASAN_POISON));
(fpi_flags & FPI_SKIP_KASAN_POISON)) ||
PageSkipKASanPoison(page);
}
static inline bool early_page_uninitialised(unsigned long pfn)
@ -1244,7 +1246,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
unsigned int order, bool check_free, fpi_t fpi_flags)
{
int bad = 0;
bool skip_kasan_poison = should_skip_kasan_poison(fpi_flags);
bool skip_kasan_poison = should_skip_kasan_poison(page, fpi_flags);
VM_BUG_ON_PAGE(PageTail(page), page);