Commit c275c5c6 authored by Peter Collingbourne's avatar Peter Collingbourne Committed by Will Deacon
Browse files

kasan: disable freed user page poisoning with HW tags 



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: default avatarPeter Collingbourne <pcc@google.com>
Reviewed-by: default avatarAndrey 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: default avatarWill Deacon <will@kernel.org>
parent 013bb59d

include/linux/gfp.h

+10 −3
Original line number Diff line number Diff line
@@ -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)

include/linux/page-flags.h

+9 −0
Original line number Diff line number Diff line
@@ -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

include/trace/events/mmflags.h

+8 −1
Original line number Diff line number Diff line
@@ -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, "|",				\

mm/kasan/hw_tags.c

+3 −0
Original line number Diff line number Diff line
@@ -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;

mm/page_alloc.c

+7 −5
Original line number Diff line number Diff line
@@ -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);