[PATCH v5 4/4] kasan: disable freed user page poisoning with HW tags
Peter Collingbourne
pcc at google.com
Tue Jun 1 12:50:49 PDT 2021
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 at google.com>
Link: https://linux-review.googlesource.com/id/I716846e2de8ef179f44e835770df7e6307be96c9
---
v4:
- move flag to GFP_HIGHUSER_MOVABLE
- remove command line flag
include/linux/gfp.h | 13 ++++++++++---
include/linux/page-flags.h | 9 +++++++++
include/trace/events/mmflags.h | 9 ++++++++-
mm/kasan/hw_tags.c | 3 +++
mm/page_alloc.c | 12 +++++++-----
5 files changed, 37 insertions(+), 9 deletions(-)
diff --git a/include/linux/gfp.h b/include/linux/gfp.h
index 68ba237365dc..e6102dfa4faa 100644
--- a/include/linux/gfp.h
+++ b/include/linux/gfp.h
@@ -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)
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 04a34c08e0a6..40e2c5000585 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -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
diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
index 629c7a0eaff2..390270e00a1d 100644
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h
@@ -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, "|", \
diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
index 41fd5326ee0a..ed5e5b833d61 100644
--- a/mm/kasan/hw_tags.c
+++ b/mm/kasan/hw_tags.c
@@ -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;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 13937e793fda..5ad76e540a22 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -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);
--
2.32.0.rc1.229.g3e70b5a671-goog
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