[PATCH v2 1/2] kasan: allow sampling page_alloc allocations for HW_TAGS

From: andrey . konovalov
Date: Sat Nov 26 2022 - 14:13:04 EST

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From: Andrey Konovalov <andreyknvl@xxxxxxxxxx>

Add a new boot parameter called kasan.page_alloc.sample, which makes
Hardware Tag-Based KASAN tag only every Nth page_alloc allocation for
allocations marked with __GFP_KASAN_SAMPLE.

As Hardware Tag-Based KASAN is intended to be used in production, its
performance impact is crucial. As page_alloc allocations tend to be big,
tagging and checking all such allocations can introduce a significant
slowdown. The new flag allows to alleviate that slowdown for chosen
allocations.

The exact performance improvement caused by using __GFP_KASAN_SAMPLE and
kasan.page_alloc.sample depends on how often the marked allocations happen
and how large the are. See the next patch for the details about marking and
sampling skb allocations.

Enabling page_alloc sampling has a downside: KASAN will miss bad accesses
to a page_alloc allocation that has not been tagged.

Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>

---

Changes v1->v2:
- Only sample allocations when __GFP_KASAN_SAMPLE is provided to
alloc_pages().
- Fix build when KASAN is disabled.
- Add more information about the flag to documentation.
- Use optimized preemption-safe approach for sampling suggested by Marco.
---
Documentation/dev-tools/kasan.rst | 8 ++++++
include/linux/gfp_types.h | 10 +++++--
include/linux/kasan.h | 18 ++++++++-----
include/trace/events/mmflags.h | 3 ++-
mm/kasan/common.c | 10 +++++--
mm/kasan/hw_tags.c | 26 ++++++++++++++++++
mm/kasan/kasan.h | 19 +++++++++++++
mm/mempool.c | 2 +-
mm/page_alloc.c | 44 +++++++++++++++++++++----------
9 files changed, 114 insertions(+), 26 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 5c93ab915049..bd6d064c7419 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -140,6 +140,14 @@ disabling KASAN altogether or controlling its features:
- ``kasan.vmalloc=off`` or ``=on`` disables or enables tagging of vmalloc
allocations (default: ``on``).

+- ``kasan.page_alloc.sample=<sampling interval>`` makes KASAN tag only every
+ Nth page_alloc allocation for allocations marked with __GFP_KASAN_SAMPLE,
+ where N is the value of the parameter (default: ``1``).
+ This parameter is intended to mitigate the performance overhead.
+ Note that enabling this parameter makes Hardware Tag-Based KASAN skip checks
+ of allocations chosen by sampling and thus miss bad accesses to these
+ allocations. Use the default value for accurate bug detection.
+
Error reports
~~~~~~~~~~~~~

diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
index d88c46ca82e1..c322cd159445 100644
--- a/include/linux/gfp_types.h
+++ b/include/linux/gfp_types.h
@@ -50,13 +50,15 @@ typedef unsigned int __bitwise gfp_t;
#define ___GFP_SKIP_ZERO 0x1000000u
#define ___GFP_SKIP_KASAN_UNPOISON 0x2000000u
#define ___GFP_SKIP_KASAN_POISON 0x4000000u
+#define ___GFP_KASAN_SAMPLE 0x8000000u
#else
#define ___GFP_SKIP_ZERO 0
#define ___GFP_SKIP_KASAN_UNPOISON 0
#define ___GFP_SKIP_KASAN_POISON 0
+#define ___GFP_KASAN_SAMPLE 0
#endif
#ifdef CONFIG_LOCKDEP
-#define ___GFP_NOLOCKDEP 0x8000000u
+#define ___GFP_NOLOCKDEP 0x10000000u
#else
#define ___GFP_NOLOCKDEP 0
#endif
@@ -243,6 +245,9 @@ typedef unsigned int __bitwise gfp_t;
*
* %__GFP_SKIP_KASAN_POISON makes KASAN skip poisoning on page deallocation.
* Typically, used for userspace pages. Only effective in HW_TAGS mode.
+ *
+ * %__GFP_KASAN_SAMPLE makes KASAN use sampling to skip poisoning and
+ * unpoisoning of page allocations. Only effective in HW_TAGS mode.
*/
#define __GFP_NOWARN ((__force gfp_t)___GFP_NOWARN)
#define __GFP_COMP ((__force gfp_t)___GFP_COMP)
@@ -251,12 +256,13 @@ typedef unsigned int __bitwise gfp_t;
#define __GFP_SKIP_ZERO ((__force gfp_t)___GFP_SKIP_ZERO)
#define __GFP_SKIP_KASAN_UNPOISON ((__force gfp_t)___GFP_SKIP_KASAN_UNPOISON)
#define __GFP_SKIP_KASAN_POISON ((__force gfp_t)___GFP_SKIP_KASAN_POISON)
+#define __GFP_KASAN_SAMPLE ((__force gfp_t)___GFP_KASAN_SAMPLE)

/* Disable lockdep for GFP context tracking */
#define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)

/* Room for N __GFP_FOO bits */
-#define __GFP_BITS_SHIFT (27 + IS_ENABLED(CONFIG_LOCKDEP))
+#define __GFP_BITS_SHIFT (28 + IS_ENABLED(CONFIG_LOCKDEP))
#define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))

/**
diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index d811b3d7d2a1..4cc946b8cbc8 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -120,12 +120,15 @@ static __always_inline void kasan_poison_pages(struct page *page,
__kasan_poison_pages(page, order, init);
}

-void __kasan_unpoison_pages(struct page *page, unsigned int order, bool init);
-static __always_inline void kasan_unpoison_pages(struct page *page,
- unsigned int order, bool init)
+bool __kasan_unpoison_pages(struct page *page, unsigned int order,
+ bool init, bool sample);
+static __always_inline bool kasan_unpoison_pages(struct page *page,
+ unsigned int order,
+ bool init, bool sample)
{
if (kasan_enabled())
- __kasan_unpoison_pages(page, order, init);
+ return __kasan_unpoison_pages(page, order, init, sample);
+ return false;
}

void __kasan_cache_create_kmalloc(struct kmem_cache *cache);
@@ -249,8 +252,11 @@ static __always_inline bool kasan_check_byte(const void *addr)
static inline void kasan_unpoison_range(const void *address, size_t size) {}
static inline void kasan_poison_pages(struct page *page, unsigned int order,
bool init) {}
-static inline void kasan_unpoison_pages(struct page *page, unsigned int order,
- bool init) {}
+static inline bool kasan_unpoison_pages(struct page *page, unsigned int order,
+ bool init, bool sample)
+{
+ return false;
+}
static inline void kasan_cache_create_kmalloc(struct kmem_cache *cache) {}
static inline void kasan_poison_slab(struct slab *slab) {}
static inline void kasan_unpoison_object_data(struct kmem_cache *cache,
diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
index e87cb2b80ed3..bcaecf859d1f 100644
--- a/include/trace/events/mmflags.h
+++ b/include/trace/events/mmflags.h
@@ -57,7 +57,8 @@
#define __def_gfpflag_names_kasan , \
gfpflag_string(__GFP_SKIP_ZERO), \
gfpflag_string(__GFP_SKIP_KASAN_POISON), \
- gfpflag_string(__GFP_SKIP_KASAN_UNPOISON)
+ gfpflag_string(__GFP_SKIP_KASAN_UNPOISON), \
+ gfpflag_string(__GFP_KASAN_SAMPLE)
#else
#define __def_gfpflag_names_kasan
#endif
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 833bf2cfd2a3..05d799ada873 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -95,19 +95,25 @@ asmlinkage void kasan_unpoison_task_stack_below(const void *watermark)
}
#endif /* CONFIG_KASAN_STACK */

-void __kasan_unpoison_pages(struct page *page, unsigned int order, bool init)
+bool __kasan_unpoison_pages(struct page *page, unsigned int order,
+ bool init, bool sample)
{
u8 tag;
unsigned long i;

if (unlikely(PageHighMem(page)))
- return;
+ return false;
+
+ if (sample && !kasan_sample_page_alloc())
+ return false;

tag = kasan_random_tag();
kasan_unpoison(set_tag(page_address(page), tag),
PAGE_SIZE << order, init);
for (i = 0; i < (1 << order); i++)
page_kasan_tag_set(page + i, tag);
+
+ return true;
}

void __kasan_poison_pages(struct page *page, unsigned int order, bool init)
diff --git a/mm/kasan/hw_tags.c b/mm/kasan/hw_tags.c
index b22c4f461cb0..5e6571820a3f 100644
--- a/mm/kasan/hw_tags.c
+++ b/mm/kasan/hw_tags.c
@@ -59,6 +59,11 @@ EXPORT_SYMBOL_GPL(kasan_mode);
/* Whether to enable vmalloc tagging. */
DEFINE_STATIC_KEY_TRUE(kasan_flag_vmalloc);

+/* Sampling interval of page_alloc allocation (un)poisoning. */
+unsigned long kasan_page_alloc_sample = 1;
+
+DEFINE_PER_CPU(long, kasan_page_alloc_skip);
+
/* kasan=off/on */
static int __init early_kasan_flag(char *arg)
{
@@ -122,6 +127,27 @@ static inline const char *kasan_mode_info(void)
return "sync";
}

+/* kasan.page_alloc.sample=<sampling interval> */
+static int __init early_kasan_flag_page_alloc_sample(char *arg)
+{
+ int rv;
+
+ if (!arg)
+ return -EINVAL;
+
+ rv = kstrtoul(arg, 0, &kasan_page_alloc_sample);
+ if (rv)
+ return rv;
+
+ if (!kasan_page_alloc_sample || kasan_page_alloc_sample > LONG_MAX) {
+ kasan_page_alloc_sample = 1;
+ return -EINVAL;
+ }
+
+ return 0;
+}
+early_param("kasan.page_alloc.sample", early_kasan_flag_page_alloc_sample);
+
/*
* kasan_init_hw_tags_cpu() is called for each CPU.
* Not marked as __init as a CPU can be hot-plugged after boot.
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index abbcc1b0eec5..ce0b30889587 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -42,6 +42,9 @@ enum kasan_mode {

extern enum kasan_mode kasan_mode __ro_after_init;

+extern unsigned long kasan_page_alloc_sample;
+DECLARE_PER_CPU(long, kasan_page_alloc_skip);
+
static inline bool kasan_vmalloc_enabled(void)
{
return static_branch_likely(&kasan_flag_vmalloc);
@@ -57,6 +60,17 @@ static inline bool kasan_sync_fault_possible(void)
return kasan_mode == KASAN_MODE_SYNC || kasan_mode == KASAN_MODE_ASYMM;
}

+static inline bool kasan_sample_page_alloc(void)
+{
+ if (this_cpu_dec_return(kasan_page_alloc_skip) < 0) {
+ this_cpu_write(kasan_page_alloc_skip,
+ kasan_page_alloc_sample - 1);
+ return true;
+ }
+
+ return false;
+}
+
#else /* CONFIG_KASAN_HW_TAGS */

static inline bool kasan_async_fault_possible(void)
@@ -69,6 +83,11 @@ static inline bool kasan_sync_fault_possible(void)
return true;
}

+static inline bool kasan_sample_page_alloc(void)
+{
+ return true;
+}
+
#endif /* CONFIG_KASAN_HW_TAGS */

#ifdef CONFIG_KASAN_GENERIC
diff --git a/mm/mempool.c b/mm/mempool.c
index 96488b13a1ef..d3b3702e5191 100644
--- a/mm/mempool.c
+++ b/mm/mempool.c
@@ -115,7 +115,7 @@ static void kasan_unpoison_element(mempool_t *pool, void *element)
kasan_unpoison_range(element, __ksize(element));
else if (pool->alloc == mempool_alloc_pages)
kasan_unpoison_pages(element, (unsigned long)pool->pool_data,
- false);
+ false, false);
}

static __always_inline void add_element(mempool_t *pool, void *element)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6e60657875d3..969b0e4f0046 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1367,6 +1367,8 @@ static int free_tail_pages_check(struct page *head_page, struct page *page)
* see the comment next to it.
* 3. Skipping poisoning is requested via __GFP_SKIP_KASAN_POISON,
* see the comment next to it.
+ * 4. The allocation is excluded from being checked due to sampling,
+ * see the call to kasan_unpoison_pages.
*
* Poisoning pages during deferred memory init will greatly lengthen the
* process and cause problem in large memory systems as the deferred pages
@@ -2476,7 +2478,8 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
{
bool init = !want_init_on_free() && want_init_on_alloc(gfp_flags) &&
!should_skip_init(gfp_flags);
- bool init_tags = init && (gfp_flags & __GFP_ZEROTAGS);
+ bool zero_tags = init && (gfp_flags & __GFP_ZEROTAGS);
+ bool reset_tags = !zero_tags;
int i;

set_page_private(page, 0);
@@ -2499,30 +2502,43 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
*/

/*
- * If memory tags should be zeroed (which happens only when memory
- * should be initialized as well).
+ * If memory tags should be zeroed
+ * (which happens only when memory should be initialized as well).
*/
- if (init_tags) {
+ if (zero_tags) {
/* Initialize both memory and tags. */
for (i = 0; i != 1 << order; ++i)
tag_clear_highpage(page + i);

- /* Note that memory is already initialized by the loop above. */
+ /* Take note that memory was initialized by the loop above. */
init = false;
}
if (!should_skip_kasan_unpoison(gfp_flags)) {
- /* Unpoison shadow memory or set memory tags. */
- kasan_unpoison_pages(page, order, init);
-
- /* Note that memory is already initialized by KASAN. */
- if (kasan_has_integrated_init())
- init = false;
- } else {
- /* Ensure page_address() dereferencing does not fault. */
+ /* Try unpoisoning (or setting tags) and initializing memory. */
+ if (kasan_unpoison_pages(page, order, init,
+ gfp_flags & __GFP_KASAN_SAMPLE)) {
+ /* Take note that memory was initialized by KASAN. */
+ if (kasan_has_integrated_init())
+ init = false;
+ /* Take note that memory tags were set by KASAN. */
+ reset_tags = false;
+ } else {
+ /*
+ * KASAN decided to exclude this allocation from being
+ * poisoned due to sampling. Skip poisoning as well.
+ */
+ SetPageSkipKASanPoison(page);
+ }
+ }
+ /*
+ * If memory tags have not been set, reset the page tags to ensure
+ * page_address() dereferencing does not fault.
+ */
+ if (reset_tags) {
for (i = 0; i != 1 << order; ++i)
page_kasan_tag_reset(page + i);
}
- /* If memory is still not initialized, do it now. */
+ /* If memory is still not initialized, initialize it now. */
if (init)
kernel_init_pages(page, 1 << order);
/* Propagate __GFP_SKIP_KASAN_POISON to page flags. */
--
2.25.1

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