[PATCH] kasan: allow sampling page_alloc allocations for HW_TAGS

From: andrey . konovalov
Date: Thu Oct 27 2022 - 16:10:35 EST

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.

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 introduces a significant
slowdown in some testing scenarios. The new flag allows to alleviate
that slowdown.

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>
---
Documentation/dev-tools/kasan.rst | 4 +++
include/linux/kasan.h | 7 ++---
mm/kasan/common.c | 9 +++++--
mm/kasan/hw_tags.c | 26 +++++++++++++++++++
mm/kasan/kasan.h | 15 +++++++++++
mm/page_alloc.c | 43 +++++++++++++++++++++----------
6 files changed, 85 insertions(+), 19 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index 5c93ab915049..bd97301845ef 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -140,6 +140,10 @@ 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 frequency>`` makes KASAN tag only
+ every Nth page_alloc allocation, where N is the value of the parameter
+ (default: ``1``).
+
Error reports
~~~~~~~~~~~~~

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index d811b3d7d2a1..d45d45dfd007 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -120,12 +120,13 @@ 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,
+bool __kasan_unpoison_pages(struct page *page, unsigned int order, bool init);
+static __always_inline bool kasan_unpoison_pages(struct page *page,
unsigned int order, bool init)
{
if (kasan_enabled())
- __kasan_unpoison_pages(page, order, init);
+ return __kasan_unpoison_pages(page, order, init);
+ return false;
}

void __kasan_cache_create_kmalloc(struct kmem_cache *cache);
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 833bf2cfd2a3..1f30080a7a4c 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -95,19 +95,24 @@ 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)
{
u8 tag;
unsigned long i;

if (unlikely(PageHighMem(page)))
- return;
+ return false;
+
+ if (!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..aa3b5a080297 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);

+/* Frequency of page_alloc allocation poisoning. */
+unsigned long kasan_page_alloc_sample = 1;
+
+DEFINE_PER_CPU(unsigned long, kasan_page_alloc_count);
+
/* 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 frequency> */
+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 = 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..ee67eb35f4a7 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(unsigned long, kasan_page_alloc_count);
+
static inline bool kasan_vmalloc_enabled(void)
{
return static_branch_likely(&kasan_flag_vmalloc);
@@ -57,6 +60,13 @@ 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)
+{
+ unsigned long *count = this_cpu_ptr(&kasan_page_alloc_count);
+
+ return (*count)++ % kasan_page_alloc_sample == 0;
+}
+
#else /* CONFIG_KASAN_HW_TAGS */

static inline bool kasan_async_fault_possible(void)
@@ -69,6 +79,11 @@ 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/page_alloc.c b/mm/page_alloc.c
index b5a6c815ae28..0b36456aedfb 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1366,6 +1366,8 @@ 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
@@ -2475,7 +2477,8 @@ 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);
@@ -2498,30 +2501,42 @@ 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)) {
+ /* 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