[PATCH] kasan: allow sampling page_alloc allocations for HW_TAGS

* [PATCH] kasan: allow sampling page_alloc allocations for HW_TAGS
@ 2022-10-27 20:10 andrey.konovalov
  2022-10-27 20:44 ` Andrew Morton
                   ` (4 more replies)
  0 siblings, 5 replies; 8+ messages in thread
From: andrey.konovalov @ 2022-10-27 20:10 UTC (permalink / raw)
  To: Marco Elver
  Cc: Andrey Konovalov, Alexander Potapenko, Dmitry Vyukov,
	Andrey Ryabinin, kasan-dev, Peter Collingbourne,
	Evgenii Stepanov, Florian Mayer, Andrew Morton, linux-mm,
	linux-kernel, Andrey Konovalov

From: Andrey Konovalov <andreyknvl@google.com>

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@google.com>
---
 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


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