kasan: allow sampling page_alloc allocations for HW_TAGS

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.

Add two new boot parameters that allow to alleviate that slowdown:

- kasan.page_alloc.sample, which makes Hardware Tag-Based KASAN tag only
every Nth page_alloc allocation with the order configured by the second
added parameter (default: tag every such allocation).

- kasan.page_alloc.sample.order, which makes sampling enabled by the first
parameter only affect page_alloc allocations with the order equal or
greater than the specified value (default: 3, see below).

The exact performance improvement caused by using the new parameters
depends on their values and the applied workload.

The chosen default value for kasan.page_alloc.sample.order is 3, which
matches both PAGE_ALLOC_COSTLY_ORDER and SKB_FRAG_PAGE_ORDER. This is
done for two reasons:

1. PAGE_ALLOC_COSTLY_ORDER is "the order at which allocations are deemed
costly to service", which corresponds to the idea that only large and
thus costly allocations are supposed to sampled.

2. One of the workloads targeted by this patch is a benchmark that sends
a large amount of data over a local loopback connection. Most multi-page
data allocations in the networking subsystem have the order of
SKB_FRAG_PAGE_ORDER (or PAGE_ALLOC_COSTLY_ORDER).

When running a local loopback test on a testing MTE-enabled device in sync
mode, enabling Hardware Tag-Based KASAN introduces a ~50% slowdown.
Applying this patch and setting kasan.page_alloc.sampling to a value
higher than 1 allows to lower the slowdown. The performance improvement
saturates around the sampling interval value of 10 with the default
sampling page order of 3. This lowers the slowdown to ~20%. The slowdown
in real scenarios involving the network will likely be better.

Enabling page_alloc sampling has a downside: KASAN misses bad accesses to
a page_alloc allocation that has not been tagged. This lowers the value
of KASAN as a security mitigation.

However, based on measuring the number of page_alloc allocations of
different orders during boot in a test build, sampling with the default
kasan.page_alloc.sample.order value affects only ~7% of allocations. The
rest ~93% of allocations are still checked deterministically.

Link: https://lkml.kernel.org/r/12[e-mail]
Signed-off-by: Andrey Konovalov <[e-mail]>
Reviewed-by: Marco Elver <[e-mail]>
Cc: Alexander Potapenko <[e-mail]>
Cc: Andrey Ryabinin <[e-mail]>
Cc: Dmitry Vyukov <[e-mail]>
Cc: Evgenii Stepanov <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Mark Brand <[e-mail]>
Cc: Peter Collingbourne <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/dev-tools/kasan.rst

···
 - ``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 with the order equal or greater than
+  ``kasan.page_alloc.sample.order``, where N is the value of the ``sample``
+  parameter (default: ``1``, or tag every such allocation).
+  This parameter is intended to mitigate the performance overhead introduced
+  by KASAN.
+  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.
+
+- ``kasan.page_alloc.sample.order=<minimum page order>`` specifies the minimum
+  order of allocations that are affected by sampling (default: ``3``).
+  Only applies when ``kasan.page_alloc.sample`` is set to a value greater
+  than ``1``.
+  This parameter is intended to allow sampling only large page_alloc
+  allocations, which is the biggest source of the performance overhead.
+
 Error reports
 ~~~~~~~~~~~~~
 

include/linux/kasan.h

···
 		__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);
···
 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)
+{
+	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,

mm/kasan/common.c

···
 }
 #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(order))
+		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)

mm/kasan/hw_tags.c

···
 /* Whether to enable vmalloc tagging. */
 DEFINE_STATIC_KEY_TRUE(kasan_flag_vmalloc);
 
+#define PAGE_ALLOC_SAMPLE_DEFAULT	1
+#define PAGE_ALLOC_SAMPLE_ORDER_DEFAULT	3
+
+/*
+ * Sampling interval of page_alloc allocation (un)poisoning.
+ * Defaults to no sampling.
+ */
+unsigned long kasan_page_alloc_sample = PAGE_ALLOC_SAMPLE_DEFAULT;
+
+/*
+ * Minimum order of page_alloc allocations to be affected by sampling.
+ * The default value is chosen to match both
+ * PAGE_ALLOC_COSTLY_ORDER and SKB_FRAG_PAGE_ORDER.
+ */
+unsigned int kasan_page_alloc_sample_order = PAGE_ALLOC_SAMPLE_ORDER_DEFAULT;
+
+DEFINE_PER_CPU(long, kasan_page_alloc_skip);
+
 /* kasan=off/on */
 static int __init early_kasan_flag(char *arg)
 {
···
 	else
 		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 = PAGE_ALLOC_SAMPLE_DEFAULT;
+		return -EINVAL;
+	}
+
+	return 0;
+}
+early_param("kasan.page_alloc.sample", early_kasan_flag_page_alloc_sample);
+
+/* kasan.page_alloc.sample.order=<minimum page order> */
+static int __init early_kasan_flag_page_alloc_sample_order(char *arg)
+{
+	int rv;
+
+	if (!arg)
+		return -EINVAL;
+
+	rv = kstrtouint(arg, 0, &kasan_page_alloc_sample_order);
+	if (rv)
+		return rv;
+
+	if (kasan_page_alloc_sample_order > INT_MAX) {
+		kasan_page_alloc_sample_order = PAGE_ALLOC_SAMPLE_ORDER_DEFAULT;
+		return -EINVAL;
+	}
+
+	return 0;
+}
+early_param("kasan.page_alloc.sample.order", early_kasan_flag_page_alloc_sample_order);
 
 /*
  * kasan_init_hw_tags_cpu() is called for each CPU.

mm/kasan/kasan.h

···
 
 extern enum kasan_mode kasan_mode __ro_after_init;
 
+extern unsigned long kasan_page_alloc_sample;
+extern unsigned int kasan_page_alloc_sample_order;
+DECLARE_PER_CPU(long, kasan_page_alloc_skip);
+
 static inline bool kasan_vmalloc_enabled(void)
 {
 	return static_branch_likely(&kasan_flag_vmalloc);
···
 	return kasan_mode == KASAN_MODE_SYNC || kasan_mode == KASAN_MODE_ASYMM;
 }
 
+static inline bool kasan_sample_page_alloc(unsigned int order)
+{
+	/* Fast-path for when sampling is disabled. */
+	if (kasan_page_alloc_sample == 1)
+		return true;
+
+	if (order < kasan_page_alloc_sample_order)
+		return true;
+
+	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)
···
 }
 
 static inline bool kasan_sync_fault_possible(void)
+{
+	return true;
+}
+
+static inline bool kasan_sample_page_alloc(unsigned int order)
 {
 	return true;
 }

mm/page_alloc.c

···
  *    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
···
 {
 	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);
···
 	 */
 
 	/*
-	 * 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. */