[PATCH v3 mm 02/13] kasan, mm: optimize kmalloc poisoning

Andrey Konovalov
2021-02-05T17:44:55Z

For allocations from kmalloc caches, kasan_kmalloc() always follows
kasan_slab_alloc(). Currenly, both of them unpoison the whole object,
which is unnecessary.

This patch provides separate implementations for both annotations:
kasan_slab_alloc() unpoisons the whole object, and kasan_kmalloc()
only poisons the redzone.

For generic KASAN, the redzone start might not be aligned to
KASAN_GRANULE_SIZE. Therefore, the poisoning is split in two parts:
kasan_poison_last_granule() poisons the unaligned part, and then
kasan_poison() poisons the rest.

This patch also clarifies alignment guarantees of each of the poisoning
functions and drops the unnecessary round_up() call for redzone_end.

With this change, the early SLUB cache annotation needs to be changed to
kasan_slab_alloc(), as kasan_kmalloc() doesn't unpoison objects now.
The number of poisoned bytes for objects in this cache stays the same, as
kmem_cache_node->object_size is equal to sizeof(struct kmem_cache_node).

Reviewed-by: Marco Elver <elver@google.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
---
mm/kasan/common.c | 93 +++++++++++++++++++++++++++++++----------------
mm/kasan/kasan.h | 43 +++++++++++++++++++++-
mm/kasan/shadow.c | 28 +++++++-------
mm/slub.c | 3 +-
4 files changed, 119 insertions(+), 48 deletions(-)

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index bfdf5464f4ef..00edbc3eb32e 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -278,21 +278,11 @@ void __kasan_poison_object_data(struct kmem_cache *cache, void *object)
* based on objects indexes, so that objects that are next to each other
* get different tags.
*/
-static u8 assign_tag(struct kmem_cache *cache, const void *object,
- bool init, bool keep_tag)
+static u8 assign_tag(struct kmem_cache *cache, const void *object, bool init)
{
if (IS_ENABLED(CONFIG_KASAN_GENERIC))
return 0xff;

- /*
- * 1. When an object is kmalloc()'ed, two hooks are called:
- * kasan_slab_alloc() and kasan_kmalloc(). We assign the
- * tag only in the first one.
- * 2. We reuse the same tag for krealloc'ed objects.
- */
- if (keep_tag)
- return get_tag(object);
-
/*
* If the cache neither has a constructor nor has SLAB_TYPESAFE_BY_RCU
* set, assign a tag when the object is being allocated (init == false).
@@ -325,7 +315,7 @@ void * __must_check __kasan_init_slab_obj(struct kmem_cache *cache,
}

/* Tag is ignored in set_tag() without CONFIG_KASAN_SW/HW_TAGS */
- object = set_tag(object, assign_tag(cache, object, true, false));
+ object = set_tag(object, assign_tag(cache, object, true));

return (void *)object;
}
@@ -413,12 +403,46 @@ static void set_alloc_info(struct kmem_cache *cache, void *object,
kasan_set_track(&alloc_meta->alloc_track, flags);
}

+void * __must_check __kasan_slab_alloc(struct kmem_cache *cache,
+ void *object, gfp_t flags)
+{
+ u8 tag;
+ void *tagged_object;
+
+ if (gfpflags_allow_blocking(flags))
+ kasan_quarantine_reduce();
+
+ if (unlikely(object == NULL))
+ return NULL;
+
+ if (is_kfence_address(object))
+ return (void *)object;
+
+ /*
+ * Generate and assign random tag for tag-based modes.
+ * Tag is ignored in set_tag() for the generic mode.
+ */
+ tag = assign_tag(cache, object, false);
+ tagged_object = set_tag(object, tag);
+
+ /*
+ * Unpoison the whole object.
+ * For kmalloc() allocations, kasan_kmalloc() will do precise poisoning.
+ */
+ kasan_unpoison(tagged_object, cache->object_size);
+
+ /* Save alloc info (if possible) for non-kmalloc() allocations. */
+ if (kasan_stack_collection_enabled())
+ set_alloc_info(cache, (void *)object, flags, false);
+
+ return tagged_object;
+}
+
static void *____kasan_kmalloc(struct kmem_cache *cache, const void *object,
- size_t size, gfp_t flags, bool is_kmalloc)
+ size_t size, gfp_t flags)
{
unsigned long redzone_start;
unsigned long redzone_end;
- u8 tag;

if (gfpflags_allow_blocking(flags))
kasan_quarantine_reduce();
@@ -429,33 +453,41 @@ static void *____kasan_kmalloc(struct kmem_cache *cache, const void *object,
if (is_kfence_address(kasan_reset_tag(object)))
return (void *)object;

+ /*
+ * The object has already been unpoisoned by kasan_slab_alloc() for
+ * kmalloc() or by ksize() for krealloc().
+ */
+
+ /*
+ * The redzone has byte-level precision for the generic mode.
+ * Partially poison the last object granule to cover the unaligned
+ * part of the redzone.
+ */
+ if (IS_ENABLED(CONFIG_KASAN_GENERIC))
+ kasan_poison_last_granule((void *)object, size);
+
+ /* Poison the aligned part of the redzone. */
redzone_start = round_up((unsigned long)(object + size),
KASAN_GRANULE_SIZE);
- redzone_end = round_up((unsigned long)object + cache->object_size,
- KASAN_GRANULE_SIZE);
- tag = assign_tag(cache, object, false, is_kmalloc);
-
- /* Tag is ignored in set_tag without CONFIG_KASAN_SW/HW_TAGS */
- kasan_unpoison(set_tag(object, tag), size);
+ redzone_end = (unsigned long)object + cache->object_size;
kasan_poison((void *)redzone_start, redzone_end - redzone_start,
KASAN_KMALLOC_REDZONE);

+ /*
+ * Save alloc info (if possible) for kmalloc() allocations.
+ * This also rewrites the alloc info when called from kasan_krealloc().
+ */
if (kasan_stack_collection_enabled())
- set_alloc_info(cache, (void *)object, flags, is_kmalloc);
+ set_alloc_info(cache, (void *)object, flags, true);

- return set_tag(object, tag);
-}
-
-void * __must_check __kasan_slab_alloc(struct kmem_cache *cache,
- void *object, gfp_t flags)
-{
- return ____kasan_kmalloc(cache, object, cache->object_size, flags, false);
+ /* Keep the tag that was set by kasan_slab_alloc(). */
+ return (void *)object;
}

void * __must_check __kasan_kmalloc(struct kmem_cache *cache, const void *object,
size_t size, gfp_t flags)
{
- return ____kasan_kmalloc(cache, object, size, flags, true);
+ return ____kasan_kmalloc(cache, object, size, flags);
}
EXPORT_SYMBOL(__kasan_kmalloc);

@@ -496,8 +528,7 @@ void * __must_check __kasan_krealloc(const void *object, size_t size, gfp_t flag
if (unlikely(!PageSlab(page)))
return __kasan_kmalloc_large(object, size, flags);
else
- return ____kasan_kmalloc(page->slab_cache, object, size,
- flags, true);
+ return ____kasan_kmalloc(page->slab_cache, object, size, flags);
}

void __kasan_kfree_large(void *ptr, unsigned long ip)
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 4fb8106f8e31..d0a3516e0909 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -355,12 +355,51 @@ static inline bool kasan_byte_accessible(const void *addr)

#else /* CONFIG_KASAN_HW_TAGS */

-void kasan_poison(const void *address, size_t size, u8 value);
-void kasan_unpoison(const void *address, size_t size);
+/**
+ * kasan_poison - mark the memory range as unaccessible
+ * @addr - range start address, must be aligned to KASAN_GRANULE_SIZE
+ * @size - range size
+ * @value - value that's written to metadata for the range
+ *
+ * The size gets aligned to KASAN_GRANULE_SIZE before marking the range.
+ */
+void kasan_poison(const void *addr, size_t size, u8 value);
+
+/**
+ * kasan_unpoison - mark the memory range as accessible
+ * @addr - range start address, must be aligned to KASAN_GRANULE_SIZE
+ * @size - range size
+ *
+ * For the tag-based modes, the @size gets aligned to KASAN_GRANULE_SIZE before
+ * marking the range.
+ * For the generic mode, the last granule of the memory range gets partially
+ * unpoisoned based on the @size.
+ */
+void kasan_unpoison(const void *addr, size_t size);
+
bool kasan_byte_accessible(const void *addr);

#endif /* CONFIG_KASAN_HW_TAGS */

+#ifdef CONFIG_KASAN_GENERIC
+
+/**
+ * kasan_poison_last_granule - mark the last granule of the memory range as
+ * unaccessible
+ * @addr - range start address, must be aligned to KASAN_GRANULE_SIZE
+ * @size - range size
+ *
+ * This function is only available for the generic mode, as it's the only mode
+ * that has partially poisoned memory granules.
+ */
+void kasan_poison_last_granule(const void *address, size_t size);
+
+#else /* CONFIG_KASAN_GENERIC */
+
+static inline void kasan_poison_last_granule(const void *address, size_t size) { }
+
+#endif /* CONFIG_KASAN_GENERIC */
+
/*
* Exported functions for interfaces called from assembly or from generated
* code. Declarations here to avoid warning about missing declarations.
diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
index 1372a2fc0ca9..1ed7817e4ee6 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -69,10 +69,6 @@ void *memcpy(void *dest, const void *src, size_t len)
return __memcpy(dest, src, len);
}

-/*
- * Poisons the shadow memory for 'size' bytes starting from 'addr'.
- * Memory addresses should be aligned to KASAN_GRANULE_SIZE.
- */
void kasan_poison(const void *address, size_t size, u8 value)
{
void *shadow_start, *shadow_end;
@@ -83,12 +79,12 @@ void kasan_poison(const void *address, size_t size, u8 value)
* addresses to this function.
*/
address = kasan_reset_tag(address);
- size = round_up(size, KASAN_GRANULE_SIZE);

/* Skip KFENCE memory if called explicitly outside of sl*b. */
if (is_kfence_address(address))
return;

+ size = round_up(size, KASAN_GRANULE_SIZE);
shadow_start = kasan_mem_to_shadow(address);
shadow_end = kasan_mem_to_shadow(address + size);

@@ -96,6 +92,16 @@ void kasan_poison(const void *address, size_t size, u8 value)
}
EXPORT_SYMBOL(kasan_poison);

+#ifdef CONFIG_KASAN_GENERIC
+void kasan_poison_last_granule(const void *address, size_t size)
+{
+ if (size & KASAN_GRANULE_MASK) {
+ u8 *shadow = (u8 *)kasan_mem_to_shadow(address + size);
+ *shadow = size & KASAN_GRANULE_MASK;
+ }
+}
+#endif
+
void kasan_unpoison(const void *address, size_t size)
{
u8 tag = get_tag(address);
@@ -115,16 +121,12 @@ void kasan_unpoison(const void *address, size_t size)
if (is_kfence_address(address))
return;

+ /* Unpoison round_up(size, KASAN_GRANULE_SIZE) bytes. */
kasan_poison(address, size, tag);

- if (size & KASAN_GRANULE_MASK) {
- u8 *shadow = (u8 *)kasan_mem_to_shadow(address + size);
-
- if (IS_ENABLED(CONFIG_KASAN_SW_TAGS))
- *shadow = tag;
- else /* CONFIG_KASAN_GENERIC */
- *shadow = size & KASAN_GRANULE_MASK;
- }
+ /* Partially poison the last granule for the generic mode. */
+ if (IS_ENABLED(CONFIG_KASAN_GENERIC))
+ kasan_poison_last_granule(address, size);
}

#ifdef CONFIG_MEMORY_HOTPLUG
diff --git a/mm/slub.c b/mm/slub.c
index 176b1cb0d006..e564008c2329 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3565,8 +3565,7 @@ static void early_kmem_cache_node_alloc(int node)
init_object(kmem_cache_node, n, SLUB_RED_ACTIVE);
init_tracking(kmem_cache_node, n);
#endif
- n = kasan_kmalloc(kmem_cache_node, n, sizeof(struct kmem_cache_node),
- GFP_KERNEL);
+ n = kasan_slab_alloc(kmem_cache_node, n, GFP_KERNEL);
page->freelist = get_freepointer(kmem_cache_node, n);
page->inuse = 1;
page->frozen = 0;
--
2.30.0.365.g02bc693789-goog