Commit a3fe7cdf authored by Andrey Konovalov's avatar Andrey Konovalov Committed by Linus Torvalds
Browse files

kasan: fix krealloc handling for tag-based mode

Right now tag-based KASAN can retag the memory that is reallocated via
krealloc and return a differently tagged pointer even if the same slab
object gets used and no reallocated technically happens.

There are a few issues with this approach.  One is that krealloc callers
can't rely on comparing the return value with the passed argument to
check whether reallocation happened.  Another is that if a caller knows
that no reallocation happened, that it can access object memory through
the old pointer, which leads to false positives.  Look at
nf_ct_ext_add() to see an example.

Fix this by keeping the same tag if the memory don't actually gets
reallocated during krealloc.

Link: http://lkml.kernel.org/r/bb2a71d17ed072bcc528cbee46fcbd71a6da3be4.1546540962.git.andreyknvl@google.com

Signed-off-by: default avatarAndrey Konovalov <andreyknvl@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 96fedce2
......@@ -347,28 +347,43 @@ void kasan_poison_object_data(struct kmem_cache *cache, void *object)
}
/*
* Since it's desirable to only call object contructors once during slab
* allocation, we preassign tags to all such objects. Also preassign tags for
* SLAB_TYPESAFE_BY_RCU slabs to avoid use-after-free reports.
* For SLAB allocator we can't preassign tags randomly since the freelist is
* stored as an array of indexes instead of a linked list. Assign tags based
* on objects indexes, so that objects that are next to each other get
* different tags.
* After a tag is assigned, the object always gets allocated with the same tag.
* The reason is that we can't change tags for objects with constructors on
* reallocation (even for non-SLAB_TYPESAFE_BY_RCU), because the constructor
* code can save the pointer to the object somewhere (e.g. in the object
* itself). Then if we retag it, the old saved pointer will become invalid.
* This function assigns a tag to an object considering the following:
* 1. A cache might have a constructor, which might save a pointer to a slab
* object somewhere (e.g. in the object itself). We preassign a tag for
* each object in caches with constructors during slab creation and reuse
* the same tag each time a particular object is allocated.
* 2. A cache might be SLAB_TYPESAFE_BY_RCU, which means objects can be
* accessed after being freed. We preassign tags for objects in these
* caches as well.
* 3. For SLAB allocator we can't preassign tags randomly since the freelist
* is stored as an array of indexes instead of a linked list. Assign tags
* 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 new)
static u8 assign_tag(struct kmem_cache *cache, const void *object,
bool init, bool krealloc)
{
/* Reuse the same tag for krealloc'ed objects. */
if (krealloc)
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).
*/
if (!cache->ctor && !(cache->flags & SLAB_TYPESAFE_BY_RCU))
return new ? KASAN_TAG_KERNEL : random_tag();
return init ? KASAN_TAG_KERNEL : random_tag();
/* For caches that either have a constructor or SLAB_TYPESAFE_BY_RCU: */
#ifdef CONFIG_SLAB
/* For SLAB assign tags based on the object index in the freelist. */
return (u8)obj_to_index(cache, virt_to_page(object), (void *)object);
#else
return new ? random_tag() : get_tag(object);
/*
* For SLUB assign a random tag during slab creation, otherwise reuse
* the already assigned tag.
*/
return init ? random_tag() : get_tag(object);
#endif
}
......@@ -384,7 +399,8 @@ void * __must_check kasan_init_slab_obj(struct kmem_cache *cache,
__memset(alloc_info, 0, sizeof(*alloc_info));
if (IS_ENABLED(CONFIG_KASAN_SW_TAGS))
object = set_tag(object, assign_tag(cache, object, true));
object = set_tag(object,
assign_tag(cache, object, true, false));
return (void *)object;
}
......@@ -450,8 +466,8 @@ bool kasan_slab_free(struct kmem_cache *cache, void *object, unsigned long ip)
return __kasan_slab_free(cache, object, ip, true);
}
void * __must_check kasan_kmalloc(struct kmem_cache *cache, const void *object,
size_t size, gfp_t flags)
static void *__kasan_kmalloc(struct kmem_cache *cache, const void *object,
size_t size, gfp_t flags, bool krealloc)
{
unsigned long redzone_start;
unsigned long redzone_end;
......@@ -469,7 +485,7 @@ void * __must_check kasan_kmalloc(struct kmem_cache *cache, const void *object,
KASAN_SHADOW_SCALE_SIZE);
if (IS_ENABLED(CONFIG_KASAN_SW_TAGS))
tag = assign_tag(cache, object, false);
tag = assign_tag(cache, object, false, krealloc);
/* Tag is ignored in set_tag without CONFIG_KASAN_SW_TAGS */
kasan_unpoison_shadow(set_tag(object, tag), size);
......@@ -481,6 +497,12 @@ void * __must_check kasan_kmalloc(struct kmem_cache *cache, const void *object,
return set_tag(object, tag);
}
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, false);
}
EXPORT_SYMBOL(kasan_kmalloc);
void * __must_check kasan_kmalloc_large(const void *ptr, size_t size,
......@@ -520,7 +542,8 @@ void * __must_check kasan_krealloc(const void *object, size_t size, gfp_t flags)
if (unlikely(!PageSlab(page)))
return kasan_kmalloc_large(object, size, flags);
else
return kasan_kmalloc(page->slab_cache, object, size, flags);
return __kasan_kmalloc(page->slab_cache, object, size,
flags, true);
}
void kasan_poison_kfree(void *ptr, unsigned long ip)
......
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