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.
Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
---
mm/kasan/common.c | 63 ++++++++++++++++++++++++++++++++---------------
1 file changed, 43 insertions(+), 20 deletions(-)
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 44390392d4c9..73c9cbfdedf4 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -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)
--
2.20.1.415.g653613c723-goog
