mm/slub: Improve redzone check and zeroing for krealloc()

For current krealloc(), one problem is its caller doesn't pass the old
request size, say the object is 64 bytes kmalloc one, but caller may
only requested 48 bytes. Then when krealloc() shrinks or grows in the
same object, or allocate a new bigger object, it lacks this 'original
size' information to do accurate data preserving or zeroing (when
__GFP_ZERO is set).

Thus with slub debug redzone and object tracking enabled, parts of the
object after krealloc() might contain redzone data instead of zeroes,
which is violating the __GFP_ZERO guarantees. Good thing is in this
case, kmalloc caches do have this 'orig_size' feature. So solve the
problem by utilize 'org_size' to do accurate data zeroing and preserving.

[Thanks to syzbot and V, Narasimhan for discovering kfence and big
 kmalloc related issues in early patch version]

Suggested-by: Vlastimil Babka <[email protected]>
Signed-off-by: Feng Tang <[email protected]>
Reviewed-by: Hyeonggon Yoo <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>

mm/slub.c

@@ -4718,34 +4718,66 @@ static __always_inline __realloc_size(2) void *
 __do_krealloc(const void *p, size_t new_size, gfp_t flags)
 {
 	void *ret;
-	size_t ks;
+	size_t ks = 0;
+	int orig_size = 0;
+	struct kmem_cache *s = NULL;
 
-	/* Check for double-free before calling ksize. */
-	if (likely(!ZERO_OR_NULL_PTR(p))) {
-		if (!kasan_check_byte(p))
-			return NULL;
-		ks = ksize(p);
-	} else
-		ks = 0;
+	if (unlikely(ZERO_OR_NULL_PTR(p)))
+		goto alloc_new;
 
-	/* If the object still fits, repoison it precisely. */
-	if (ks >= new_size) {
-		/* Zero out spare memory. */
-		if (want_init_on_alloc(flags)) {
-			kasan_disable_current();
-			memset(kasan_reset_tag(p) + new_size, 0, ks - new_size);
-			kasan_enable_current();
+	/* Check for double-free. */
+	if (!kasan_check_byte(p))
+		return NULL;
+
+	if (is_kfence_address(p)) {
+		ks = orig_size = kfence_ksize(p);
+	} else {
+		struct folio *folio;
+
+		folio = virt_to_folio(p);
+		if (unlikely(!folio_test_slab(folio))) {
+			/* Big kmalloc object */
+			WARN_ON(folio_size(folio) <= KMALLOC_MAX_CACHE_SIZE);
+			WARN_ON(p != folio_address(folio));
+			ks = folio_size(folio);
+		} else {
+			s = folio_slab(folio)->slab_cache;
+			orig_size = get_orig_size(s, (void *)p);
+			ks = s->object_size;
 		}
+	}
 
-		p = kasan_krealloc((void *)p, new_size, flags);
-		return (void *)p;
+	/* If the old object doesn't fit, allocate a bigger one */
+	if (new_size > ks)
+		goto alloc_new;
+
+	/* Zero out spare memory. */
+	if (want_init_on_alloc(flags)) {
+		kasan_disable_current();
+		if (orig_size && orig_size < new_size)
+			memset(kasan_reset_tag(p) + orig_size, 0, new_size - orig_size);
+		else
+			memset(kasan_reset_tag(p) + new_size, 0, ks - new_size);
+		kasan_enable_current();
 	}
 
+	/* Setup kmalloc redzone when needed */
+	if (s && slub_debug_orig_size(s)) {
+		set_orig_size(s, (void *)p, new_size);
+		if (s->flags & SLAB_RED_ZONE && new_size < ks)
+			memset_no_sanitize_memory(kasan_reset_tag(p) + new_size,
+						SLUB_RED_ACTIVE, ks - new_size);
+	}
+
+	p = kasan_krealloc(p, new_size, flags);
+	return (void *)p;
+
+alloc_new:
 	ret = kmalloc_node_track_caller_noprof(new_size, flags, NUMA_NO_NODE, _RET_IP_);
 	if (ret && p) {
 		/* Disable KASAN checks as the object's redzone is accessed. */
 		kasan_disable_current();
-		memcpy(ret, kasan_reset_tag(p), ks);
+		memcpy(ret, kasan_reset_tag(p), orig_size ?: ks);
 		kasan_enable_current();
 	}
 
@@ -4766,16 +4798,20 @@ __do_krealloc(const void *p, size_t new_size, gfp_t flags)
  * memory allocation is flagged with __GFP_ZERO. Otherwise, it is possible that
  * __GFP_ZERO is not fully honored by this API.
  *
- * This is the case, since krealloc() only knows about the bucket size of an
- * allocation (but not the exact size it was allocated with) and hence
- * implements the following semantics for shrinking and growing buffers with
- * __GFP_ZERO.
+ * When slub_debug_orig_size() is off, krealloc() only knows about the bucket
+ * size of an allocation (but not the exact size it was allocated with) and
+ * hence implements the following semantics for shrinking and growing buffers
+ * with __GFP_ZERO.
  *
  *         new             bucket
  * 0       size             size
  * |--------|----------------|
  * |  keep  |      zero      |
  *
+ * Otherwise, the original allocation size 'orig_size' could be used to
+ * precisely clear the requested size, and the new size will also be stored
+ * as the new 'orig_size'.
+ *
  * In any case, the contents of the object pointed to are preserved up to the
  * lesser of the new and old sizes.
  *