skbuff: Introduce slab_build_skb()

syzkaller reported:

BUG: KASAN: slab-out-of-bounds in __build_skb_around+0x235/0x340 net/core/skbuff.c:294
Write of size 32 at addr ffff88802aa172c0 by task syz-executor413/5295

For bpf_prog_test_run_skb(), which uses a kmalloc()ed buffer passed to
build_skb().

When build_skb() is passed a frag_size of 0, it means the buffer came
from kmalloc. In these cases, ksize() is used to find its actual size,
but since the allocation may not have been made to that size, actually
perform the krealloc() call so that all the associated buffer size
checking will be correctly notified (and use the "new" pointer so that
compiler hinting works correctly). Split this logic out into a new
interface, slab_build_skb(), but leave the original 0 checking for now
to catch any stragglers.

Reported-by: [e-mail]
Link: https://groups.google.com/g/syzkaller-bugs/c/UnIKxTtU5-0/m/-wbXinkgAQAJ
Fixes: 38931d8989b5 ("mm: Make ksize() a reporting-only function")
Cc: Pavel Begunkov <[e-mail]>
Cc: pepsipu <[e-mail]>
Cc: [e-mail]
Cc: Vlastimil Babka <[e-mail]>
Cc: kasan-dev <[e-mail]>
Cc: Andrii Nakryiko <[e-mail]>
Cc: [e-mail]
Cc: Daniel Borkmann <[e-mail]>
Cc: Hao Luo <[e-mail]>
Cc: Jesper Dangaard Brouer <[e-mail]>
Cc: John Fastabend <[e-mail]>
Cc: [e-mail]
Cc: KP Singh <[e-mail]>
Cc: [e-mail]
Cc: Stanislav Fomichev <[e-mail]>
Cc: [e-mail]
Cc: Yonghong Song <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/broadcom/bnx2.c

···
 
 	dma_unmap_single(&bp->pdev->dev, dma_addr, bp->rx_buf_use_size,
 			 DMA_FROM_DEVICE);
-	skb = build_skb(data, 0);
+	skb = slab_build_skb(data);
 	if (!skb) {
 		kfree(data);
 		goto error;

drivers/net/ethernet/qlogic/qed/qed_ll2.c

···
 	dma_unmap_single(&cdev->pdev->dev, buffer->phys_addr,
 			 cdev->ll2->rx_size, DMA_FROM_DEVICE);
 
-	skb = build_skb(buffer->data, 0);
+	skb = slab_build_skb(buffer->data);
 	if (!skb) {
 		DP_INFO(cdev, "Failed to build SKB\n");
 		kfree(buffer->data);

include/linux/skbuff.h

···
 void skb_attempt_defer_free(struct sk_buff *skb);
 
 struct sk_buff *napi_build_skb(void *data, unsigned int frag_size);
+struct sk_buff *slab_build_skb(void *data);
 
 /**
  * alloc_skb - allocate a network buffer

net/bpf/test_run.c

···
 	}
 	sock_init_data(NULL, sk);
 
-	skb = build_skb(data, 0);
+	skb = slab_build_skb(data);
 	if (!skb) {
 		kfree(data);
 		kfree(ctx);

net/core/skbuff.c

···
 	return skb;
 }
 
-/* Caller must provide SKB that is memset cleared */
-static void __build_skb_around(struct sk_buff *skb, void *data,
-			       unsigned int frag_size)
+static inline void __finalize_skb_around(struct sk_buff *skb, void *data,
+					 unsigned int size)
 {
 	struct skb_shared_info *shinfo;
-	unsigned int size = frag_size ? : ksize(data);
 
 	size -= SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
 
···
 	skb_set_kcov_handle(skb, kcov_common_handle());
 }
 
+static inline void *__slab_build_skb(struct sk_buff *skb, void *data,
+				     unsigned int *size)
+{
+	void *resized;
+
+	/* Must find the allocation size (and grow it to match). */
+	*size = ksize(data);
+	/* krealloc() will immediately return "data" when
+	 * "ksize(data)" is requested: it is the existing upper
+	 * bounds. As a result, GFP_ATOMIC will be ignored. Note
+	 * that this "new" pointer needs to be passed back to the
+	 * caller for use so the __alloc_size hinting will be
+	 * tracked correctly.
+	 */
+	resized = krealloc(data, *size, GFP_ATOMIC);
+	WARN_ON_ONCE(resized != data);
+	return resized;
+}
+
+/* build_skb() variant which can operate on slab buffers.
+ * Note that this should be used sparingly as slab buffers
+ * cannot be combined efficiently by GRO!
+ */
+struct sk_buff *slab_build_skb(void *data)
+{
+	struct sk_buff *skb;
+	unsigned int size;
+
+	skb = kmem_cache_alloc(skbuff_head_cache, GFP_ATOMIC);
+	if (unlikely(!skb))
+		return NULL;
+
+	memset(skb, 0, offsetof(struct sk_buff, tail));
+	data = __slab_build_skb(skb, data, &size);
+	__finalize_skb_around(skb, data, size);
+
+	return skb;
+}
+EXPORT_SYMBOL(slab_build_skb);
+
+/* Caller must provide SKB that is memset cleared */
+static void __build_skb_around(struct sk_buff *skb, void *data,
+			       unsigned int frag_size)
+{
+	unsigned int size = frag_size;
+
+	/* frag_size == 0 is considered deprecated now. Callers
+	 * using slab buffer should use slab_build_skb() instead.
+	 */
+	if (WARN_ONCE(size == 0, "Use slab_build_skb() instead"))
+		data = __slab_build_skb(skb, data, &size);
+
+	__finalize_skb_around(skb, data, size);
+}
+
 /**
  * __build_skb - build a network buffer
  * @data: data buffer provided by caller
- * @frag_size: size of data, or 0 if head was kmalloced
+ * @frag_size: size of data (must not be 0)
  *
  * Allocate a new &sk_buff. Caller provides space holding head and
- * skb_shared_info. @data must have been allocated by kmalloc() only if
- * @frag_size is 0, otherwise data should come from the page allocator
- *  or vmalloc()
+ * skb_shared_info. @data must have been allocated from the page
+ * allocator or vmalloc(). (A @frag_size of 0 to indicate a kmalloc()
+ * allocation is deprecated, and callers should use slab_build_skb()
+ * instead.)
  * The return is the new skb buffer.
  * On a failure the return is %NULL, and @data is not freed.
  * Notes :