net: convert sk_buff_fclones.fclone_ref from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <[e-mail]>
Signed-off-by: Hans Liljestrand <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: David Windsor <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/skbuff.h

···
 
 	struct sk_buff	skb2;
 
-	atomic_t	fclone_ref;
+	refcount_t	fclone_ref;
 };
 
 /**
···
 	fclones = container_of(skb, struct sk_buff_fclones, skb1);
 
 	return skb->fclone == SKB_FCLONE_ORIG &&
-	       atomic_read(&fclones->fclone_ref) > 1 &&
+	       refcount_read(&fclones->fclone_ref) > 1 &&
 	       fclones->skb2.sk == sk;
 }
 

net/core/skbuff.c

···
 
 		kmemcheck_annotate_bitfield(&fclones->skb2, flags1);
 		skb->fclone = SKB_FCLONE_ORIG;
-		atomic_set(&fclones->fclone_ref, 1);
+		refcount_set(&fclones->fclone_ref, 1);
 
 		fclones->skb2.fclone = SKB_FCLONE_CLONE;
 	}
···
 		 * This test would have no chance to be true for the clone,
 		 * while here, branch prediction will be good.
 		 */
-		if (atomic_read(&fclones->fclone_ref) == 1)
+		if (refcount_read(&fclones->fclone_ref) == 1)
 			goto fastpath;
 		break;
 
···
 		fclones = container_of(skb, struct sk_buff_fclones, skb2);
 		break;
 	}
-	if (!atomic_dec_and_test(&fclones->fclone_ref))
+	if (!refcount_dec_and_test(&fclones->fclone_ref))
 		return;
 fastpath:
 	kmem_cache_free(skbuff_fclone_cache, fclones);
···
 		return NULL;
 
 	if (skb->fclone == SKB_FCLONE_ORIG &&
-	    atomic_read(&fclones->fclone_ref) == 1) {
+	    refcount_read(&fclones->fclone_ref) == 1) {
 		n = &fclones->skb2;
-		atomic_set(&fclones->fclone_ref, 2);
+		refcount_set(&fclones->fclone_ref, 2);
 	} else {
 		if (skb_pfmemalloc(skb))
 			gfp_mask |= __GFP_MEMALLOC;