xsk: Fix xsk_build_skb() error: 'skb' dereferencing possible ERR_PTR()

Currently, xsk_build_skb() is a function that builds skb in two possible
ways and then is ended with common error handling.

We can distinguish four possible error paths and handling in xsk_build_skb():

 1. sock_alloc_send_skb fails: Retry (skb is NULL).
 2. skb_store_bits fails : Free skb and retry.
 3. MAX_SKB_FRAGS exceeded: Free skb, cleanup and drop packet.
 4. alloc_page fails for frag: Retry page allocation w/o freeing skb

1] and 3] can happen in xsk_build_skb_zerocopy(), which is one of the
two code paths responsible for building skb. Common error path in
xsk_build_skb() assumes that in case errno != -EAGAIN, skb is a valid
pointer, which is wrong as kernel test robot reports that in
xsk_build_skb_zerocopy() other errno values are returned for skb being
NULL.

To fix this, set -EOVERFLOW as error when MAX_SKB_FRAGS are exceeded
and packet needs to be dropped in both xsk_build_skb() and
xsk_build_skb_zerocopy() and use this to distinguish against all other
error cases. Also, add explicit kfree_skb() for 3] so that handling
of 1], 2], and 3] becomes identical where allocation needs to be retried.

Fixes: cf24f5a ("xsk: add support for AF_XDP multi-buffer on Tx path")
Reported-by: kernel test robot <[email protected]>
Reported-by: Dan Carpenter <[email protected]>
Signed-off-by: Tirthendu Sarkar <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Magnus Karlsson <[email protected]>
Closes: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/bpf/[email protected]

net/xdp/xsk.c

@@ -602,7 +602,7 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
 
 	for (copied = 0, i = skb_shinfo(skb)->nr_frags; copied < len; i++) {
 		if (unlikely(i >= MAX_SKB_FRAGS))
-			return ERR_PTR(-EFAULT);
+			return ERR_PTR(-EOVERFLOW);
 
 		page = pool->umem->pgs[addr >> PAGE_SHIFT];
 		get_page(page);
@@ -655,15 +655,17 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 			skb_put(skb, len);
 
 			err = skb_store_bits(skb, 0, buffer, len);
-			if (unlikely(err))
+			if (unlikely(err)) {
+				kfree_skb(skb);
 				goto free_err;
+			}
 		} else {
 			int nr_frags = skb_shinfo(skb)->nr_frags;
 			struct page *page;
 			u8 *vaddr;
 
 			if (unlikely(nr_frags == (MAX_SKB_FRAGS - 1) && xp_mb_desc(desc))) {
-				err = -EFAULT;
+				err = -EOVERFLOW;
 				goto free_err;
 			}
 
@@ -690,12 +692,14 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 	return skb;
 
 free_err:
-	if (err == -EAGAIN) {
-		xsk_cq_cancel_locked(xs, 1);
-	} else {
-		xsk_set_destructor_arg(skb);
-		xsk_drop_skb(skb);
+	if (err == -EOVERFLOW) {
+		/* Drop the packet */
+		xsk_set_destructor_arg(xs->skb);
+		xsk_drop_skb(xs->skb);
 		xskq_cons_release(xs->tx);
+	} else {
+		/* Let application retry */
+		xsk_cq_cancel_locked(xs, 1);
 	}
 
 	return ERR_PTR(err);
@@ -738,7 +742,7 @@ static int __xsk_generic_xmit(struct sock *sk)
 		skb = xsk_build_skb(xs, &desc);
 		if (IS_ERR(skb)) {
 			err = PTR_ERR(skb);
-			if (err == -EAGAIN)
+			if (err != -EOVERFLOW)
 				goto out;
 			err = 0;
 			continue;