[PATCH net 2/4] xsk: free the skb when hitting the upper bound MAX_SKB_FRAGS

[Jason Xing <kerneljasonxing@gmail.com>]

From: Jason Xing <kernelxing@tencent.com>

Fix it by explicitly adding kfree_skb() before returning back to its
caller.

How to reproduce it in virtio_net:
1. the current skb is the first one (which means xs->skb is NULL) and
   hit the limit MAX_SKB_FRAGS.
2. xsk_build_skb_zerocopy() returns -EOVERFLOW.
3. the caller xsk_build_skb() clears skb by using 'skb = NULL;'. This
   is why bug can be triggered.
4. there is no chance to free this skb anymore.

Note that if in this case the xs->skb is not NULL, xsk_build_skb() will
call xsk_drop_skb(xs->skb) to do the right thing.

Fixes: cf24f5a5feea ("xsk: add support for AF_XDP multi-buffer on Tx path")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 net/xdp/xsk.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 8fcde34aec7b..5d3dbb118730 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -778,8 +778,11 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
 	addr = buffer - pool->addrs;
 
 	for (copied = 0, i = skb_shinfo(skb)->nr_frags; copied < len; i++) {
-		if (unlikely(i >= MAX_SKB_FRAGS))
+		if (unlikely(i >= MAX_SKB_FRAGS)) {
+			if (!xs->skb)
+				kfree_skb(skb);
 			return ERR_PTR(-EOVERFLOW);
+		}
 
 		page = pool->umem->pgs[addr >> PAGE_SHIFT];
 		get_page(page);
-- 
2.41.3