xsk: free the skb when hitting the upper bound MAX_SKB_FRAGS

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")
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://patch.msgid.link/20260502200722.53960-3-kerneljasonxing@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 887abed254668093045d16e37b0dd30f35ccbc30..d706b1e0bf602931254d8206296a22f0fcd88d6c 100644 (file)
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -856,8 +856,11 @@ 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);