[PATCH net v4 6/8] xsk: free the skb when hitting the upper bound MAX_SKB_FRAGS

Jason Xing
2026-04-24T05:38:55Z

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>
---
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 c49b58199d2f..5e6326e076ab 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -776,8 +776,11 @@ 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