Subject: [PATCH net v4 3/8] xsk: fix use-after-free of xs->skb in xsk_build_skb() free_err path

From: Jason Xing <[email-masked]>
Date: Fri, 24 Apr 2026 13:38:11 +0800

From: Jason Xing <[email-masked]>

When xsk_build_skb() processes multi-buffer packets in copy mode, the
first descriptor stores data into the skb linear area without adding
any frags, so nr_frags stays at 0. The caller then sets xs->skb = skb
to accumulate subsequent descriptors.

If a continuation descriptor fails (e.g. alloc_page returns NULL with
-EAGAIN), we jump to free_err where the condition:

  if (skb && !skb_shinfo(skb)->nr_frags)
      kfree_skb(skb);

evaluates to true because nr_frags is still 0 (the first descriptor
used the linear area, not frags). This frees the skb while xs->skb
still points to it, creating a dangling pointer. On the next transmit
attempt or socket close, xs->skb is dereferenced, causing a
use-after-free or double-free.

Fix by using a !xs->skb check to handle first frag situation, ensuring
we only free skbs that were freshly allocated in this call
(xs->skb is NULL) and never free an in-progress multi-buffer skb that
the caller still references.

Closes: https://lore.kernel.org/all/[email-masked]/
Fixes: 6b9c129c2f93 ("xsk: remove @first_frag from xsk_build_skb()")
Acked-by: Stanislav Fomichev <[email-masked]>
Signed-off-by: Jason Xing <[email-masked]>
---
 net/xdp/xsk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 6521604f8d42..d23d1b14b8b4 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -889,7 +889,7 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 	return skb;
 
 free_err:
-	if (skb && !skb_shinfo(skb)->nr_frags)
+	if (skb && !xs->skb)
 		kfree_skb(skb);
 
 	if (err == -EOVERFLOW) {
-- 
2.41.3