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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 adding a !xs->skb check to the condition, 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/20260415082654.21026-4-kerneljasonxing@gmail.com/ > Fixes: 6b9c129c2f93 ("xsk: remove @first_frag from xsk_build_skb()") > Signed-off-by: Jason Xing > --- > 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..4fdd1a45a9bd 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 && !skb_shinfo(skb)->nr_frags) > kfree_skb(skb); > > if (err == -EOVERFLOW) { > -- > 2.41.3 Now "!skb_shinfo(skb)->nr_frags" feels redundant? It's either "skb && !xs->skb" and we own the kfree. or "xs->skb != NULL" and we want xsk_drop_skb? Or am I missing something?