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Thu, 23 Apr 2026 22:38:39 -0700 (PDT) From: Jason Xing To: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, bjorn@kernel.org, magnus.karlsson@intel.com, maciej.fijalkowski@intel.com, jonathan.lemon@gmail.com, sdf@fomichev.me, ast@kernel.org, daniel@iogearbox.net, hawk@kernel.org, john.fastabend@gmail.com, aleksander.lobakin@intel.com Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, Jason Xing Subject: [PATCH net v4 3/8] xsk: fix use-after-free of xs->skb in xsk_build_skb() free_err path Date: Fri, 24 Apr 2026 13:38:11 +0800 Message-Id: <20260424053816.27965-4-kerneljasonxing@gmail.com> X-Mailer: git-send-email 2.33.0 In-Reply-To: <20260424053816.27965-1-kerneljasonxing@gmail.com> References: <20260424053816.27965-1-kerneljasonxing@gmail.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Jason Xing 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/20260415082654.21026-4-kerneljasonxing@gmail.com/ Fixes: 6b9c129c2f93 ("xsk: remove @first_frag from xsk_build_skb()") Acked-by: Stanislav Fomichev 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..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