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Tue, 19 May 2026 17:43:11 -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, horms@kernel.org, andrew+netdev@lunn.ch Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, Jason Xing Subject: [PATCH net v4 2/5] xsk: fix buffer leak in xsk_drop_skb() for AF_XDP multi-buffer Tx Date: Wed, 20 May 2026 08:42:41 +0800 Message-Id: <20260520004244.55663-3-kerneljasonxing@gmail.com> X-Mailer: git-send-email 2.33.0 In-Reply-To: <20260520004244.55663-1-kerneljasonxing@gmail.com> References: <20260520004244.55663-1-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 This patch is inspired by the check[1] from sashiko. It says when overflow happens, the address of cq to be published is invalid. Actually the severer thing is the whole process of publishing the address of cq in this particular case is not right: it should truely publish the address and advance the cached_prod in cq as long as it reads descriptors from txq. The following is the full analysis. xsk_drop_skb() is called in three places, which all discard a partially built multi-buffer skb: 1) xsk_build_skb() -EOVERFLOW error path: packet exceeds MAX_SKB_FRAGS 2) __xsk_generic_xmit() post-loop cleanup: an invalid descriptor in the TX ring prevents the partial packet from completing 3) xsk_release(): socket close while xs->skb holds an incomplete packet In all three cases, the TX descriptors for the already-processed frags have been consumed from the TX ring (xskq_cons_release), and CQ slots have been reserved. However, xsk_drop_skb() calls xsk_consume_skb() which cancels the CQ reservations via xsk_cq_cancel_locked(). Since the buffer addresses never appear in the completion queue, userspace permanently loses track of these buffers. Fix this by letting consume_skb() trigger the existing xsk_destruct_skb destructor, which already submits buffer addresses to the CQ via xsk_cq_submit_addr_locked(). Note that cancelling the descriptors back to the TX ring (via xskq_cons_cancel_n) is not a appropriate option because an oversized packet that always exceeds MAX_SKB_FRAGS would be retried indefinitely, which is an obviously deadlock bug in the TX path. Also move the desc->addr assignment in xsk_build_skb() above the overflow check so that the current descriptor's address is recorded before a potential -EOVERFLOW jump to free_err, consistent with the zerocopy path in xsk_build_skb_zerocopy(). [1]: https://lore.kernel.org/all/20260425041726.85FB3C2BCB2@smtp.kernel.org/ Fixes: cf24f5a5feea ("xsk: add support for AF_XDP multi-buffer on Tx path") Signed-off-by: Jason Xing --- net/xdp/xsk.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c index f8c8a8c9dfba..0a6203c42576 100644 --- a/net/xdp/xsk.c +++ b/net/xdp/xsk.c @@ -793,8 +793,11 @@ static void xsk_consume_skb(struct sk_buff *skb) static void xsk_drop_skb(struct sk_buff *skb) { - xdp_sk(skb->sk)->tx->invalid_descs += xsk_get_num_desc(skb); - xsk_consume_skb(skb); + struct xdp_sock *xs = xdp_sk(skb->sk); + + xs->tx->invalid_descs += xsk_get_num_desc(skb); + consume_skb(skb); + xs->skb = NULL; } static int xsk_skb_metadata(struct sk_buff *skb, void *buffer, @@ -876,7 +879,7 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs, return ERR_PTR(-ENOMEM); /* in case of -EOVERFLOW that could happen below, - * xsk_consume_skb() will release this node as whole skb + * xsk_drop_skb() will release this node as whole skb * would be dropped, which implies freeing all list elements */ xsk_addr->addrs[xsk_addr->num_descs] = desc->addr; @@ -968,6 +971,8 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs, goto free_err; } + xsk_addr->addrs[xsk_addr->num_descs] = desc->addr; + if (unlikely(nr_frags == (MAX_SKB_FRAGS - 1) && xp_mb_desc(desc))) { err = -EOVERFLOW; goto free_err; @@ -985,8 +990,6 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs, skb_add_rx_frag(skb, nr_frags, page, 0, len, PAGE_SIZE); refcount_add(PAGE_SIZE, &xs->sk.sk_wmem_alloc); - - xsk_addr->addrs[xsk_addr->num_descs] = desc->addr; } } -- 2.43.7