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Tue, 01 Sep 2026 00:37:27 -0700 (PDT) Date: Tue, 1 Sep 2026 16:37:22 +0900 From: Hyunwoo Kim To: davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, ncardwell@google.com, dsahern@kernel.org, idosch@nvidia.com, kuniyu@google.com, horms@kernel.org, willemb@google.com, andrew+netdev@lunn.ch, kees@kernel.org, jiayuan.chen@linux.dev Cc: kerneljasonxing@gmail.com, ij@kernel.org, martin.lau@kernel.org, shakeel.butt@linux.dev, matttbe@kernel.org, martineau@kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, imv4bel@gmail.com Subject: Re: [PATCH net v2 6/8] tcp: fix use-after-free in the lockless listener path Message-ID: References: <20260824033331.1084971-1-imv4bel@gmail.com> <20260824033331.1084971-7-imv4bel@gmail.com> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260824033331.1084971-7-imv4bel@gmail.com> On Mon, Aug 24, 2026 at 12:32:50PM +0900, Hyunwoo Kim wrote: > tcp_v{4,6}_rcv() calls tcp_v{4,6}_do_rcv() without holding the socket > lock when sk->sk_state is TCP_LISTEN. Every other path into > tcp_v{4,6}_do_rcv() holds it. > > tcp_v{4,6}_do_rcv() and tcp_rcv_state_process() below it read > sk->sk_state again. A listener can leave TCP_LISTEN through > connect(AF_UNSPEC), and if that happens in between, the second read > returns a different state. > > tcp_rcv_established() or tcp_rcv_state_process() then runs without the > lock. If the second read returns TCP_SYN_SENT, the incoming SYN is > treated as a crossed SYN and reaches tcp_send_synack(). When the SYN skb > at the head of the retransmit queue is skb_cloned(), that function > replaces it with a copy and releases the original with > tcp_rtx_queue_unlink_and_free(). > > The original is the skb that a thread on another CPU is transmitting > right now in __tcp_transmit_skb(). skb_cloned() is true because the > clone made for that transmit is still alive. Once the transmit returns, > tcp_update_skb_after_send() calls list_move_tail() on the skb's > tcp_tsorted_anchor. > > In short: > > socket(AF_INET) -> bind() -> listen() // the socket that changes state > socket(AF_INET) -> bind() -> listen() // the peer > > Several threads keep opening new sockets and connecting to the first > socket's address. > > Another thread repeats this on the first socket: > connect(AF_UNSPEC) // TCP_LISTEN -> TCP_CLOSE > connect(peer address) // TCP_CLOSE -> TCP_SYN_SENT > // another CPU still sees a listener, handles > // one of those SYNs without the lock and > // releases the SYN skb that this connect() > // is transmitting > // -> use-after-free > connect(AF_UNSPEC) > listen() // TCP_LISTEN again > > KASAN log: > > BUG: KASAN: slab-use-after-free in __list_del_entry_valid_or_report+0x14/0x140 > Read of size 8 at addr ffff88800a5d1460 by task poc/125 > ... > Call Trace: > __list_del_entry_valid_or_report+0x14/0x140 > tcp_update_skb_after_send+0x62/0x170 > __tcp_transmit_skb+0xe33/0x1e40 > tcp_connect+0x1b67/0x2490 > tcp_v4_connect+0x998/0xab0 > __inet_stream_connect+0x22c/0x700 > inet_stream_connect+0x48/0x70 > __sys_connect+0x101/0x130 > ... > Allocated by task 125: > __alloc_skb+0xd1/0x370 > tcp_stream_alloc_skb+0x2d/0x2b0 > tcp_connect+0x72d/0x2490 > tcp_v4_connect+0x998/0xab0 > __inet_stream_connect+0x22c/0x700 > inet_stream_connect+0x48/0x70 > __sys_connect+0x101/0x130 > ... > The buggy address belongs to the object at ffff88800a5d1400 > which belongs to the cache skbuff_fclone_cache of size 472 > > Instead of taking the lock, keep the lockless path from reading > sk->sk_state again to decide how to process the packet. Move the > TCP_LISTEN handling out of tcp_rcv_state_process() into > tcp_rcv_listen_state_process(), and let the TCP_LISTEN branch of > tcp_v{4,6}_rcv() call a new tcp_v{4,6}_rcv_listen(). Listener processing > does not change. The TCP_LISTEN arm of tcp_v{4,6}_do_rcv() is left > alone, because a socket can finish listen() after the state check and a > backlogged skb is then processed there. > > Fixes: e994b2f0fb92 ("tcp: do not lock listener to process SYN packets") > Cc: stable@vger.kernel.org > Signed-off-by: Hyunwoo Kim Looking at this further, unhashing the listener and then calling synchronize_net() lets the disconnect path handle it. MPTCP needs a fix too, though, because it closes and reuses the first subflow directly without going through tcp_disconnect(). This also closes the trigger path for patches 4, 5 and 8. I would still keep those, since they add no work to the fast path and they remove the root cause itself. Their changelogs would have to change though. diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index b4237d0e994d6f..ae6ab3b22beb53 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -3373,6 +3373,8 @@ int tcp_disconnect(struct sock *sk, int flags) /* ABORT function of RFC793 */ if (old_state == TCP_LISTEN) { + /* Wait for lockless listener receive paths to finish. */ + synchronize_net(); inet_csk_listen_stop(sk); } else if (unlikely(tp->repair)) { WRITE_ONCE(sk->sk_err, ECONNABORTED); diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c index b474d03620a75d..e41a5e1bf8dbf3 100644 --- a/net/mptcp/protocol.c +++ b/net/mptcp/protocol.c @@ -3432,7 +3432,7 @@ static __poll_t mptcp_check_readable(struct sock *sk) return mptcp_epollin_ready(sk) ? EPOLLIN | EPOLLRDNORM : 0; } -static void mptcp_check_listen_stop(struct sock *sk) +static void mptcp_check_listen_stop(struct sock *sk, bool sync_net) { struct sock *ssk; @@ -3446,6 +3446,9 @@ static void mptcp_check_listen_stop(struct sock *sk) lock_sock_nested(ssk, SINGLE_DEPTH_NESTING); tcp_set_state(ssk, TCP_CLOSE); + if (sync_net) + /* Wait for lockless listener receive paths to finish. */ + synchronize_net(); mptcp_subflow_queue_clean(sk, ssk); inet_csk_listen_stop(ssk); mptcp_event_pm_listener(ssk, MPTCP_EVENT_LISTENER_CLOSED); @@ -3462,7 +3465,7 @@ bool __mptcp_close(struct sock *sk, long timeout) WRITE_ONCE(sk->sk_shutdown, SHUTDOWN_MASK); if ((1 << sk->sk_state) & (TCPF_LISTEN | TCPF_CLOSE)) { - mptcp_check_listen_stop(sk); + mptcp_check_listen_stop(sk, false); mptcp_set_state(sk, TCP_CLOSE); goto cleanup; } @@ -3593,7 +3596,7 @@ static int mptcp_disconnect(struct sock *sk, int flags) if (msk->fastopening) return -EBUSY; - mptcp_check_listen_stop(sk); + mptcp_check_listen_stop(sk, true); mptcp_set_state(sk, TCP_CLOSE); mptcp_stop_rtx_timer(sk); Best regards, Hyunwoo Kim