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Tsirkin" , Jason Wang Cc: netdev@vger.kernel.org, virtualization@lists.linux.dev, kvm@vger.kernel.org Subject: [RFC PATCH] vsock: keep SOCK_SEQPACKET message boundaries on interrupted send Date: Fri, 18 Sep 2026 00:01:00 +0200 Message-ID: <20260917220101.55744-1-bartlomiej.dmitruk@isec.pl> X-Mailer: git-send-email 2.46.2 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit A credit-limited SOCK_SEQPACKET send transmits fragments as credit becomes available, and the VIRTIO_VSOCK_SEQ_EOM flag is set only on the fragment where msg_data_left() reaches 0. If vsock_connectible_sendmsg() exits via out_err after a partial send -- notably the non-terminal -EINTR path (signal_pending while blocked for credit), but also sk_err / peer RCV_SHUTDOWN -- the already-transmitted fragments carry no EOM. The receiver only advances msg_count / sets msg_ready on an EOM skb, so the orphaned fragments are silently merged into the *next* message, violating SOCK_SEQPACKET atomicity. Reproduced on vsock_loopback: an -EINTR-aborted partial send of 'A's is merged into the next 'B' message; recv() returns one [A...][B...] message instead of just the 'B' message (reproducer + before/after below the ---). Since a SEQPACKET message is bounded by min(peer_buf_alloc, buf_alloc) (EMSGSIZE gate in virtio_transport_seqpacket_enqueue()), the sender can wait for room for the whole remaining message before enqueuing, so the message is committed atomically or not at all; an error while waiting then leaves nothing on the wire. SOCK_STREAM behaviour is unchanged (min_space == 1 reproduces the old "wait while space == 0"). RFC: an alternative is to close a partially-sent message with an explicit EOM on the error path; feedback on the preferred approach welcome. Signed-off-by: Bartłomiej Dmitruk --- Testing (vsock_loopback, unprivileged, single process; reproducer in t_vuln004.c): receiver buf_alloc=16384; a 14000-byte message is left unread to shrink credit to 2384; an 8000-byte message is sent with alarm(1) so the send is interrupted (-EINTR) after a partial 2384-byte fragment; the 14000 message is drained; then a 1000-byte message is sent. before: recv() returns one 3384-byte message = 2384 'A' (orphan) + 1000 'B' after: recv() returns one 1000-byte message = 1000 'B' (boundary intact) Normal SEQPACKET send/recv (the 14000- and 1000-byte whole messages) works unchanged. diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c --- a/net/vmw_vsock/af_vsock.c +++ b/net/vmw_vsock/af_vsock.c @@ -2250,9 +2250,16 @@ while (total_written < len) { ssize_t written; + /* For SEQPACKET wait until the whole remaining message fits, so + * it is enqueued atomically. A credit-limited partial send that + * then errors out (e.g. -EINTR) would otherwise leave EOM-less + * fragments that the peer merges into the next message. + */ + s64 min_space = (sk->sk_type == SOCK_SEQPACKET) ? + (s64)(len - total_written) : 1; add_wait_queue(sk_sleep(sk), &wait); - while (vsock_stream_has_space(vsk) == 0 && + while (vsock_stream_has_space(vsk) < min_space && sk->sk_err == 0 && !(sk->sk_shutdown & SEND_SHUTDOWN) && !(vsk->peer_shutdown & RCV_SHUTDOWN)) {