[PATCH v2] vsock: keep SOCK_SEQPACKET message boundaries on interrupted send

["Bartłomiej Dmitruk" <bartlomiej.dmitruk@isec.pl>]

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.

Wait until the whole remaining SEQPACKET message fits before enqueuing, so a
message is committed atomically (with its EOM) or not started; 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").

To avoid blocking forever on a message that can never fit -- a peer can
advertise a small buf_alloc, or the message can simply exceed the transmit
buffer -- reject such a message up front with -EMSGSIZE (the same error the
transport already returns for an oversized message) instead of waiting. This
needs the transport's maximum message size, added as a new optional
seqpacket_max_size() transport op implemented by the virtio/loopback
transports.

Signed-off-by: Bartłomiej Dmitruk <bartlomiej.dmitruk@isec.pl>
Assisted-by: Claude (Anthropic)
---
Changes in v2:
 - Fix an indefinite wait the v1 approach introduced for oversized
   SOCK_SEQPACKET messages (reported by the Sashiko AI review on v1): the
   atomic-wait could never be satisfied when len > the transport's max
   message size (e.g. a peer advertising a small peer_buf_alloc), so the
   sender blocked instead of returning -EMSGSIZE. v2 checks this up front via
   the new seqpacket_max_size() op and returns -EMSGSIZE without waiting.
 - v1: https://lore.kernel.org/netdev/20260917220101.55744-1-bartlomiej.dmitruk@isec.pl/

Testing (vsock_loopback, unprivileged):
 - Interrupted partial send (-EINTR) no longer leaks orphan bytes into the
   next message (recv() returns only the next message).
 - Oversized message and small-peer_buf_alloc cases return -EMSGSIZE promptly
   instead of blocking.
 - Normal SEQPACKET send/recv unaffected.

diff --git a/include/linux/virtio_vsock.h b/include/linux/virtio_vsock.h
index f91704731..e96790c03 100644
--- a/include/linux/virtio_vsock.h
+++ b/include/linux/virtio_vsock.h
@@ -219,6 +219,7 @@ virtio_transport_seqpacket_dequeue(struct vsock_sock *vsk,
 s64 virtio_transport_stream_has_data(struct vsock_sock *vsk);
 s64 virtio_transport_stream_has_space(struct vsock_sock *vsk);
 u32 virtio_transport_seqpacket_has_data(struct vsock_sock *vsk);
+u32 virtio_transport_seqpacket_max_size(struct vsock_sock *vsk);
 
 ssize_t virtio_transport_unsent_bytes(struct vsock_sock *vsk);
 
diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
index 5549298c1..d94c613ef 100644
--- a/include/net/af_vsock.h
+++ b/include/net/af_vsock.h
@@ -143,6 +143,7 @@ struct vsock_transport {
 				 size_t len);
 	bool (*seqpacket_allow)(struct vsock_sock *vsk, u32 remote_cid);
 	u32 (*seqpacket_has_data)(struct vsock_sock *vsk);
+	u32 (*seqpacket_max_size)(struct vsock_sock *vsk);
 
 	/* Notification. */
 	int (*notify_poll_in)(struct vsock_sock *, size_t, bool *);
diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index f840498b5..ac64cb957 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -2250,9 +2250,32 @@ static int vsock_connectible_sendmsg(struct socket *sock, struct msghdr *msg,
 
 	while (total_written < len) {
 		ssize_t written;
+		s64 min_space;
+
+		if (sk->sk_type == SOCK_SEQPACKET) {
+			/* A SEQPACKET message must be delivered atomically, so
+			 * wait until the whole remaining message fits before
+			 * enqueuing.  Otherwise a credit-limited partial send that
+			 * later errors out (e.g. -EINTR) leaves EOM-less fragments
+			 * that the peer merges into the next message.
+			 *
+			 * Reject a message that can never fit up front so the wait
+			 * below cannot block forever (a peer may advertise a small
+			 * buf_alloc); this mirrors the -EMSGSIZE the transport
+			 * returns for an oversized message.
+			 */
+			if (transport->seqpacket_max_size &&
+			    len > transport->seqpacket_max_size(vsk)) {
+				err = -EMSGSIZE;
+				goto out_err;
+			}
+			min_space = len - total_written;
+		} else {
+			min_space = 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)) {
diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index 4f9aa9c4c..b7d587a20 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -585,6 +585,7 @@ static struct virtio_transport virtio_transport = {
 		.seqpacket_enqueue        = virtio_transport_seqpacket_enqueue,
 		.seqpacket_allow          = virtio_transport_seqpacket_allow,
 		.seqpacket_has_data       = virtio_transport_seqpacket_has_data,
+		.seqpacket_max_size       = virtio_transport_seqpacket_max_size,
 
 		.msgzerocopy_allow        = virtio_transport_msgzerocopy_allow,
 
diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index f225f53ed..e10e7b958 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -994,6 +994,19 @@ virtio_transport_seqpacket_enqueue(struct vsock_sock *vsk,
 }
 EXPORT_SYMBOL_GPL(virtio_transport_seqpacket_enqueue);
 
+u32 virtio_transport_seqpacket_max_size(struct vsock_sock *vsk)
+{
+	struct virtio_vsock_sock *vvs = vsk->trans;
+	u32 max_size;
+
+	spin_lock_bh(&vvs->tx_lock);
+	max_size = virtio_transport_tx_buf_size(vvs);
+	spin_unlock_bh(&vvs->tx_lock);
+
+	return max_size;
+}
+EXPORT_SYMBOL_GPL(virtio_transport_seqpacket_max_size);
+
 int
 virtio_transport_dgram_dequeue(struct vsock_sock *vsk,
 			       struct msghdr *msg,
diff --git a/net/vmw_vsock/vsock_loopback.c b/net/vmw_vsock/vsock_loopback.c
index 8068d1b6e..b9a0cc861 100644
--- a/net/vmw_vsock/vsock_loopback.c
+++ b/net/vmw_vsock/vsock_loopback.c
@@ -90,6 +90,7 @@ static struct virtio_transport loopback_transport = {
 		.seqpacket_enqueue        = virtio_transport_seqpacket_enqueue,
 		.seqpacket_allow          = vsock_loopback_seqpacket_allow,
 		.seqpacket_has_data       = virtio_transport_seqpacket_has_data,
+		.seqpacket_max_size       = virtio_transport_seqpacket_max_size,
 
 		.msgzerocopy_allow        = vsock_loopback_msgzerocopy_allow,