[PATCH net v3 1/2] vsock/virtio: validate packet source for connected sockets

[Daehyeon Ko <4ncienth@gmail.com>]

virtio_transport_recv_pkt() looks up sockets first by the full source and
destination tuple, then by destination only in the bound table. The
fallback is needed for listening and connecting sockets, but sockets remain
in the bound table after connect(), so it can also return a non-listening
socket.

The fallback does not validate the source address. In TCP_SYN_SENT, a
RESPONSE from an unrelated source can transition the victim socket to
TCP_ESTABLISHED while its stored remote address remains unchanged.
Subsequent RW packets from that source are delivered through the same
destination-only fallback.

This was reproduced with capability-empty processes under different UIDs.
The attacker discovered the target tuple through unprivileged AF_VSOCK
sock_diag and caused the victim socket to read 16 attacker-chosen bytes;
the intended peer-side socket read 0 of those 16 bytes.

Add vsock_check_source() to validate the transport, source port and source
CID against the peer stored in a non-listening socket. The local transport
is the CID exception because its packets are generated internally with
VMADDR_CID_LOCAL as their source, including connections using CID aliases.

Use the helper after lock_sock() in the virtio receive path.

Fixes: 06a8fc78367d ("VSOCK: Introduce virtio_vsock_common.ko")
Closes: https://lore.kernel.org/netdev/20260813121236.2328599-1-4ncienth@gmail.com/
Cc: stable@vger.kernel.org
Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Assisted-by: Codex:gpt-5.6-sol
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
 include/net/af_vsock.h                  |  3 +++
 net/vmw_vsock/af_vsock.c                | 32 +++++++++++++++++++++++++
 net/vmw_vsock/virtio_transport_common.c |  3 ++-
 3 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
index 3357ee62d..5549298c1 100644
--- a/include/net/af_vsock.h
+++ b/include/net/af_vsock.h
@@ -229,6 +229,9 @@ struct sock *vsock_find_bound_socket_net(struct sockaddr_vm *addr,
 struct sock *vsock_find_connected_socket_net(struct sockaddr_vm *src,
 					     struct sockaddr_vm *dst,
 					     struct net *net);
+bool vsock_check_source(const struct vsock_sock *vsk,
+			const struct vsock_transport *transport,
+			const struct sockaddr_vm *src);
 void vsock_remove_sock(struct vsock_sock *vsk);
 void vsock_for_each_connected_socket(struct vsock_transport *transport,
 				     void (*fn)(struct sock *sk));
diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index a33b2a2d3..f840498b5 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -438,6 +438,38 @@ struct sock *vsock_find_connected_socket(struct sockaddr_vm *src,
 }
 EXPORT_SYMBOL_GPL(vsock_find_connected_socket);
 
+/**
+ * vsock_check_source - validate a packet source against a socket peer
+ * @vsk: socket receiving the packet
+ * @transport: transport receiving the packet
+ * @src: source address from the packet
+ *
+ * Return: true if the packet arrived on the socket's assigned transport and
+ * its source matches the stored peer. Loopback packets are generated
+ * internally and always use the local CID as their source, including
+ * connections using a valid CID alias.
+ *
+ * The caller must hold the socket lock and must not call this for listening
+ * sockets, which accept packets from any source and have no assigned
+ * transport.
+ */
+bool vsock_check_source(const struct vsock_sock *vsk,
+			const struct vsock_transport *transport,
+			const struct sockaddr_vm *src)
+{
+	if (vsk->transport != transport)
+		return false;
+
+	if (src->svm_port != vsk->remote_addr.svm_port)
+		return false;
+
+	if (src->svm_cid == vsk->remote_addr.svm_cid)
+		return true;
+
+	return transport->get_local_cid() == VMADDR_CID_LOCAL;
+}
+EXPORT_SYMBOL_GPL(vsock_check_source);
+
 void vsock_remove_sock(struct vsock_sock *vsk)
 {
 	/* Transport reassignment must not remove the binding. */
diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index e4ebaa70f..6301c108a 100644
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/net/vmw_vsock/virtio_transport_common.c
@@ -1823,7 +1823,8 @@ void virtio_transport_recv_pkt(struct virtio_transport *t,
 	 * lock_sock (note: listener sockets are not assigned to any transport)
 	 */
 	if (sock_flag(sk, SOCK_DONE) ||
-	    (sk->sk_state != TCP_LISTEN && vsk->transport != &t->transport)) {
+	    (sk->sk_state != TCP_LISTEN &&
+	     !vsock_check_source(vsk, &t->transport, &src))) {
 		(void)virtio_transport_reset_no_sock(t, skb, net);
 		release_sock(sk);
 		sock_put(sk);
-- 
2.54.0