vsock/virtio: fix potential underflow in virtio_transport_get_credit()

The credit calculation in virtio_transport_get_credit() uses unsigned
arithmetic:

  ret = vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);

If the peer shrinks its advertised buffer (peer_buf_alloc) while bytes
are in flight, the subtraction can underflow and produce a large
positive value, potentially allowing more data to be queued than the
peer can handle.

Reuse virtio_transport_has_space() which already handles this case and
add a comment to make it clear why we are doing that.

Fixes: 06a8fc7 ("VSOCK: Introduce virtio_vsock_common.ko")
Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Melbin K Mathew <mlbnkm1@gmail.com>
[Stefano: use virtio_transport_has_space() instead of duplicating the code]
[Stefano: tweak the commit message]
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Luigi Leonardi <leonardi@redhat.com>
Link: https://patch.msgid.link/20260121093628.9941-2-sgarzare@redhat.com
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: melbinkm

net/vmw_vsock/virtio_transport_common.c

@@ -28,6 +28,7 @@
 
 static void virtio_transport_cancel_close_work(struct vsock_sock *vsk,
 					       bool cancel_timeout);
+static s64 virtio_transport_has_space(struct virtio_vsock_sock *vvs);
 
 static const struct virtio_transport *
 virtio_transport_get_ops(struct vsock_sock *vsk)
@@ -499,9 +500,7 @@ u32 virtio_transport_get_credit(struct virtio_vsock_sock *vvs, u32 credit)
 		return 0;
 
 	spin_lock_bh(&vvs->tx_lock);
-	ret = vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);
-	if (ret > credit)
-		ret = credit;
+	ret = min_t(u32, credit, virtio_transport_has_space(vvs));
 	vvs->tx_cnt += ret;
 	vvs->bytes_unsent += ret;
 	spin_unlock_bh(&vvs->tx_lock);
@@ -877,11 +876,14 @@ u32 virtio_transport_seqpacket_has_data(struct vsock_sock *vsk)
 }
 EXPORT_SYMBOL_GPL(virtio_transport_seqpacket_has_data);
 
-static s64 virtio_transport_has_space(struct vsock_sock *vsk)
+static s64 virtio_transport_has_space(struct virtio_vsock_sock *vvs)
 {
-	struct virtio_vsock_sock *vvs = vsk->trans;
 	s64 bytes;
 
+	/* Use s64 arithmetic so if the peer shrinks peer_buf_alloc while
+	 * we have bytes in flight (tx_cnt - peer_fwd_cnt), the subtraction
+	 * does not underflow.
+	 */
 	bytes = (s64)vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);
 	if (bytes < 0)
 		bytes = 0;
@@ -895,7 +897,7 @@ s64 virtio_transport_stream_has_space(struct vsock_sock *vsk)
 	s64 bytes;
 
 	spin_lock_bh(&vvs->tx_lock);
-	bytes = virtio_transport_has_space(vsk);
+	bytes = virtio_transport_has_space(vvs);
 	spin_unlock_bh(&vvs->tx_lock);
 
 	return bytes;
@@ -1492,7 +1494,7 @@ static bool virtio_transport_space_update(struct sock *sk,
 	spin_lock_bh(&vvs->tx_lock);
 	vvs->peer_buf_alloc = le32_to_cpu(hdr->buf_alloc);
 	vvs->peer_fwd_cnt = le32_to_cpu(hdr->fwd_cnt);
-	space_available = virtio_transport_has_space(vsk);
+	space_available = virtio_transport_has_space(vvs);
 	spin_unlock_bh(&vvs->tx_lock);
 	return space_available;
 }