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: 06a8fc78367d ("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>

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index 26b979ad71f09e9710900f94fead52c98e675673..6175124d63d34f807b376743c0d4183d90c65ec1 100644 (file)
--- a/net/vmw_vsock/virtio_transport_common.c
+++ b/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;
 }