From: Melbin K Mathew <redacted>

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 <redacted>
[Stefano: use virtio_transport_has_space() instead of duplicating the code]
[Stefano: tweak the commit message]
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
net/vmw_vsock/virtio_transport_common.c | 16 +++++++++-------
1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
index 26b979ad71f0..6175124d63d3 100644
--- 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;
}
-- 2.52.0