[PATCH 6.1 151/280] vsock/virtio: fix potential underflow in virtio_transport_get_credit()

Greg Kroah-Hartman
2026-02-04T15:11:35Z

From: Melbin K Mathew <mlbnkm1@gmail.com>

[ Upstream commit 3ef3d52a1a9860d094395c7a3e593f3aa26ff012 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 a65da57fe26fd..76329191102a0 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)
@@ -284,9 +285,7 @@ u32 virtio_transport_get_credit(struct virtio_vsock_sock *vvs, u32 credit)
u32 ret;

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;
spin_unlock_bh(&vvs->tx_lock);

@@ -588,11 +587,14 @@ 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;
@@ -606,7 +608,7 @@ 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;
@@ -1207,7 +1209,7 @@ 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;
}
--
2.51.0