vsock/virtio: cap TX credit to local buffer size
CVE: CVE-2026-23086
Component: kernel
Severity: Moderate
CVSS v3: 6.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Release date: 2026-02-04T00:00:00+00:00

In the Linux kernel, the following vulnerability has been resolved:

vsock/virtio: cap TX credit to local buffer size

The virtio transports derives its TX credit directly from peer_buf_alloc,
which is set from the remote endpoint's SO_VM_SOCKETS_BUFFER_SIZE value.

On the host side this means that the amount of data we are willing to
queue for a connection is scaled by a guest-chosen buffer size, rather
than the host's own vsock configuration. A malicious guest can advertise
a large buffer and read slowly, causing the host to allocate a
correspondingly large amount of sk_buff memory.
The same thing would happen in the guest with a malicious host, since
virtio transports share the same code base.

Introduce a small helper, virtio_transport_tx_buf_size(), that
returns min(peer_buf_alloc, buf_alloc), and use it wherever we consume
peer_buf_alloc.

This ensures the effective TX window is bounded by both the peer's
advertised buffer and our own buf_alloc (already clamped to
buffer_max_size via SO_VM_SOCKETS_BUFFER_MAX_SIZE), so a remote peer
cannot force the other to queue more data than allowed by its own
vsock settings.

On an unpatched Ubuntu 22.04 host (~64 GiB RAM), running a PoC with
32 guest vsock connections advertising 2 GiB each and reading slowly
drove Slab/SUnreclaim from ~0.5 GiB to ~57 GiB; the system only
recovered after killing the QEMU process. That said, if QEMU memory is
limited with cgroups, the maximum memory used will be limited.

With this patch applied:

  Before:
    MemFree:        ~61.6 GiB
    Slab:           ~142 MiB
    SUnreclaim:     ~117 MiB

  After 32 high-credit connections:
    MemFree:        ~61.5 GiB
    Slab:           ~178 MiB
    SUnreclaim:     ~152 MiB

Only ~35 MiB increase in Slab/SUnreclaim, no host OOM, and the guest
remains responsive.

Compatibility with non-virtio transports:

  - VMCI uses the AF_VSOCK buffer knobs to size its queue pairs per
    socket based on the local vsk->buffer_* values; the remote side
    cannot enlarge those queues beyond what the local endpoint
    configured.

  - Hyper-V's vsock transport uses fixed-size VMBus ring buffers and
    an MTU bound; there is no peer-controlled credit field comparable
    to peer_buf_alloc, and the remote endpoint cannot drive in-flight
    kernel memory above those ring sizes.

  - The loopback path reuses virtio_transport_common.c, so it
    naturally follows the same semantics as the virtio transport.

This change is limited to virtio_transport_common.c and thus affects
virtio-vsock, vhost-vsock, and loopback, bringing them in line with the
"remote window intersected with local policy" behaviour that VMCI and
Hyper-V already effectively have.

[Stefano: small adjustments after changing the previous patch]
[Stefano: tweak the commit message]

Statement: The bug is in the Virtual Socket Protocol (that is a socket protocol similar to TCP/IP allowing communication between Virtual Machines and hypervisor or host). A memory exhaustion denial of service can be triggered in virtio vsock because the transmit credit was derived directly from the peer advertised peer_buf_alloc value. A malicious peer can advertise a very large receive buffer and then read slowly. This causes the other endpoint to queue far more sk_buff data than its local vsock policy intended and can drive slab usage to extreme levels leading to OOM and service disruption. For the CVSS the PR is N because an unprivileged process that can open an AF_VSOCK connection can influence SO_VM_SOCKETS_BUFFER_SIZE on its endpoint. The issue is reachable across the vsock channel which often crosses VM boundaries between a guest and a host even though it is not a traditional IP network. Impact is denial of service through memory pressure and potential process termination such as killing QEMU on the host.

Remediation:
To mitigate this issue, prevent module vsock from being loaded. Please see https://access.redhat.com/solutions/41278 for how to blacklist a kernel module to prevent it from loading automatically.