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Message-ID: <20251211080251-mutt-send-email-mst@kernel.org>
Date: Thu, 11 Dec 2025 08:05:11 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Melbin K Mathew <mlbnkm1@...il.com>
Cc: stefanha@...hat.com, sgarzare@...hat.com, kvm@...r.kernel.org,
netdev@...r.kernel.org, virtualization@...ts.linux.dev,
linux-kernel@...r.kernel.org, jasowang@...hat.com,
xuanzhuo@...ux.alibaba.com, eperezma@...hat.com,
davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
pabeni@...hat.com, horms@...nel.org
Subject: Re: [PATCH net v3] vsock/virtio: cap TX credit to local buffer size
On Thu, Dec 11, 2025 at 01:51:04PM +0100, Melbin K Mathew wrote:
> The virtio vsock transport currently derives its TX credit directly from
> peer_buf_alloc, which is populated from the remote endpoint's
> SO_VM_SOCKETS_BUFFER_SIZE value.
>
> On the host side, this means the amount of data we are willing to queue
> for a given connection is scaled purely by a peer-chosen value, rather
> than by the host's own vsock buffer configuration. A guest that
> advertises a very large buffer and reads slowly can cause the host to
> allocate a correspondingly large amount of sk_buff memory for that
> connection.
>
> In practice, a malicious guest can:
>
> - set a large AF_VSOCK buffer size (e.g. 2 GiB) with
> SO_VM_SOCKETS_BUFFER_MAX_SIZE / SO_VM_SOCKETS_BUFFER_SIZE, and
>
> - open multiple connections to a host vsock service that sends data
> while the guest drains slowly.
>
> On an unconstrained host this can drive Slab/SUnreclaim into the tens of
> GiB range, causing allocation failures and OOM kills in unrelated host
> processes while the offending VM remains running.
>
> On non-virtio transports and compatibility:
>
> - VMCI uses the AF_VSOCK buffer knobs to size its queue pairs per
> socket based on the local vsk->buffer_* values; the remote side
> can’t 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 can’t 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.
>
> Make virtio-vsock consistent with that model by intersecting the peer’s
> advertised receive window with the local vsock buffer size when
> computing TX credit. We introduce a small helper and use it in
> virtio_transport_get_credit(), virtio_transport_has_space() and
> virtio_transport_seqpacket_enqueue(), so that:
>
> effective_tx_window = min(peer_buf_alloc, buf_alloc)
>
> This prevents a remote endpoint from forcing us to queue more data than
> our own configuration allows, while preserving the existing credit
> semantics and keeping virtio-vsock compatible with the other transports.
>
> 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 and the system only
> recovered after killing the QEMU process.
>
> With this patch applied, rerunning the same PoC yields:
>
> Before:
> MemFree: ~61.6 GiB
> MemAvailable: ~62.3 GiB
> Slab: ~142 MiB
> SUnreclaim: ~117 MiB
>
> After 32 high-credit connections:
> MemFree: ~61.5 GiB
> MemAvailable: ~62.3 GiB
> Slab: ~178 MiB
> SUnreclaim: ~152 MiB
>
> i.e. only ~35 MiB increase in Slab/SUnreclaim, no host OOM, and the
> guest remains responsive.
>
> Fixes: 06a8fc78367d ("VSOCK: Introduce virtio_vsock_common.ko")
> Suggested-by: Stefano Garzarella <sgarzare@...hat.com>
> Signed-off-by: Melbin K Mathew <mlbnkm1@...il.com>
> ---
> net/vmw_vsock/virtio_transport_common.c | 27 ++++++++++++++++++++++---
> 1 file changed, 24 insertions(+), 3 deletions(-)
>
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index dcc8a1d58..02eeb96dd 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -491,6 +491,25 @@ void virtio_transport_consume_skb_sent(struct sk_buff *skb, bool consume)
> }
> EXPORT_SYMBOL_GPL(virtio_transport_consume_skb_sent);
>
> +/* Return the effective peer buffer size for TX credit computation.
> + *
> + * The peer advertises its receive buffer via peer_buf_alloc, but we
> + * cap that to our local buf_alloc (derived from
> + * SO_VM_SOCKETS_BUFFER_SIZE and already clamped to buffer_max_size)
> + * so that a remote endpoint cannot force us to queue more data than
> + * our own configuration allows.
> + */
> +static u32 virtio_transport_tx_buf_alloc(struct virtio_vsock_sock *vvs)
> +{
> + return min(vvs->peer_buf_alloc, vvs->buf_alloc);
> +}
> +
> u32 virtio_transport_get_credit(struct virtio_vsock_sock *vvs, u32 credit)
> {
> u32 ret;
> @@ -499,7 +518,8 @@ 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);
> + ret = virtio_transport_tx_buf_alloc(vvs) -
> + (vvs->tx_cnt - vvs->peer_fwd_cnt);
> if (ret > credit)
> ret = credit;
> vvs->tx_cnt += ret;
> @@ -831,7 +851,7 @@ virtio_transport_seqpacket_enqueue(struct vsock_sock *vsk,
>
> spin_lock_bh(&vvs->tx_lock);
>
> - if (len > vvs->peer_buf_alloc) {
> + if (len > virtio_transport_tx_buf_alloc(vvs)) {
> spin_unlock_bh(&vvs->tx_lock);
> return -EMSGSIZE;
> }
> @@ -882,7 +902,8 @@ static s64 virtio_transport_has_space(struct vsock_sock *vsk)
> struct virtio_vsock_sock *vvs = vsk->trans;
> s64 bytes;
>
> - bytes = (s64)vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);
> + bytes = (s64)virtio_transport_tx_buf_alloc(vvs) -
> + (vvs->tx_cnt - vvs->peer_fwd_cnt);
> if (bytes < 0)
> bytes = 0;
>
Acked-by: Michael S. Tsirkin <mst@...hat.com>
Looking at this, why is one place casting to s64 the other is not?
> --
> 2.34.1
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