netdev - Re: [PATCH net] vsock/virtio: cap TX credit to local buffer size

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Message-ID: <7m3k4fhwnyvocrpwyzkm2xpgssik7lph3f5kqagqifllijvhbq@5opsnu2vcgic>
Date: Wed, 10 Dec 2025 15:22:38 +0100
From: Stefano Garzarella <sgarzare@...hat.com>
To: Melbin K Mathew <mlbnkm1@...il.com>
Cc: netdev@...r.kernel.org, virtualization@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, stefanha@...hat.com, mst@...hat.com, jasowang@...hat.com
Subject: Re: [PATCH net] vsock/virtio: cap TX credit to local buffer size

On Wed, Dec 10, 2025 at 02:32:59PM +0100, Melbin K Mathew wrote:
>The virtio vsock transport currently 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.
>
>Introduce a small helper, virtio_transport_peer_buf_alloc(), that
>returns min(peer_buf_alloc, buf_alloc), and use it wherever we consume
>peer_buf_alloc:
>
>  - virtio_transport_get_credit()
>  - virtio_transport_has_space()
>  - virtio_transport_seqpacket_enqueue()
>
>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 guest
>cannot force the host to queue more data than allowed by the host's
>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 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: d021c344051a ("VSOCK: Introduce VM Sockets")
>Reported-by: Melbin K Mathew <mlbnkm1@...il.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(-)

Thanks, in addition to Michael comments:

Please run `./scripts/checkpatch.pl`, there are several warnings/errors 
mainly related to spaces/tabs.

Also please use `./scripts/get_maintainer.pl` since several maintainers 
are missing in CC.

>
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index dcc8a1d58..f5afedf01 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_peer_buf_alloc(struct virtio_vsock_sock *vvs)

This maybe can be called virtio_transport_tx_buf_alloc or something like 
that, to make it clear that should be used in the TX path. (not a strong 
opinion)

Thanks,
Stefano

>+{
>+	u32 peer  = vvs->peer_buf_alloc;
>+	u32 local = vvs->buf_alloc;
>+
>+	if (peer > local)
>+		return local;
>+	return peer;
>+}
>+
> 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_peer_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_peer_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_peer_buf_alloc(vvs) -
>+               (vvs->tx_cnt - vvs->peer_fwd_cnt);
> 	if (bytes < 0)
> 		bytes = 0;
>
>-- 
>2.34.1
>