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 <[email protected]>
Signed-off-by: Melbin K Mathew <[email protected]>
---
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
