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Wed, 21 Jan 2026 04:10:30 -0800 (PST) Date: Wed, 21 Jan 2026 13:10:28 +0100 From: Luigi Leonardi To: Stefano Garzarella Cc: netdev@vger.kernel.org, Simon Horman , Eric Dumazet , "Michael S. Tsirkin" , Arseniy Krasnov , "David S. Miller" , virtualization@lists.linux.dev, Paolo Abeni , Jakub Kicinski , Stefan Hajnoczi , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Claudio Imbrenda , Jason Wang , Xuan Zhuo , Eugenio =?utf-8?B?UMOpcmV6?= , Asias He , Melbin K Mathew Subject: Re: [PATCH net v6 3/4] vsock/virtio: cap TX credit to local buffer size Message-ID: References: <20260121093628.9941-1-sgarzare@redhat.com> <20260121093628.9941-4-sgarzare@redhat.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20260121093628.9941-4-sgarzare@redhat.com> On Wed, Jan 21, 2026 at 10:36:27AM +0100, Stefano Garzarella wrote: >From: Melbin K Mathew > >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. > >Fixes: 06a8fc78367d ("VSOCK: Introduce virtio_vsock_common.ko") >Suggested-by: Stefano Garzarella >Signed-off-by: Melbin K Mathew >[Stefano: small adjustments after changing the previous patch] >[Stefano: tweak the commit message] >Signed-off-by: Stefano Garzarella >--- > net/vmw_vsock/virtio_transport_common.c | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > >diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c >index 6175124d63d3..d3e26025ef58 100644 >--- a/net/vmw_vsock/virtio_transport_common.c >+++ b/net/vmw_vsock/virtio_transport_common.c >@@ -821,6 +821,15 @@ virtio_transport_seqpacket_dequeue(struct vsock_sock *vsk, > } > EXPORT_SYMBOL_GPL(virtio_transport_seqpacket_dequeue); > >+static u32 virtio_transport_tx_buf_size(struct virtio_vsock_sock *vvs) >+{ >+ /* The peer advertises its receive buffer via peer_buf_alloc, but we >+ * cap it to our local buf_alloc so a remote peer cannot force us to >+ * queue more data than our own buffer configuration allows. >+ */ >+ return min(vvs->peer_buf_alloc, vvs->buf_alloc); >+} >+ > int > virtio_transport_seqpacket_enqueue(struct vsock_sock *vsk, > struct msghdr *msg, >@@ -830,7 +839,7 @@ 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_size(vvs)) { > spin_unlock_bh(&vvs->tx_lock); > return -EMSGSIZE; > } >@@ -884,7 +893,8 @@ static s64 virtio_transport_has_space(struct virtio_vsock_sock *vvs) > * 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); >+ bytes = (s64)virtio_transport_tx_buf_size(vvs) - >+ (vvs->tx_cnt - vvs->peer_fwd_cnt); > if (bytes < 0) > bytes = 0; > >-- >2.52.0 > LGTM! Reviewed-by: Luigi Leonardi