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([2a01:4f9:6a:4e9f::2]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-5990da790efsm2591419e87.102.2025.12.17.10.12.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 17 Dec 2025 10:12:31 -0800 (PST) From: Melbin K Mathew To: stefanha@redhat.com, sgarzare@redhat.com Cc: kvm@vger.kernel.org, netdev@vger.kernel.org, virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, mst@redhat.com, jasowang@redhat.com, xuanzhuo@linux.alibaba.com, eperezma@redhat.com, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, Melbin K Mathew Subject: [PATCH net v4 2/4] vsock/virtio: cap TX credit to local buffer size Date: Wed, 17 Dec 2025 19:12:04 +0100 Message-Id: <20251217181206.3681159-3-mlbnkm1@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20251217181206.3681159-1-mlbnkm1@gmail.com> References: <20251217181206.3681159-1-mlbnkm1@gmail.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit The virtio vsock transport 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_tx_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; the system only recovered after killing the QEMU process. 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 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 --- net/vmw_vsock/virtio_transport_common.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-) diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c index d692b227912d..92575e9d02cd 100644 --- a/net/vmw_vsock/virtio_transport_common.c +++ b/net/vmw_vsock/virtio_transport_common.c @@ -491,6 +491,18 @@ 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. + * + * 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. + */ +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; @@ -508,7 +520,7 @@ u32 virtio_transport_get_credit(struct virtio_vsock_sock *vvs, u32 credit) * its advertised buffer while data is in flight). */ inflight = vvs->tx_cnt - vvs->peer_fwd_cnt; - bytes = (s64)vvs->peer_buf_alloc - inflight; + bytes = (s64)virtio_transport_tx_buf_alloc(vvs) - inflight; if (bytes < 0) bytes = 0; @@ -842,7 +854,7 @@ 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; } @@ -893,7 +905,7 @@ 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; -- 2.34.1