{"id":"CVE-2026-23086","summary":"vsock/virtio: cap TX credit to local buffer size","details":"In the Linux kernel, the following vulnerability has been resolved:\n\nvsock/virtio: cap TX credit to local buffer size\n\nThe virtio transports derives its TX credit directly from peer_buf_alloc,\nwhich is set from the remote endpoint's SO_VM_SOCKETS_BUFFER_SIZE value.\n\nOn the host side this means that the amount of data we are willing to\nqueue for a connection is scaled by a guest-chosen buffer size, rather\nthan the host's own vsock configuration. A malicious guest can advertise\na large buffer and read slowly, causing the host to allocate a\ncorrespondingly large amount of sk_buff memory.\nThe same thing would happen in the guest with a malicious host, since\nvirtio transports share the same code base.\n\nIntroduce a small helper, virtio_transport_tx_buf_size(), that\nreturns min(peer_buf_alloc, buf_alloc), and use it wherever we consume\npeer_buf_alloc.\n\nThis ensures the effective TX window is bounded by both the peer's\nadvertised buffer and our own buf_alloc (already clamped to\nbuffer_max_size via SO_VM_SOCKETS_BUFFER_MAX_SIZE), so a remote peer\ncannot force the other to queue more data than allowed by its own\nvsock settings.\n\nOn an unpatched Ubuntu 22.04 host (~64 GiB RAM), running a PoC with\n32 guest vsock connections advertising 2 GiB each and reading slowly\ndrove Slab/SUnreclaim from ~0.5 GiB to ~57 GiB; the system only\nrecovered after killing the QEMU process. That said, if QEMU memory is\nlimited with cgroups, the maximum memory used will be limited.\n\nWith this patch applied:\n\n  Before:\n    MemFree:        ~61.6 GiB\n    Slab:           ~142 MiB\n    SUnreclaim:     ~117 MiB\n\n  After 32 high-credit connections:\n    MemFree:        ~61.5 GiB\n    Slab:           ~178 MiB\n    SUnreclaim:     ~152 MiB\n\nOnly ~35 MiB increase in Slab/SUnreclaim, no host OOM, and the guest\nremains responsive.\n\nCompatibility with non-virtio transports:\n\n  - VMCI uses the AF_VSOCK buffer knobs to size its queue pairs per\n    socket based on the local vsk-\u003ebuffer_* values; the remote side\n    cannot enlarge those queues beyond what the local endpoint\n    configured.\n\n  - Hyper-V's vsock transport uses fixed-size VMBus ring buffers and\n    an MTU bound; there is no peer-controlled credit field comparable\n    to peer_buf_alloc, and the remote endpoint cannot drive in-flight\n    kernel memory above those ring sizes.\n\n  - The loopback path reuses virtio_transport_common.c, so it\n    naturally follows the same semantics as the virtio transport.\n\nThis change is limited to virtio_transport_common.c and thus affects\nvirtio-vsock, vhost-vsock, and loopback, bringing them in line with the\n\"remote window intersected with local policy\" behaviour that VMCI and\nHyper-V already effectively have.\n\n[Stefano: small adjustments after changing the previous patch]\n[Stefano: tweak the commit message]","modified":"2026-04-02T17:30:36.740799Z","published":"2026-02-04T16:08:10.047Z","related":["SUSE-SU-2026:0962-1","SUSE-SU-2026:1078-1","SUSE-SU-2026:1081-1","SUSE-SU-2026:20667-1","SUSE-SU-2026:20720-1","SUSE-SU-2026:20838-1","SUSE-SU-2026:20845-1","SUSE-SU-2026:20876-1","SUSE-SU-2026:20931-1","openSUSE-SU-2026:20416-1"],"database_specific":{"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/23xxx/CVE-2026-23086.json","cna_assigner":"Linux"},"references":[{"type":"WEB","url":"https://git.kernel.org/stable/c/84ef86aa7120449828d1e0ce438c499014839711"},{"type":"WEB","url":"https://git.kernel.org/stable/c/8ee784fdf006cbe8739cfa093f54d326cbf54037"},{"type":"WEB","url":"https://git.kernel.org/stable/c/c0e42fb0e054c2b2ec4ee80f48ccd256ae0227ce"},{"type":"WEB","url":"https://git.kernel.org/stable/c/d9d5f222558b42f6277eafaaa6080966faf37676"},{"type":"WEB","url":"https://git.kernel.org/stable/c/fef7110ae5617555c792a2bb4d27878d84583adf"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/23xxx/CVE-2026-23086.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-23086"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"06a8fc78367d070720af960dcecec917d3ae5f3b"},{"fixed":"fef7110ae5617555c792a2bb4d27878d84583adf"},{"fixed":"d9d5f222558b42f6277eafaaa6080966faf37676"},{"fixed":"c0e42fb0e054c2b2ec4ee80f48ccd256ae0227ce"},{"fixed":"84ef86aa7120449828d1e0ce438c499014839711"},{"fixed":"8ee784fdf006cbe8739cfa093f54d326cbf54037"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-23086.json"}}],"schema_version":"1.7.5"}