Saint OVSwrap, Who Wrapped the Length Around

Patron of Open vSwitch, sixteen-bit length fields, and limits removed with too much trust.

Life

In July 2026 Asim Manizada revealed OVSwrap (CVE-2026-64531), a flaw in Open vSwitch in the Linux kernel. Nested actions use a sixteen-bit length field: without a check, an oversized action made the counter wrap around, and the parser resumed reading inside attacker-chosen data, leading to kernel memory reads and writes and root privileges. The faulty assignment had existed for years, but became exploitable only when an old limit was removed in 2025.

Prayer

O Saint OVSwrap, who wraps lengths past their maximum, teach our parsers to reject what does not fit their fields. Amen.

How to celebrate

Celebrated by updating the kernel to the fixed versions and, where it is not needed, unloading or blocking the openvswitch module.

A length that wraps, a parser that slips.

Technical record

Disclosed on: July 28 2026
CVE: CVE-2026-64531
CVSS: 7.8
Product: Linux · Linux

In the Linux kernel, the following vulnerability has been resolved: net: openvswitch: reject oversized nested action attrs Open vSwitch stores generated flow actions as nlattrs, whose nla_len field is u16. Commit a1e64addf3ff ("net: openvswitch: remove misbehaving actions length check") allowed the total sw_flow_actions stream to grow beyond 64 KiB, which is valid, but also removed the last guard preventing a generated nested action attribute from exceeding U16_MAX. An oversized generated container can thus be closed with a truncated nla_len. A later dump or teardown then walks a structurally different stream than the one that was validated. In particular, an oversized nested CLONE/CT action may cause subsequent bytes in the generated stream to be interpreted as independent actions. Keep the larger total-action-stream behavior, but make nested action close reject generated containers that do not fit in nla_len, and return the error through all callers. For recursive SAMPLE, CLONE, DEC_TTL, and CHECK_PKT_LEN builders, trim resource-owning action-list tails in reverse construction order before discarding failed wrappers, so resources copied into the rejected tails are released before the wrappers are removed. Most failed outer wrappers are discarded by truncating actions_len after child resources have been released. CHECK_PKT_LEN also trims its parent after branch resources are gone. SET/TUNNEL close failures unwind their known tun_dst ownership directly, and SET_TO_MASKED has no external ownership and truncates on close failure.

Sources: heyitsas.im

Canonized during the vacant see

Saint OVSwrap, Who Wrapped the Length Around

Life

Prayer

How to celebrate

Technical record

Related saints

Sharing the feast day

Same product: Linux kernel

Same year: 2026