CVE-2026-31669

In the Linux kernel, the following vulnerability has been resolved:

mptcp: fix slab-use-after-free in __inet_lookup_established

The ehash table lookups are lockless and rely on SLAB_TYPESAFE_BY_RCU to guarantee socket memory stability during RCU read-side critical sections. Both tcp_prot and tcpv6_prot have their slab caches created with this flag via proto_register().

However, MPTCPs mptcp_subflow_init() copies tcpv6_prot into tcpv6_prot_override during inet_init() (fs_initcall, level 5), before inet6_init() (module_init/device_initcall, level 6) has called proto_register(&tcpv6_prot). At that point, tcpv6_prot.slab is still NULL, so tcpv6_prot_override.slab remains NULL permanently.

This causes MPTCP v6 subflow child sockets to be allocated via kmalloc (falling into kmalloc-4k) instead of the TCPv6 slab cache. The kmalloc-4k cache lacks SLAB_TYPESAFE_BY_RCU, so when these sockets are freed without SOCK_RCU_FREE (which is cleared for child sockets by design), the memory can be immediately reused. Concurrent ehash lookups under rcu_read_lock can then access freed memory, triggering a slab-use-after-free in __inet_lookup_established.

Fix this by splitting the IPv6-specific initialization out of mptcp_subflow_init() into a new mptcp_subflow_v6_init(), called from mptcp_proto_v6_init() before protocol registration. This ensures tcpv6_prot_override.slab correctly inherits the SLAB_TYPESAFE_BY_RCU slab cache.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/15fa9ead4d5e6b6b9c794e84144146c917f2cb62
• https://git.kernel.org/stable/c/3fd6547f5b8ac99687be6d937a0321efda760597
• https://git.kernel.org/stable/c/9b55b253907e7431210483519c5ad711a37dafa1
• https://git.kernel.org/stable/c/b313e9037d98c13938740e5ebda7852929366dff
• https://git.kernel.org/stable/c/eb9c6aeb512f877cf397deb1e4526f646c70e4a7
• https://git.kernel.org/stable/c/f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47
• https://git.kernel.org/stable/c/fb1f54b7d16f393b8b65d328410f78b4beea8fcc