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

Overview

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, MPTCP's 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.

Affected

• Linux >= b19bc2945b40b9fd38e835700907ffe8534ef0de < f6e1f25fa5e733570f6d6fe37a4dfed2a0deba47
• Linux >= b19bc2945b40b9fd38e835700907ffe8534ef0de < fb1f54b7d16f393b8b65d328410f78b4beea8fcc
• Linux >= b19bc2945b40b9fd38e835700907ffe8534ef0de < 3fd6547f5b8ac99687be6d937a0321efda760597
• Linux >= b19bc2945b40b9fd38e835700907ffe8534ef0de < eb9c6aeb512f877cf397deb1e4526f646c70e4a7
• Linux >= b19bc2945b40b9fd38e835700907ffe8534ef0de < 15fa9ead4d5e6b6b9c794e84144146c917f2cb62
• Linux >= b19bc2945b40b9fd38e835700907ffe8534ef0de < b313e9037d98c13938740e5ebda7852929366dff
• Linux >= b19bc2945b40b9fd38e835700907ffe8534ef0de < 9b55b253907e7431210483519c5ad711a37dafa1
• Linux 5.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.