Re: [PATCH net v2] mptcp: fix slab-use-after-free in __inet_lookup_established

[Matthieu Baerts]

Hi Jiayuan,

On 06/04/2026 05:15, Jiayuan Chen wrote:
> 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.

Thank you for this v2, it looks good to me:

Reviewed-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>

@Netdev maintainers: this patch can be applied in 'net' directly.

Cheers,
Matt
-- 
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