Re: [PATCH net v1] net: mptcp: fix slab-use-after-free in __inet_lookup_established

[Matthieu Baerts <matttbe@kernel.org>]

Hi Jiayuan,

On 03/04/2026 15:07, 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.

Good catch! Thank you for this patch.

> Fix this by splitting the IPv6-specific initialization out of
> mptcp_subflow_init() into a new mptcp_subflow_v6_init(), which
> is called from mptcpv6_init() after proto_register(&tcpv6_prot)
> has completed. This ensures tcpv6_prot_override.slab correctly
> inherits the SLAB_TYPESAFE_BY_RCU slab cache.

The split makes sense anyway: better to regroup all v6-related init steps.

> Fixes: b19bc2945b40 ("mptcp: implement delegated actions")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  net/mptcp/ctrl.c     |  6 +++++-
>  net/mptcp/protocol.h |  1 +
>  net/mptcp/subflow.c  | 15 +++++++++------
>  3 files changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/net/mptcp/ctrl.c b/net/mptcp/ctrl.c
> index d96130e49942..5887ddcdb875 100644
> --- a/net/mptcp/ctrl.c
> +++ b/net/mptcp/ctrl.c
> @@ -583,7 +583,11 @@ int __init mptcpv6_init(void)
>  	int err;
>  
>  	err = mptcp_proto_v6_init();
> +	if (err)
> +		return err;
>  
> -	return err;
> +	mptcp_subflow_v6_init();

I think it would be better to move this to mptcp_proto_v6_init, similar
to what is done with mptcp_subflow_init, from mptcp_proto_init.

From there, you can even call it before registering the protocol, at the
beginning, so before inet6_register_protosw, which seems more logical
and similar to what is done in v4. WDYT?

If you send a v2, can you please remove the 'net:' prefix please?
'mptcp:' is enough:

  [PATCH net v2] mptcp: fix (...)

Also, can you add a "Cc: stable" tag please?

Cheers,
Matt
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