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

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 4/4/26 7:15 PM, Matthieu Baerts wrote:
> 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


Hi Matt,

Thanks for the feedback! I'll try all your suggestions in v2.

Thanks!

Jiayuan