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

[Jiayuan Chen]

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@xxxxxxxxx>
> > ---
> >   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