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

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

On 4/3/26 9:07 PM, 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(), 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.
>
> 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();
> +
> +	return 0;
>   }
>   #endif
> diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
> index 0bd1ee860316..ec15e503da8b 100644
> --- a/net/mptcp/protocol.h
> +++ b/net/mptcp/protocol.h
> @@ -875,6 +875,7 @@ static inline void mptcp_subflow_tcp_fallback(struct sock *sk,
>   void __init mptcp_proto_init(void);
>   #if IS_ENABLED(CONFIG_MPTCP_IPV6)
>   int __init mptcp_proto_v6_init(void);
> +void __init mptcp_subflow_v6_init(void);
>   #endif
>   
>   struct sock *mptcp_sk_clone_init(const struct sock *sk,
> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index 6716970693e9..4ff5863aa9fd 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -2165,7 +2165,15 @@ void __init mptcp_subflow_init(void)
>   	tcp_prot_override.psock_update_sk_prot = NULL;
>   #endif
>   
> +	mptcp_diag_subflow_init(&subflow_ulp_ops);
> +
> +	if (tcp_register_ulp(&subflow_ulp_ops) != 0)
> +		panic("MPTCP: failed to register subflows to ULP\n");
> +}
> +
>   #if IS_ENABLED(CONFIG_MPTCP_IPV6)
> +void __init mptcp_subflow_v6_init(void)
> +{
>   	/* In struct mptcp_subflow_request_sock, we assume the TCP request sock
>   	 * structures for v4 and v6 have the same size. It should not changed in
>   	 * the future but better to make sure to be warned if it is no longer
> @@ -2204,10 +2212,5 @@ void __init mptcp_subflow_init(void)
>   	/* Disable sockmap processing for subflows */
>   	tcpv6_prot_override.psock_update_sk_prot = NULL;
>   #endif
> -#endif
> -
> -	mptcp_diag_subflow_init(&subflow_ulp_ops);
> -
> -	if (tcp_register_ulp(&subflow_ulp_ops) != 0)
> -		panic("MPTCP: failed to register subflows to ULP\n");
>   }
> +#endif



I think the AI review is not accurate here.
https://sashiko.dev/#/patchset/20260403130734.93981-1-jiayuan.chen%40linux.dev

Userspace programs only start running after all initcalls have 
completed, so there is no race condition.