Re: [PATCH net] soreuseport: Fix use-after-free when a socket is added to socks[] twice

[Kuniyuki Iwashima <kuniyu@google.com>]

From: Norbert Szetei <norbert@doyensec.com>
Date: Mon, 28 Sep 2026 19:22:38 +0200
> reuseport_stop_listen_sock() moves a shutdown()ed listener from the
> listening section of reuse->socks[] to the closed section: it removes the
> socket with __reuseport_detach_sock() and adds it back with
> __reuseport_add_closed_sock(). The return value of the removal is
> discarded and the add runs unconditionally.
> 
> The socket need not be in the listening section. inet_unhash() calls
> reuseport_stop_listen_sock() for a listener whenever sk->sk_reuseport_cb
> is set, but inet_hash() enters the reuseport path only when
> sk->sk_reuseport is set, and SO_REUSEPORT can be cleared in any state.

This has long been a known problem, and I think it's time
to fix it instead of working around it:

diff --git a/net/core/sock.c b/net/core/sock.c
index 2948dffcc3e1..a33cdf99368d 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1324,6 +1324,8 @@ int sk_setsockopt(struct sock *sk, int level, int optname,
 	case SO_REUSEPORT:
 		if (valbool && !sk_is_inet(sk))
 			ret = -EOPNOTSUPP;
+		else if (!valbool && rcu_access_pointer(sk->sk_reuseport_cb))
+			ret = -EBUSY;
 		else
 			sk->sk_reuseport = valbool;
 		break;


> Clearing it between shutdown() and listen() makes that listen() skip
> reuseport_add_sock(), and with it reuseport_resurrect(), so the socket is
> hashed as a listener while it is still in the closed section. On the next
> shutdown() __reuseport_detach_sock() does not find it in the listening
> section, returns false, and __reuseport_add_closed_sock() adds a second
> copy of it to socks[].
> 
> sk_destruct() calls reuseport_detach_sock(), which removes one of the two
> entries. reuseport_grow() then dereferences the other one, because its
> loop runs over every slot up to reuse->max_socks:
> 
>   BUG: KASAN: slab-use-after-free in reuseport_grow (net/core/sock_reuseport.c:291)
>   Write of size 8 at addr ffff888132359988 by task poc/621
> 
>    reuseport_grow (net/core/sock_reuseport.c:291)
>    reuseport_add_sock (net/core/sock_reuseport.c:350)
>    inet_hash (net/ipv4/inet_hashtables.c:810)
>    inet_csk_listen_start (net/ipv4/inet_connection_sock.c:1359)
>    __inet_listen_sk (net/ipv4/af_inet.c:225)
>    inet_listen (net/ipv4/af_inet.c:247)
>    __sys_listen (net/socket.c:2014)
> 
>   Allocated by task 621:
>    sk_prot_alloc (net/core/sock.c:2246)
>    sk_alloc (net/core/sock.c:2308)
>    inet_create (net/ipv4/af_inet.c:333)
> 
>   Freed by task 0:
>    slab_free_after_rcu_debug (mm/slub.c:6570)
>    rcu_core (kernel/rcu/tree.c:2919)
> 
>   The buggy address is located 904 bytes inside of
>    freed 2624-byte region [ffff888132359600, ffff88813235a040)
> 
> Only move the socket to the closed section when __reuseport_detach_sock()
> reports that it was removed from the listening section.
> 
> Fixes: 333bb73f620e ("tcp: Keep TCP_CLOSE sockets in the reuseport group.")
> Assisted-by: LLM
> Signed-off-by: Norbert Szetei <norbert@doyensec.com>
> ---
>  net/core/sock_reuseport.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/sock_reuseport.c b/net/core/sock_reuseport.c
> index 29948cb44b7d..031b641be317 100644
> --- a/net/core/sock_reuseport.c
> +++ b/net/core/sock_reuseport.c
> @@ -479,8 +479,8 @@ void reuseport_stop_listen_sock(struct sock *sk)
>  			 */
>  			bpf_sk_reuseport_detach(sk);
>  
> -			__reuseport_detach_sock(sk, reuse);
> -			__reuseport_add_closed_sock(sk, reuse);
> +			if (__reuseport_detach_sock(sk, reuse))
> +				__reuseport_add_closed_sock(sk, reuse);
>  
>  			spin_unlock_bh(&reuseport_lock);
>  			return;
> -- 
> 2.55.0
>