[rds-devel] [PATCH net v3] net/rds: fix recursive lock in rds_tcp_conn_slots_available

[Allison Henderson]

On Thu, 2026-02-19 at 08:57 +0100, Fernando Fernandez Mancera wrote:
> syzbot reported a recursive lock warning in rds_tcp_get_peer_sport() as
> it calls inet6_getname() which acquires the socket lock that was already
> held by __release_sock().
> 
>  kworker/u8:6/2985 is trying to acquire lock:
>  ffff88807a07aa20 (k-sk_lock-AF_INET6){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1709 [inline]
>  ffff88807a07aa20 (k-sk_lock-AF_INET6){+.+.}-{0:0}, at: inet6_getname+0x15d/0x650 net/ipv6/af_inet6.c:533
> 
>  but task is already holding lock:
>  ffff88807a07aa20 (k-sk_lock-AF_INET6){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1709 [inline]
>  ffff88807a07aa20 (k-sk_lock-AF_INET6){+.+.}-{0:0}, at: tcp_sock_set_cork+0x2c/0x2e0 net/ipv4/tcp.c:3694
>    lock_sock_nested+0x48/0x100 net/core/sock.c:3780
>    lock_sock include/net/sock.h:1709 [inline]
>    inet6_getname+0x15d/0x650 net/ipv6/af_inet6.c:533
>    rds_tcp_get_peer_sport net/rds/tcp_listen.c:70 [inline]
>    rds_tcp_conn_slots_available+0x288/0x470 net/rds/tcp_listen.c:149
>    rds_recv_hs_exthdrs+0x60f/0x7c0 net/rds/recv.c:265
>    rds_recv_incoming+0x9f6/0x12d0 net/rds/recv.c:389
>    rds_tcp_data_recv+0x7f1/0xa40 net/rds/tcp_recv.c:243
>    __tcp_read_sock+0x196/0x970 net/ipv4/tcp.c:1702
>    rds_tcp_read_sock net/rds/tcp_recv.c:277 [inline]
>    rds_tcp_data_ready+0x369/0x950 net/rds/tcp_recv.c:331
>    tcp_rcv_established+0x19e9/0x2670 net/ipv4/tcp_input.c:6675
>    tcp_v6_do_rcv+0x8eb/0x1ba0 net/ipv6/tcp_ipv6.c:1609
>    sk_backlog_rcv include/net/sock.h:1185 [inline]
>    __release_sock+0x1b8/0x3a0 net/core/sock.c:3213
> 
> Reading from the socket struct directly is safe from possible paths. For
> rds_tcp_accept_one(), the socket has just been accepted and is not yet
> exposed to concurrent access. For rds_tcp_conn_slots_available(), direct
> access avoids the recursive deadlock seen during backlog processing
> where the socket lock is already held from the __release_sock().
> 
> However, rds_tcp_conn_slots_available() is also called from the normal
> softirq path via tcp_data_ready() where the lock is not held. This is
> also safe because inet_dport is a stable 16 bits field. A READ_ONCE()
> annotation as the value might be accessed lockless in a concurrent
> access context.
> 
> Note that it is also safe to call rds_tcp_conn_slots_available() from
> rds_conn_shutdown() because the fan-out is disabled.
> 
> Fixes: 9d27a0fb122f ("net/rds: Trigger rds_send_ping() more than once")
> Reported-by: syzbot+5efae91f60932839f0a5 at syzkaller.appspotmail.com
> Closes: https://urldefense.com/v3/__https://syzkaller.appspot.com/bug?extid=5efae91f60932839f0a5__;!!ACWV5N9M2RV99hQ!J8DoczeYGSPP6QsBvwYQaxjJ0hVCVc6popPLkw4DNanrXOKGk9ilfqD3dHQ0xwgfwZe3Gatnp-8PmidwnWlezs0$ 
> Signed-off-by: Fernando Fernandez Mancera <fmancera at suse.de>

This looks good to me.  Thank you!
Reviewed-by: Allison Henderson <achender at kernel.org>

> ---
> v2: clarified commit message and add a comment around
> rds_conn_shutdown() path 
> v3: used READ_ONCE() for lockless read and adjusted commit message
> ---
>  net/rds/connection.c |  3 +++
>  net/rds/tcp_listen.c | 28 +++++-----------------------
>  2 files changed, 8 insertions(+), 23 deletions(-)
> 
> diff --git a/net/rds/connection.c b/net/rds/connection.c
> index 185f73b01694..a542f94c0214 100644
> --- a/net/rds/connection.c
> +++ b/net/rds/connection.c
> @@ -455,6 +455,9 @@ void rds_conn_shutdown(struct rds_conn_path *cp)
>  		rcu_read_unlock();
>  	}
>  
> +	/* we do not hold the socket lock here but it is safe because
> +	 * fan-out is disabled when calling conn_slots_available()
> +	 */
>  	if (conn->c_trans->conn_slots_available)
>  		conn->c_trans->conn_slots_available(conn, false);
>  }
> diff --git a/net/rds/tcp_listen.c b/net/rds/tcp_listen.c
> index 6fb5c928b8fd..dce7ac9d3197 100644
> --- a/net/rds/tcp_listen.c
> +++ b/net/rds/tcp_listen.c
> @@ -59,30 +59,12 @@ void rds_tcp_keepalive(struct socket *sock)
>  static int
>  rds_tcp_get_peer_sport(struct socket *sock)
>  {
> -	union {
> -		struct sockaddr_storage storage;
> -		struct sockaddr addr;
> -		struct sockaddr_in sin;
> -		struct sockaddr_in6 sin6;
> -	} saddr;
> -	int sport;
> -
> -	if (kernel_getpeername(sock, &saddr.addr) >= 0) {
> -		switch (saddr.addr.sa_family) {
> -		case AF_INET:
> -			sport = ntohs(saddr.sin.sin_port);
> -			break;
> -		case AF_INET6:
> -			sport = ntohs(saddr.sin6.sin6_port);
> -			break;
> -		default:
> -			sport = -1;
> -		}
> -	} else {
> -		sport = -1;
> -	}
> +	struct sock *sk = sock->sk;
> +
> +	if (!sk)
> +		return -1;
>  
> -	return sport;
> +	return ntohs(READ_ONCE(inet_sk(sk)->inet_dport));
>  }
>  
>  /* rds_tcp_accept_one_path(): if accepting on cp_index > 0, make sure the