Re: [PATCH net] sctp: Hold RCU read lock while iterating over address list

[Kuniyuki Iwashima <kuniyu@google.com>]

From: Stefan Wiehler <stefan.wiehler@nokia.com>
Date: Thu, 23 Oct 2025 21:18:08 +0200
> With CONFIG_PROVE_RCU_LIST=y and by executing
> 
>   $ netcat -l --sctp &
>   $ netcat --sctp localhost &
>   $ ss --sctp
> 
> one can trigger the following Lockdep-RCU splat(s):
> 
>   WARNING: suspicious RCU usage
>   6.18.0-rc1-00093-g7f864458e9a6 #5 Not tainted
>   -----------------------------
>   net/sctp/diag.c:76 RCU-list traversed in non-reader section!!
> 
>   other info that might help us debug this:
> 
>   rcu_scheduler_active = 2, debug_locks = 1
>   2 locks held by ss/215:
>    #0: ffff9c740828bec0 (nlk_cb_mutex-SOCK_DIAG){+.+.}-{4:4}, at: __netlink_dump_start+0x84/0x2b0
>    #1: ffff9c7401d72cd0 (sk_lock-AF_INET6){+.+.}-{0:0}, at: sctp_sock_dump+0x38/0x200
> 
>   stack backtrace:
>   CPU: 0 UID: 0 PID: 215 Comm: ss Not tainted 6.18.0-rc1-00093-g7f864458e9a6 #5 PREEMPT(voluntary)
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
>   Call Trace:
>    <TASK>
>    dump_stack_lvl+0x5d/0x90
>    lockdep_rcu_suspicious.cold+0x4e/0xa3
>    inet_sctp_diag_fill.isra.0+0x4b1/0x5d0
>    sctp_sock_dump+0x131/0x200
>    sctp_transport_traverse_process+0x170/0x1b0
>    ? __pfx_sctp_sock_filter+0x10/0x10
>    ? __pfx_sctp_sock_dump+0x10/0x10
>    sctp_diag_dump+0x103/0x140
>    __inet_diag_dump+0x70/0xb0
>    netlink_dump+0x148/0x490
>    __netlink_dump_start+0x1f3/0x2b0
>    inet_diag_handler_cmd+0xcd/0x100
>    ? __pfx_inet_diag_dump_start+0x10/0x10
>    ? __pfx_inet_diag_dump+0x10/0x10
>    ? __pfx_inet_diag_dump_done+0x10/0x10
>    sock_diag_rcv_msg+0x18e/0x320
>    ? __pfx_sock_diag_rcv_msg+0x10/0x10
>    netlink_rcv_skb+0x4d/0x100
>    netlink_unicast+0x1d7/0x2b0
>    netlink_sendmsg+0x203/0x450
>    ____sys_sendmsg+0x30c/0x340
>    ___sys_sendmsg+0x94/0xf0
>    __sys_sendmsg+0x83/0xf0
>    do_syscall_64+0xbb/0x390
>    entry_SYSCALL_64_after_hwframe+0x77/0x7f
>    ...
>    </TASK>
> 
> Fixes: 8f840e47f190 ("sctp: add the sctp_diag.c file")
> Signed-off-by: Stefan Wiehler <stefan.wiehler@nokia.com>
> ---
> It might be sufficient to add a check for one of the already held locks,
> but I lack the domain knowledge to be sure about that...
> ---
>  net/sctp/diag.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/net/sctp/diag.c b/net/sctp/diag.c
> index 996c2018f0e6..1a8761f87bf1 100644
> --- a/net/sctp/diag.c
> +++ b/net/sctp/diag.c
> @@ -73,19 +73,23 @@ static int inet_diag_msg_sctpladdrs_fill(struct sk_buff *skb,
>  	struct nlattr *attr;
>  	void *info = NULL;
>  
> +	rcu_read_lock();
>  	list_for_each_entry_rcu(laddr, address_list, list)
>  		addrcnt++;
> +	rcu_read_unlock();
>  
>  	attr = nla_reserve(skb, INET_DIAG_LOCALS, addrlen * addrcnt);
>  	if (!attr)
>  		return -EMSGSIZE;
>  
>  	info = nla_data(attr);
> +	rcu_read_lock();
>  	list_for_each_entry_rcu(laddr, address_list, list) {
>  		memcpy(info, &laddr->a, sizeof(laddr->a));
>  		memset(info + sizeof(laddr->a), 0, addrlen - sizeof(laddr->a));
>  		info += addrlen;

looks like TOCTOU issue exists here, we should check
the boundary like this:

		if (!--addrcnt)
			break;

Otherwise KASAN would complain about an out-of-bound write.

>  	}
> +	rcu_read_unlock();
>  
>  	return 0;
>  }
> -- 
> 2.51.0