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

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Message-ID: <CAAVpQUBxfpYHaSxS8o8SAecT27YtrNhcVY9O=rSYFr3GshF0_Q@mail.gmail.com>
Date: Thu, 23 Oct 2025 16:59:50 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: stefan.wiehler@...ia.com
Cc: davem@...emloft.net, edumazet@...gle.com, horms@...nel.org, 
	kuba@...nel.org, linux-kernel@...r.kernel.org, linux-sctp@...r.kernel.org, 
	lucien.xin@...il.com, netdev@...r.kernel.org, pabeni@...hat.com
Subject: Re: [PATCH net] sctp: Hold RCU read lock while iterating over address list

On Thu, Oct 23, 2025 at 4:53 PM Kuniyuki Iwashima <kuniyu@...gle.com> wrote:
>
> From: Stefan Wiehler <stefan.wiehler@...ia.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@...ia.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.

Or if lock_sock() is enough, we should use the plain
list_for_each_entry(), or list_for_each_entry_rcu() with
lockdep_sock_is_held() as the 4th arg.