On Thu, Oct 23, 2025 at 4:53 PM Kuniyuki Iwashima [off-list ref] wrote:

From: Stefan Wiehler <redacted>
Date: Thu, 23 Oct 2025 21:18:08 +0200

quoted

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 <redacted>
---
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.