sctp: add mutual exclusion in proc_sctp_do_udp_port()

JIRA: https://issues.redhat.com/browse/RHEL-84590
Tested: compile only

Conflicts:
- context difference due to missing 78eb4ea from upstream.

commit 1020630
Author: Eric Dumazet <edumazet@google.com>
Date:   Mon Mar 31 09:15:32 2025 +0000

    sctp: add mutual exclusion in proc_sctp_do_udp_port()

    We must serialize calls to sctp_udp_sock_stop() and sctp_udp_sock_start()
    or risk a crash as syzbot reported:

    Oops: general protection fault, probably for non-canonical address 0xdffffc000000000d: 0000 [#1] SMP KASAN PTI
    KASAN: null-ptr-deref in range [0x0000000000000068-0x000000000000006f]
    CPU: 1 UID: 0 PID: 6551 Comm: syz.1.44 Not tainted 6.14.0-syzkaller-g7f2ff7b62617 #0 PREEMPT(full)
    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
     RIP: 0010:kernel_sock_shutdown+0x47/0x70 net/socket.c:3653
    Call Trace:
     <TASK>
      udp_tunnel_sock_release+0x68/0x80 net/ipv4/udp_tunnel_core.c:181
      sctp_udp_sock_stop+0x71/0x160 net/sctp/protocol.c:930
      proc_sctp_do_udp_port+0x264/0x450 net/sctp/sysctl.c:553
      proc_sys_call_handler+0x3d0/0x5b0 fs/proc/proc_sysctl.c:601
      iter_file_splice_write+0x91c/0x1150 fs/splice.c:738
      do_splice_from fs/splice.c:935 [inline]
      direct_splice_actor+0x18f/0x6c0 fs/splice.c:1158
      splice_direct_to_actor+0x342/0xa30 fs/splice.c:1102
      do_splice_direct_actor fs/splice.c:1201 [inline]
      do_splice_direct+0x174/0x240 fs/splice.c:1227
      do_sendfile+0xafd/0xe50 fs/read_write.c:1368
      __do_sys_sendfile64 fs/read_write.c:1429 [inline]
      __se_sys_sendfile64 fs/read_write.c:1415 [inline]
      __x64_sys_sendfile64+0x1d8/0x220 fs/read_write.c:1415
      do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]

    Fixes: 046c052 ("sctp: enable udp tunneling socks")
    Reported-by: syzbot+fae49d997eb56fa7c74d@syzkaller.appspotmail.com
    Closes: https://lore.kernel.org/netdev/67ea5c01.050a0220.1547ec.012b.GAE@google.com/T/#u
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
    Acked-by: Xin Long <lucien.xin@gmail.com>
    Link: https://patch.msgid.link/20250331091532.224982-1-edumazet@google.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Xin Long <lxin@redhat.com>

Author: Xin Long

net/sctp/sysctl.c

@@ -529,6 +529,8 @@ static int proc_sctp_do_auth(struct ctl_table *ctl, int write,
 	return ret;
 }
 
+static DEFINE_MUTEX(sctp_sysctl_mutex);
+
 static int proc_sctp_do_udp_port(struct ctl_table *ctl, int write,
 				 void *buffer, size_t *lenp, loff_t *ppos)
 {
@@ -553,6 +555,7 @@ static int proc_sctp_do_udp_port(struct ctl_table *ctl, int write,
 		if (new_value > max || new_value < min)
 			return -EINVAL;
 
+		mutex_lock(&sctp_sysctl_mutex);
 		net->sctp.udp_port = new_value;
 		sctp_udp_sock_stop(net);
 		if (new_value) {
@@ -565,6 +568,7 @@ static int proc_sctp_do_udp_port(struct ctl_table *ctl, int write,
 		lock_sock(sk);
 		sctp_sk(sk)->udp_port = htons(net->sctp.udp_port);
 		release_sock(sk);
+		mutex_unlock(&sctp_sysctl_mutex);
 	}
 
 	return ret;