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: [email protected]
Closes: https://lore.kernel.org/netdev/[email protected]/T/#u
Signed-off-by: Eric Dumazet <[email protected]>
Cc: Marcelo Ricardo Leitner <[email protected]>
Acked-by: Xin Long <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Eric Dumazet

net/sctp/sysctl.c

@@ -525,6 +525,8 @@ static int proc_sctp_do_auth(const struct ctl_table *ctl, int write,
 	return ret;
 }
 
+static DEFINE_MUTEX(sctp_sysctl_mutex);
+
 static int proc_sctp_do_udp_port(const struct ctl_table *ctl, int write,
 				 void *buffer, size_t *lenp, loff_t *ppos)
 {
@@ -549,6 +551,7 @@ static int proc_sctp_do_udp_port(const 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) {
@@ -561,6 +564,7 @@ static int proc_sctp_do_udp_port(const 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;