mISDN: annotate data-race around dev->work

dev->work can re read locklessly in mISDN_read()
and mISDN_poll(). Add READ_ONCE()/WRITE_ONCE() annotations.

BUG: KCSAN: data-race in mISDN_ioctl / mISDN_read

write to 0xffff88812d848280 of 4 bytes by task 10864 on cpu 1:
  misdn_add_timer drivers/isdn/mISDN/timerdev.c:175 [inline]
  mISDN_ioctl+0x2fb/0x550 drivers/isdn/mISDN/timerdev.c:233
  vfs_ioctl fs/ioctl.c:51 [inline]
  __do_sys_ioctl fs/ioctl.c:597 [inline]
  __se_sys_ioctl+0xce/0x140 fs/ioctl.c:583
  __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:583
  x64_sys_call+0x14b0/0x3000 arch/x86/include/generated/asm/syscalls_64.h:17
  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
  do_syscall_64+0xd8/0x2c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88812d848280 of 4 bytes by task 10857 on cpu 0:
  mISDN_read+0x1f2/0x470 drivers/isdn/mISDN/timerdev.c:112
  do_loop_readv_writev fs/read_write.c:847 [inline]
  vfs_readv+0x3fb/0x690 fs/read_write.c:1020
  do_readv+0xe7/0x210 fs/read_write.c:1080
  __do_sys_readv fs/read_write.c:1165 [inline]
  __se_sys_readv fs/read_write.c:1162 [inline]
  __x64_sys_readv+0x45/0x50 fs/read_write.c:1162
  x64_sys_call+0x2831/0x3000 arch/x86/include/generated/asm/syscalls_64.h:20
  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
  do_syscall_64+0xd8/0x2c0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00000000 -> 0x00000001

Fixes: 1b2b03f ("Add mISDN core files")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260118132528.2349573-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

drivers/isdn/mISDN/timerdev.c

@@ -109,14 +109,14 @@ mISDN_read(struct file *filep, char __user *buf, size_t count, loff_t *off)
 		spin_unlock_irq(&dev->lock);
 		if (filep->f_flags & O_NONBLOCK)
 			return -EAGAIN;
-		wait_event_interruptible(dev->wait, (dev->work ||
+		wait_event_interruptible(dev->wait, (READ_ONCE(dev->work) ||
 						     !list_empty(list)));
 		if (signal_pending(current))
 			return -ERESTARTSYS;
 		spin_lock_irq(&dev->lock);
 	}
 	if (dev->work)
-		dev->work = 0;
+		WRITE_ONCE(dev->work, 0);
 	if (!list_empty(list)) {
 		timer = list_first_entry(list, struct mISDNtimer, list);
 		list_del(&timer->list);
@@ -141,13 +141,16 @@ mISDN_poll(struct file *filep, poll_table *wait)
 	if (*debug & DEBUG_TIMER)
 		printk(KERN_DEBUG "%s(%p, %p)\n", __func__, filep, wait);
 	if (dev) {
+		u32 work;
+
 		poll_wait(filep, &dev->wait, wait);
 		mask = 0;
-		if (dev->work || !list_empty(&dev->expired))
+		work = READ_ONCE(dev->work);
+		if (work || !list_empty(&dev->expired))
 			mask |= (EPOLLIN | EPOLLRDNORM);
 		if (*debug & DEBUG_TIMER)
 			printk(KERN_DEBUG "%s work(%d) empty(%d)\n", __func__,
-			       dev->work, list_empty(&dev->expired));
+			       work, list_empty(&dev->expired));
 	}
 	return mask;
 }
@@ -172,7 +175,7 @@ misdn_add_timer(struct mISDNtimerdev *dev, int timeout)
 	struct mISDNtimer	*timer;
 
 	if (!timeout) {
-		dev->work = 1;
+		WRITE_ONCE(dev->work, 1);
 		wake_up_interruptible(&dev->wait);
 		id = 0;
 	} else {