netdev - [PATCH net] mISDN: Fix data race in timer handling

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Message-Id: <20260127071927.1520272-1-zeri@umich.edu>
Date: Tue, 27 Jan 2026 02:19:27 -0500
From: Henry Zhang <henryzhangjcle@...il.com>
To: netdev@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	tglx@...utronix.de,
	mingo@...nel.org,
	syzbot+c6e7bcea7ffb7ff46acb@...kaller.appspotmail.com,
	Henry Zhang <zeri@...ch.edu>
Subject: [PATCH net] mISDN: Fix data race in timer handling

KCSAN reported a data race where misdn_add_timer() writes dev->work
while mISDN_read() reads it without holding the spinlock:

write to 0xffff88812d848280 of 4 bytes by task 10864 on cpu 1:
 misdn_add_timer drivers/isdn/mISDN/timerdev.c:175 [inline]
...

read to 0xffff88812d848280 of 4 bytes by task 10857 on cpu 0:
 mISDN_read+0x1f2/0x470 drivers/isdn/mISDN/timerdev.c:112
...

dev->work is read locklessly in wait_event_interruptible() and
mISDN_poll(). In mISDN_read(), the result is rechecked under dev->lock.
In mISDN_poll(), a stale value may cause a spurious EPOLLIN or a missed
wake, but wake_up_interruptible() will correct this. In both cases, the
race is benign, so we can annotate these with READ_ONCE/WRITE_ONCE.

Reported-by: syzbot+c6e7bcea7ffb7ff46acb@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c6e7bcea7ffb7ff46acb

Signed-off-by: Henry Zhang <zeri@...ch.edu>
---
 drivers/isdn/mISDN/timerdev.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/isdn/mISDN/timerdev.c b/drivers/isdn/mISDN/timerdev.c
index df98144a9539..f98f2e0cfb9f 100644
--- a/drivers/isdn/mISDN/timerdev.c
+++ b/drivers/isdn/mISDN/timerdev.c
@@ -109,7 +109,7 @@ 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;
@@ -143,11 +143,11 @@ mISDN_poll(struct file *filep, poll_table *wait)
 	if (dev) {
 		poll_wait(filep, &dev->wait, wait);
 		mask = 0;
-		if (dev->work || !list_empty(&dev->expired))
+		if (READ_ONCE(dev->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));
+			       READ_ONCE(dev->work), list_empty(&dev->expired));
 	}
 	return mask;
 }
@@ -172,7 +172,7 @@ 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 {
-- 
2.34.1