[PATCH] tty: ldisc: fix deadlock between ldisc_sem and rtnl_mutex

[Yun Zhou]

syzbot reported a circular lock dependency involving tty ldisc_sem and
the networking rtnl_mutex. The full chain is:

  rtnl_mutex --> nft_commit_mutex --> ... --> ep->mtx --> ldisc_sem --> rtnl_mutex

The last edge (ldisc_sem -> rtnl_mutex) is created because tty line
discipline .open() callbacks (slcan, slip) call register_netdev() which
acquires rtnl_mutex, and .open() runs under ldisc_sem write lock in
tty_set_ldisc().

Fix by moving the .open() call outside the ldisc_sem write lock. The
ldisc .open() is initialization of the NEW discipline after the old one
has been closed - there is no need for ldisc_sem protection at this
point since:

 - tty_lock is held throughout, preventing concurrent tty_set_ldisc,
   hangup, or close
 - tty->ldisc is set to NULL during the window, so concurrent readers
   (tty_ldisc_ref, tty_ldisc_ref_wait) see NULL and return immediately,
   which callers already handle as a hangup condition
 - tty buffer data stays queued until the ldisc is installed

The sequence becomes:
  1. Hold ldisc_sem(write): close old ldisc, set tty->ldisc = NULL
  2. Release ldisc_sem(write)
  3. Call new_ldisc->ops->open() without ldisc_sem
  4. Re-acquire ldisc_sem(write): install new ldisc (or restore old)
  5. Release ldisc_sem(write)

Reported-by: syzbot+de610eeef174bd59a8a3@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=de610eeef174bd59a8a3
Signed-off-by: Yun Zhou <yun.zhou@xxxxxxxxxxxxx>
---
 drivers/tty/tty_ldisc.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 27fe8236f662..248a6995cc53 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -556,15 +556,28 @@ int tty_set_ldisc(struct tty_struct *tty, int disc)
 	/* Shutdown the old discipline. */
 	tty_ldisc_close(tty, old_ldisc);
 
-	/* Now set up the new line discipline. */
-	tty->ldisc = new_ldisc;
+	/* Clear tty->ldisc so concurrent readers back off during transition */
+	tty->ldisc = NULL;
 	tty_set_termios_ldisc(tty, disc);
+	tty_ldisc_unlock(tty);
 
+	/*
+	 * Open the new discipline outside ldisc_sem. The ldisc .open()
+	 * may acquire locks (e.g., rtnl_mutex) that would create circular
+	 * dependencies if taken under ldisc_sem. tty_lock is still held,
+	 * preventing concurrent ldisc changes and hangup.
+	 */
 	retval = tty_ldisc_open(tty, new_ldisc);
+
+	tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);
+
 	if (retval < 0) {
 		/* Back to the old one or N_TTY if we can't */
 		tty_ldisc_put(new_ldisc);
 		tty_ldisc_restore(tty, old_ldisc);
+	} else {
+		/* Success - install new ldisc */
+		tty->ldisc = new_ldisc;
 	}
 
 	if (tty->ldisc->ops->num != old_ldisc->ops->num && tty->ops->set_ldisc) {
-- 
2.43.0