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

On 7/16/26 15:57, Greg KH wrote:
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> On Thu, Jul 16, 2026 at 02:47:19PM +0800, Yun Zhou wrote:
>> 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: [email protected]
>> Closes: https://syzkaller.appspot.com/bug?extid=de610eeef174bd59a8a3
>> Signed-off-by: Yun Zhou <[email protected]>
>> ---
>>   drivers/tty/tty_ldisc.c | 17 +++++++++++++++--
>>   1 file changed, 15 insertions(+), 2 deletions(-)
> 
> What commit caused this to be a problem and why have we not seen this in
> any real-world usages?
> 

The circular dependency has existed for a long time - it just requires
ldisc_sem -> rtnl_mutex (from slcan/slip registering a netdev in
.open()) and the reverse path through nft_commit_mutex, epoll, and
tty_poll back to ldisc_sem.

The recent dev_instance_lock series (5326fefb9fe8 "net: hold instance
lock around NETDEV_DOWN/GOING_DOWN") increased lockdep's observability
by adding lock acquisitions in more notifier paths, making it easier for
lockdep to collect all edges in a single run. It did not create the
cycle.

We have not seen this in real-world usage because triggering the actual
deadlock requires 6 unrelated subsystems to contend simultaneously -
something only a fuzzer like syzkaller would construct.

> 
>>
>> 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);
> 
> Now you are calling open when previously we were not, are you sure this
> isn't going to cause problems?
> 

This is not a new .open() call - it is the same tty_ldisc_open() that
was always called here. The change only moves it outside ldisc_sem.
tty_lock is still held throughout, so .open() sees the same environment
as before.

>> +
>> +     tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);
> 
> Why that 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;
>>        }
> 
> Does open cause anything else to be incremented that you have to clean
> up when done that you aren't doing here?
> 

tty_ldisc_open() only sets the TTY_LDISC_OPEN flag bit and calls
ld->ops->open(). On failure it clears the flag itself.

No refcounts or other state are incremented by tty_ldisc_open() that
would need additional cleanup.

BR,
Yun