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

On Fri, Jul 17, 2026 at 11:03:18AM +0800, Zhou, Yun wrote:
> 
> 
> On 7/16/26 15:57, Greg KH wrote:
> > CAUTION: This email comes from a non Wind River email account!
> > Do not click links or open attachments unless you recognize the sender and know the content is safe.
> > 
> > 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.

Ah, you are right, my bad read of the patch, sorry.

thanks,

greg k-h