Re: [PATCH] tty races

[Jason Baron]

On Mon, 2 May 2005, Andrew Morton wrote:

> Jason Baron <jbaron@xxxxxxxxxx> wrote:
> >
> > 
> > On Mon, 25 Apr 2005, Andrew Morton wrote:
> > 
> > > Jason Baron <jbaron@xxxxxxxxxx> wrote:
> > > >
> > > > There are a couple of tty race conditions, which lead to inconsistent tty 
> > > >  reference counting and tty layer oopses.
> > > > 
> > > >  The first is a tty_open vs. tty_close race in drivers/char/tty.io.c. 
> > > >  Basically, from the time that the tty->count is deemed to be 1 and that we 
> > > >  are going to free it to the time that TTY_CLOSING bit is set, needs to be 
> > > >  atomic with respect to the manipulation of tty->count in init_dev(). This 
> > > >  atomicity was previously guarded by the BKL. However, this is no longer 
> > > >  true with the addition of a down() call in the middle of the 
> > > >  release_dev()'s atomic path. So either the down() needs to be moved 
> > > >  outside the atomic patch or dropped. I would vote for simply dropping it 
> > > >  as i don't see why it is necessary.
> > > 
> > > The release_dev() changes looks very fishy to me.  It _removes_ locking. 
> > > If that fixes the testcase then one of two things is happening:
> > > 
> > > a) we have lock_kernel() coverage and the down()'s sleeping breaks the
> > >    lock_kenrel() coverage or
> > > 
> > > b) we don't have lock_kernel() coverage, but removing the down() just
> > >    alters the timing and makes the race less probable.
> > > 
> > > I think it's b).  lock_kernel() coverage in there is very incomplete on the
> > > open() side.
> > > 
> > 
> > The patch was written for case a. Indeed lock_kernel() may appear 
> > incomplete on the open side, but it protects paths where we don't sleep. 
> > So, the 'fast_track' path in 'init_dev', is protected against the 
> > release_dev path from setting the 'tty_closing' local variable to the 
> > setting of the TTY_CLOSING flag. Thus, i believe the dropping of the 
> > down() is correct. 
> 
> I don't see anywhere which takes lock_kernel() on the tty_open() path.
> 

fs/char_dev.c:chrdev_open():        

	if (filp->f_op->open) {
                lock_kernel();
                ret = filp->f_op->open(inode,filp);
                unlock_kernel();
        }


> The normal release_dev() path takes lock_kernel(), but two error-path
> callers of lock_kernel() also appear to not take lock_kernel().
> 

these are both on open paths.

> > This was the previous locking model for open vs. close afaict, before the 
> > down() was introduced in the release_dev path that was supposed to be 
> > atomic with respect to init_dev().
> 
> We want to move away from lock_kernel()-based locking.
> 

I completely agree, but unfortunately lock_kernel() is currently used 
extensively throughout the tty layer. 

> > 
> > > I think it would be better to _increase_ the tty_sem coverage in
> > > release_dev() and to make sure that all callers of init_dev() are using
> > > tty_sem (they are).
> > > 
> > > One approach would be to require that all callers of release_dev() hold
> > > tty_sem, and make release_dev() drop and reacquire tty_sem in those cases
> > > where release_dev() needs to go to sleep when waiting for other threads of
> > > control to reelase the tty's resources.
> > > 
> > 
> > Indeed, the situation would be improved if it was held around the 
> > driver->close() routine. This routine does sometimes look at tty->count 
> > value, see con_close(), where in fact the tty_sem is added to avoid just 
> > this problem. However, it is incorrect as one can see in release_dev() the 
> > schedule(), can cause the tty->count to change via tty_open(). However, i 
> > think this is an extremely rare corner case, b/c con_close() keys off 
> > tty->count of 1, which implies that this is the last close() and thus the 
> > schedule for 'write_wait' would seem impossible, although AL Viro has 
> > said that it is possible in this case. Thus, dropping the tty_sem and 
> > reacquiring it, probably isn't good, b/c the driver->close() routines can 
> > free resources based upon tty->count==1. 
> 
> Maybe we can just hold tty_sem across that schedule() in release_dev().
> 
> If not, then maybe retest ->count and take avoiding action if it looks like
> some other thread is trying to resurrect the tty.  Obviously this is a much
> poorer approach.
> 
> > The patch was written as the least invasive and low risk way to fix a 
> > nasty race condition, which has the potential to corrupt data. The oops in 
> > vt_ioctl has also been seen on system boots with some frequency. The patch 
> > imo, returns the the tty_open vs. tty_close paths to their original 
> > locking assumptions which have been well tested.
> > 
> 
> I don't think it does, and the original lock_kernel-based locking is
> obsolete.
> 
> Please, let's do this properly, with real locks.
> 

lock_kernel() is used extensively throughout the tty layer. We can 
re-write the locking for the layer, but I'd like to see this bug fix in 
2.6.12, if that isn't done in time.

thanks,

-Jason
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