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SubjectRe: [PATCH] tty races
Jason Baron <jbaron@redhat.com> wrote:
>
>
> On Mon, 25 Apr 2005, Andrew Morton wrote:
>
> > Jason Baron <jbaron@redhat.com> 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.

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

> 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 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.

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