Re: [PATCH] race condition with drivers/char/vt.c (bug in vt_ioctl.c)

[Ingo Molnar <mingo@elte.hu>]

* Steven Rostedt <rostedt@goodmis.org> wrote:

> I googled a little and found where this may have already happened in 
> the main line kernel:
> 
> http://seclists.org/lists/linux-kernel/2005/Aug/1603.html
> 
> So here's my proposal: 
> 
>   Instead of checking for tty->count == 1 in con_open, which we see is
> not reliable.  Just check for tty->driver_data == NULL.
> 
> This should work since it should always be NULL when we need to assign 
> it.  If we switch the events of the race, so that the init_dev went 
> first, the driver_data would not be NULL and would not need to be 
> allocated, because after init_dev tty->count would be greater than 1 
> (this is assuming the case that it is already allocated) and the 
> con_close would not deallocate it.  The tty_sem and console_sem and 
> order of events protect the tty->driver_data but not the tty->count.
> 
> Without the patch, I was able to get the system to BUG on bootup every 
> other time.  With the patch applied, I was able to bootup 6 out of 6 
> times without a single crash.

cool fix. I'm wondering, there's a whole lot of other 'tty->count == 1' 
checks in drivers/char/*.c, could some of those be racy too?

	Ingo
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