linux-kernel - Re: [06/11] tty: fix race in tty

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Date:	Tue, 26 Jan 2010 14:11:28 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
cc:	Greg KH <gregkh@...e.de>, linux-kernel@...r.kernel.org,
	stable@...nel.org, stable-review@...nel.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Al Viro <viro@...IV.linux.org.uk>,
	Tavis Ormandy <taviso@...gle.com>,
	Jeff Dike <jdike@...toit.com>, Julien Tinnes <jln@...gle.com>,
	Matt Mackall <mpm@...enic.com>
Subject: Re: [06/11] tty: fix race in tty_fasync



On Tue, 26 Jan 2010, Eric W. Biederman wrote:

> Greg KH <gregkh@...e.de> writes:
> 
> > 2.6.27-stable review patch.  If anyone has any objections, please let us know.
> 
> Only that __f_setown by way of f_modown unconditionally enables interrupts.  So
> without touching f_modown as well in mainline we have nasty sounding lockdep warnings.

Hmm. That seems to be true in mainline too, isn't it?

So now we have:
 - tty_fasync() gets tty->ctrl_lock, with spin_lock_irqsave()

 - it then calls __f_setown()

 - which calls f_modown(),

 - which does

	write_lock_irq(&filp->f_owner.lock);
	..
	write_unlock_irq(&filp->f_owner.lock);

   which in turn enables interrupts while we still hold ctrl_lock.

so that whole commit 70362511806 was/is buggy in mainline too. 

The minimal fix is likely to just make f_modown() use 
write_lock_irqsave/restore. Greg?

		Linus
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