linux-kernel - Re: [BUG] signal: sighand unprotected when accessed by /proc

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Date:	Tue, 3 Jun 2014 22:01:25 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: [BUG] signal: sighand unprotected when accessed by /proc

On 06/03, Linus Torvalds wrote:
>
> On Tue, Jun 3, 2014 at 10:26 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > looks like, SLAB_DESTROY_BY_RCU logic is broken?
>
> I haven't looked at the code yet, but SLAB_DESTROY_BY_RCU can be
> subtle and very dangerous.
>
> The danger is that the *slab* itself is free'd by RCU, but individual
> allocations can (and do) get re-used FOR THE SAME OBJECT TYPE without
> waiting for RCU!
>
> This is subtle. It means that most people who think that "it's free'd
> by RCU" get it wrong. Because individual allocations really aren't at
> all RCU-free'd, it's just that the underlying memory is guaranteed to
> not change type or be entirely thrown away until after a RCU grace
> period.

Yes, exactly. And unless you use current->sighand (which is obviously
stable) you need lock_task_sighand() which relies on ->siglock initialized
by sighand_ctor().

> Without looking at the code, it sounds like somebody may doing things
> to "sighand->lock->wait_list" that they shouldn't do. We've had cases
> like that before, and most of them have been changed to *not* use
> SLAB_DESTROY_BY_RCU, and instead make each individual allocation be
> RCU-free'd (which is a lot simpler to think about, because then you
> don't have the whole re-use issue).

Sure, we only need to change __cleanup_sighand() to use call_rcu().
But I am not sure this makes sense, I mean, I do not think this can
make something more simple/clear.

> And this could easily be an RT issue, if the RT code does some
> re-initialization of the rtmutex that replaces the spinlock we have.

Unlikely... this should be done by sighand_ctor() anyway.

I'll try to recheck rt_mutex_unlock() tomorrow. _Perhaps_ rcu_read_unlock()
should be shifted from lock_task_sighand() to unlock_task_sighand() to
ensure that rt_mutex_unlock() does nothihg with this memory after it
makes another lock/unlock possible.

But if we need this (currently I do not think so), this doesn't depend on
SLAB_DESTROY_BY_RCU. And, at first glance, in this case rcu_read_unlock_special()
might be wrong too.

Oleg.

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