linux-kernel - Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand unprotected when accessed by /proc)

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20140610130138.GC3213@twins.programming.kicks-ass.net>
Date:	Tue, 10 Jun 2014 15:01:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Clark Williams <williams@...hat.com>
Subject: Re: safety of *mutex_unlock() (Was: [BUG] signal: sighand
 unprotected when accessed by /proc)

On Tue, Jun 10, 2014 at 05:52:35AM -0700, Paul E. McKenney wrote:
> On Tue, Jun 10, 2014 at 10:37:26AM +0200, Peter Zijlstra wrote:
> > On Mon, Jun 09, 2014 at 09:26:13AM -0700, Paul E. McKenney wrote:
> > > That would indeed be a bad thing, as it could potentially lead to
> > > use-after-free bugs.  Though one could argue that any code that resulted
> > > in use-after-free would be quite aggressive.  But still...
> > 
> > Let me hijack this thread for yet another issue... So I had an RCU
> > related use-after-free the other day, and while Sasha was able to
> > trigger it quite easily, I had a multi-day struggle to reproduce.
> > 
> > Once I figured out what the exact problem was it was also clear to me
> > why it was so hard for me to reproduce.
> > 
> > So normally its easier to trigger races on bigger machines, more cpus,
> > more concurrency, more races, all good.
> > 
> > _However_ with RCU the grace period machinery is slower the bigger the
> > machine, so bigger machine, slower grace period, slower RCU free, less
> > likely to hit use-after-free.
> > 
> > So I was thinking, and I know you all will go kick me for this because
> > the very last thing we need is what I'm about to propose: more RCU
> > flavours :-).
> > 
> > How about an rcu_read_unlock() reference counted RCU variant that's
> > ultra aggressive in doing the callbacks in order to better trigger such
> > issues?
> 
> If you are using synchronize_rcu() for the update side, then I suggest
> rcutorture.gp_exp=1 to force use expediting throughout.

No such luck, this was regular kfree() from call_rcu(). And the callback
execution was typically delayed long enough to never 'see' the
use-after-free.

Content of type "application/pgp-signature" skipped