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Date: Wed, 1 Nov 2017 09:05:42 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: dvyukov@google.com, hpa@zytor.com, linux-kernel@vger.kernel.org,
        bot+2af19c9e1ffe4d4ee1d16c56ae7580feaee75765@syzkaller.appspotmail.com,
        torvalds@linux-foundation.org, peterz@infradead.org,
        gratian.crisan@ni.com, linux-tip-commits@vger.kernel.org
Subject: Re: [tip:core/urgent] futex: Fix more put_pi_state() vs.
 exit_pi_state_list() races
Message-ID: <20171101080542.c2yugwpeiy3jpfc7@gmail.com>
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* Thomas Gleixner <tglx@linutronix.de> wrote:

> On Tue, 31 Oct 2017, tip-bot for Peter Zijlstra wrote:
> > Commit-ID:  d5f6ac33189af48a0dc011190af5144947a30a76
> > Gitweb:     https://git.kernel.org/tip/d5f6ac33189af48a0dc011190af5144947a30a76
> > Author:     Peter Zijlstra <peterz@infradead.org>
> > AuthorDate: Tue, 31 Oct 2017 11:18:53 +0100
> > Committer:  Ingo Molnar <mingo@kernel.org>
> > CommitDate: Tue, 31 Oct 2017 11:54:52 +0100
> > 
> > futex: Fix more put_pi_state() vs. exit_pi_state_list() races
> > 
> > Dmitry (through syzbot) reported being able to trigger the WARN in
> > get_pi_state() and a use-after-free on:
> > 
> > 	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
> > 
> > Both are due to this race:
> > 
> >   exit_pi_state_list()				put_pi_state()
> > 
> >   lock(&curr->pi_lock)
> >   while() {
> > 	pi_state = list_first_entry(head);
> > 	hb = hash_futex(&pi_state->key);
> > 	unlock(&curr->pi_lock);
> > 
> > 						dec_and_test(&pi_state->refcount);
> > 
> > 	lock(&hb->lock)
> > 	lock(&pi_state->pi_mutex.wait_lock)	// uaf if pi_state free'd
> > 	lock(&curr->pi_lock);
> > 
> > 	....
> > 
> > 	unlock(&curr->pi_lock);
> > 	get_pi_state();				// WARN; refcount==0
> > 
> > The problem is we take the reference count too late, and don't allow it
> > being 0. Fix it by using inc_not_zero() and simply retrying the loop
> > when we fail to get a refcount. In that case put_pi_state() should
> > remove the entry from the list.
> > 
> > Reported-by: Dmitry Vyukov <dvyukov@google.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Gratian Crisan <gratian.crisan@ni.com>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: dvhart@infradead.org
> > Cc: syzbot <bot+2af19c9e1ffe4d4ee1d16c56ae7580feaee75765@syzkaller.appspotmail.com>
> > Cc: syzkaller-bugs@googlegroups.com
> > Fixes: c74aef2d06a9 ("futex: Fix pi_state->owner serialization")
> > Link: http://lkml.kernel.org/r/20171031101853.xpfh72y643kdfhjs@hirez.programming.kicks-ass.net
> > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> 
> That lacks a stable tag.
> 
> Other than that a late:
> 
> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Ok, I've amended the commit with these two changes.

Thanks,

	Ingo