Re: [tip:core/urgent] futex: Fix more put_pi_state() vs. exit_pi_state_list() races

[Thomas Gleixner <tglx@linutronix.de>]

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>