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Re: [PATCH] futex: Fix potential use-after-free in FUTEX_REQUEUE_PI

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  Re: [PATCH] futex: Fix potential use-after-free in FUTEX_REQUEUE_PI Darren Hart <dvhart@infradead.org> - 2016-12-01 06:00 +0100
    Re: [PATCH] futex: Fix potential use-after-free in FUTEX_REQUEUE_PI Peter Zijlstra <peterz@infradead.org> - 2016-12-01 06:40 +0100

#1533805 — Re: [PATCH] futex: Fix potential use-after-free in FUTEX_REQUEUE_PI

On Thu, Nov 24, 2016 at 04:38:08PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 24, 2016 at 04:19:41PM +0100, Thomas Gleixner wrote:
> > On Thu, 24 Nov 2016, Peter Zijlstra wrote:
> > 
> > > 
> > > While working on the futex code, I stumbled over this potential
> > > use-after-free scenario.
> > > 
> > > pi_mutex is a pointer into pi_state, which we drop the reference on in
> > > unqueue_me_pi(). So any access to that pointer after that is bad.
> > > 
> > > Since other sites already do rt_mutex_unlock() with hb->lock held, see
> > > for example futex_lock_pi(), simply move the unlock before
> > > unqueue_me_pi().
> > > 
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > > ---
> > >  kernel/futex.c | 22 +++++++++++++---------
> > >  1 file changed, 13 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/kernel/futex.c b/kernel/futex.c
> > > index 2c4be467fecd..d5a81339209f 100644
> > > --- a/kernel/futex.c
> > > +++ b/kernel/futex.c
> > > @@ -2813,7 +2813,6 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
> > >  {
> > >  	struct hrtimer_sleeper timeout, *to = NULL;
> > >  	struct rt_mutex_waiter rt_waiter;
> > > -	struct rt_mutex *pi_mutex = NULL;
> > >  	struct futex_hash_bucket *hb;
> > >  	union futex_key key2 = FUTEX_KEY_INIT;
> > >  	struct futex_q q = futex_q_init;
> > > @@ -2905,6 +2904,8 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
> > >  			spin_unlock(q.lock_ptr);
> > 
> > In this path the fixup can return -EFAIL as well, so it should drop rtmutex
> > too if it owns it. We should move the rtmutex drop into the fixup functions...
> 
> Urgh, so would really like to avoid doing that, I'll have to instantly
> drag it back out again :/

Why would you have to drag it back out again? Something else you're working on?

-- 
Darren Hart
Intel Open Source Technology Center

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#1533816

On Wed, Nov 30, 2016 at 08:55:30PM -0800, Darren Hart wrote:
> On Thu, Nov 24, 2016 at 04:38:08PM +0100, Peter Zijlstra wrote:

> > > In this path the fixup can return -EFAIL as well, so it should drop rtmutex
> > > too if it owns it. We should move the rtmutex drop into the fixup functions...
> > 
> > Urgh, so would really like to avoid doing that, I'll have to instantly
> > drag it back out again :/
> 
> Why would you have to drag it back out again? Something else you're working on?

Yeah, the very reason I've been staring at this mess in the first place
:-)

So I could point you at the patches; and I will, see:

  https://lkml.kernel.org/r/20161021122735.GA3117@twins.programming.kicks-ass.net

but the TL;DR version is that we must not rt_mutex_unlock() while
holding hb->lock, because on RT hb->lock is itself a rt_mutex which
gives rise to some very fun prio inversions.

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