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Message-ID: <20161201060745.GB124104@f23x64.localdomain>
Date: Wed, 30 Nov 2016 22:07:46 -0800
From: Darren Hart <dvhart@...radead.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: 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@...radead.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...
I traced through the possible return codes and found:
fixup_pi_state_owner
see below
rt_mutex_finish_proxy_lock
__rt_mutex_slowlock
EINTR
ETIMEDOUT
(ignored if fixup_owner fails)
fixup_owner
fixup_pi_state_owner
fault_in_user_writeable
fixup_user_fault
EFAULT
ENOMEM
EHWPOISON
>
> Urgh, so would really like to avoid doing that, I'll have to instantly
> drag it back out again :/
>
> Also, the fixup_owner() fail in futex_lock_pi() will unlock the rt_mutex
> on _any_ fail, not only -EFAULT, should we not do the same?
>
I don't see why we should treat ENOMEM or EHWPOISON any differently from EFAULT
in this situation.
> ---
> Subject: futex: Fix potential use-after-free in FUTEX_REQUEUE_PI
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Thu, 24 Nov 2016 15:42:35 +0100
>
> 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().
>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: dvhart@...radead.org
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> kernel/futex.c | 22 +++++++++++++---------
> 1 file changed, 13 insertions(+), 9 deletions(-)
>
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -2813,7 +2813,6 @@ static int futex_wait_requeue_pi(u32 __u
> {
> 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;
> @@ -2897,6 +2896,8 @@ static int futex_wait_requeue_pi(u32 __u
> if (q.pi_state && (q.pi_state->owner != current)) {
> spin_lock(q.lock_ptr);
> ret = fixup_pi_state_owner(uaddr2, &q, current);
> + if (ret && rt_mutex_owner(&q.pi_state->pi_mutex) == current)
> + rt_mutex_unlock(&q.pi_state->pi_mutex);
> /*
> * Drop the reference to the pi state which
> * the requeue_pi() code acquired for us.
> @@ -2905,6 +2906,8 @@ static int futex_wait_requeue_pi(u32 __u
> spin_unlock(q.lock_ptr);
> }
> } else {
> + struct rt_mutex *pi_mutex;
> +
> /*
> * We have been woken up by futex_unlock_pi(), a timeout, or a
> * signal. futex_unlock_pi() will not destroy the lock_ptr nor
> @@ -2928,18 +2931,19 @@ static int futex_wait_requeue_pi(u32 __u
> if (res)
> ret = (res < 0) ? res : 0;
>
> + /*
> + * If fixup_pi_state_owner() faulted and was unable to handle
faulted or failed ?
> + * the fault, unlock the rt_mutex and return the fault to
propagate the error to userspace
> + * userspace.
> + */
> + if (ret && rt_mutex_owner(pi_mutex) == current)
> + rt_mutex_unlock(pi_mutex);
> +
> /* Unqueue and drop the lock. */
> unqueue_me_pi(&q);
> }
>
> - /*
> - * If fixup_pi_state_owner() faulted and was unable to handle the
> - * fault, unlock the rt_mutex and return the fault to userspace.
> - */
> - if (ret == -EFAULT) {
> - if (pi_mutex && rt_mutex_owner(pi_mutex) == current)
> - rt_mutex_unlock(pi_mutex);
> - } else if (ret == -EINTR) {
> + if (ret == -EINTR) {
> /*
> * We've already been requeued, but cannot restart by calling
> * futex_lock_pi() directly. We could restart this syscall, but
>
--
Darren Hart
Intel Open Source Technology Center
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