[PATCH] Fix: membarrier: racy access to p->mm in membarrier_global_expedited()

[PATCH] Fix: membarrier: racy access to p->mm in membarrier_global_expedited()

[Mathieu Desnoyers]

Jann Horn identified a racy access to p->mm in the global expedited
command of the membarrier system call.

The suggested fix is to hold the task_lock() around the accesses to
p->mm and to the mm_struct membarrier_state field to guarantee the
existence of the mm_struct.

Link: https://lore.kernel.org/lkml/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Yu3cw@mail.gmail.com
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Tested-by: Jann Horn <jannh@google.com>
CC: Jann Horn <jannh@google.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Peter Zijlstra (Intel) <peterz@infradead.org>
CC: Ingo Molnar <mingo@kernel.org>
CC: Andrea Parri <parri.andrea@gmail.com>
CC: Andy Lutomirski <luto@kernel.org>
CC: Avi Kivity <avi@scylladb.com>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Boqun Feng <boqun.feng@gmail.com>
CC: Dave Watson <davejwatson@fb.com>
CC: David Sehr <sehr@google.com>
CC: H. Peter Anvin <hpa@zytor.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Maged Michael <maged.michael@gmail.com>
CC: Michael Ellerman <mpe@ellerman.id.au>
CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Paul Mackerras <paulus@samba.org>
CC: Russell King <linux@armlinux.org.uk>
CC: Will Deacon <will.deacon@arm.com>
CC: stable@vger.kernel.org # v4.16+
CC: linux-api@vger.kernel.org
---
 kernel/sched/membarrier.c | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index 76e0eaf4654e..305fdcc4c5f7 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -81,12 +81,27 @@ static int membarrier_global_expedited(void)
 
 		rcu_read_lock();
 		p = task_rcu_dereference(&cpu_rq(cpu)->curr);
-		if (p && p->mm && (atomic_read(&p->mm->membarrier_state) &
-				   MEMBARRIER_STATE_GLOBAL_EXPEDITED)) {
-			if (!fallback)
-				__cpumask_set_cpu(cpu, tmpmask);
-			else
-				smp_call_function_single(cpu, ipi_mb, NULL, 1);
+		/*
+		 * Skip this CPU if the runqueue's current task is NULL or if
+		 * it is a kernel thread.
+		 */
+		if (p && READ_ONCE(p->mm)) {
+			bool mm_match;
+
+			/*
+			 * Read p->mm and access membarrier_state while holding
+			 * the task lock to ensure existence of mm.
+			 */
+			task_lock(p);
+			mm_match = p->mm && (atomic_read(&p->mm->membarrier_state) &
+					     MEMBARRIER_STATE_GLOBAL_EXPEDITED);
+			task_unlock(p);
+			if (mm_match) {
+				if (!fallback)
+					__cpumask_set_cpu(cpu, tmpmask);
+				else
+					smp_call_function_single(cpu, ipi_mb, NULL, 1);
+			}
 		}
 		rcu_read_unlock();
 	}
-- 
2.17.1

Re: [PATCH] Fix: membarrier: racy access to p->mm in membarrier_global_expedited()

[Paul E. McKenney]

On Mon, Jan 28, 2019 at 05:07:07PM -0500, Mathieu Desnoyers wrote:
> Jann Horn identified a racy access to p->mm in the global expedited
> command of the membarrier system call.
> 
> The suggested fix is to hold the task_lock() around the accesses to
> p->mm and to the mm_struct membarrier_state field to guarantee the
> existence of the mm_struct.
> 
> Link: https://lore.kernel.org/lkml/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Yu3cw@mail.gmail.com
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Tested-by: Jann Horn <jannh@google.com>
> CC: Jann Horn <jannh@google.com>
> CC: Thomas Gleixner <tglx@linutronix.de>
> CC: Peter Zijlstra (Intel) <peterz@infradead.org>
> CC: Ingo Molnar <mingo@kernel.org>
> CC: Andrea Parri <parri.andrea@gmail.com>
> CC: Andy Lutomirski <luto@kernel.org>
> CC: Avi Kivity <avi@scylladb.com>
> CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> CC: Boqun Feng <boqun.feng@gmail.com>
> CC: Dave Watson <davejwatson@fb.com>
> CC: David Sehr <sehr@google.com>
> CC: H. Peter Anvin <hpa@zytor.com>
> CC: Linus Torvalds <torvalds@linux-foundation.org>
> CC: Maged Michael <maged.michael@gmail.com>
> CC: Michael Ellerman <mpe@ellerman.id.au>
> CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> CC: Paul Mackerras <paulus@samba.org>
> CC: Russell King <linux@armlinux.org.uk>
> CC: Will Deacon <will.deacon@arm.com>
> CC: stable@vger.kernel.org # v4.16+
> CC: linux-api@vger.kernel.org
> ---
>  kernel/sched/membarrier.c | 27 +++++++++++++++++++++------
>  1 file changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
> index 76e0eaf4654e..305fdcc4c5f7 100644
> --- a/kernel/sched/membarrier.c
> +++ b/kernel/sched/membarrier.c
> @@ -81,12 +81,27 @@ static int membarrier_global_expedited(void)
> 
>  		rcu_read_lock();
>  		p = task_rcu_dereference(&cpu_rq(cpu)->curr);
> -		if (p && p->mm && (atomic_read(&p->mm->membarrier_state) &
> -				   MEMBARRIER_STATE_GLOBAL_EXPEDITED)) {
> -			if (!fallback)
> -				__cpumask_set_cpu(cpu, tmpmask);
> -			else
> -				smp_call_function_single(cpu, ipi_mb, NULL, 1);
> +		/*
> +		 * Skip this CPU if the runqueue's current task is NULL or if
> +		 * it is a kernel thread.
> +		 */
> +		if (p && READ_ONCE(p->mm)) {
> +			bool mm_match;
> +
> +			/*
> +			 * Read p->mm and access membarrier_state while holding
> +			 * the task lock to ensure existence of mm.
> +			 */
> +			task_lock(p);
> +			mm_match = p->mm && (atomic_read(&p->mm->membarrier_state) &

Are we guaranteed that this p->mm will be the same as the one loaded via
READ_ONCE() above?  Either way, wouldn't it be better to READ_ONCE() it a
single time and use the same value everywhere?

							Thanx, Paul

> +					     MEMBARRIER_STATE_GLOBAL_EXPEDITED);
> +			task_unlock(p);
> +			if (mm_match) {
> +				if (!fallback)
> +					__cpumask_set_cpu(cpu, tmpmask);
> +				else
> +					smp_call_function_single(cpu, ipi_mb, NULL, 1);
> +			}
>  		}
>  		rcu_read_unlock();
>  	}
> -- 
> 2.17.1
> 

Re: [PATCH] Fix: membarrier: racy access to p->mm in membarrier_global_expedited()

[Jann Horn]

On Mon, Jan 28, 2019 at 11:39 PM Paul E. McKenney <paulmck@linux.ibm.com> wrote:
> On Mon, Jan 28, 2019 at 05:07:07PM -0500, Mathieu Desnoyers wrote:
> > Jann Horn identified a racy access to p->mm in the global expedited
> > command of the membarrier system call.
> >
> > The suggested fix is to hold the task_lock() around the accesses to
> > p->mm and to the mm_struct membarrier_state field to guarantee the
> > existence of the mm_struct.
> >
> > Link: https://lore.kernel.org/lkml/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Yu3cw@mail.gmail.com
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
[...]
> > --- a/kernel/sched/membarrier.c
> > +++ b/kernel/sched/membarrier.c
> > @@ -81,12 +81,27 @@ static int membarrier_global_expedited(void)
> >
> >               rcu_read_lock();
> >               p = task_rcu_dereference(&cpu_rq(cpu)->curr);
> > -             if (p && p->mm && (atomic_read(&p->mm->membarrier_state) &
> > -                                MEMBARRIER_STATE_GLOBAL_EXPEDITED)) {
> > -                     if (!fallback)
> > -                             __cpumask_set_cpu(cpu, tmpmask);
> > -                     else
> > -                             smp_call_function_single(cpu, ipi_mb, NULL, 1);
> > +             /*
> > +              * Skip this CPU if the runqueue's current task is NULL or if
> > +              * it is a kernel thread.
> > +              */
> > +             if (p && READ_ONCE(p->mm)) {
> > +                     bool mm_match;
> > +
> > +                     /*
> > +                      * Read p->mm and access membarrier_state while holding
> > +                      * the task lock to ensure existence of mm.
> > +                      */
> > +                     task_lock(p);
> > +                     mm_match = p->mm && (atomic_read(&p->mm->membarrier_state) &
>
> Are we guaranteed that this p->mm will be the same as the one loaded via
> READ_ONCE() above?

No; the way I read it, that's just an optimization and has no effect
on correctness.

> Either way, wouldn't it be better to READ_ONCE() it a
> single time and use the same value everywhere?

No; the first READ_ONCE() returns a pointer that you can't access
because it wasn't read under a lock. You can only use it for a NULL
check.

Re: [PATCH] Fix: membarrier: racy access to p->mm in membarrier_global_expedited()

[Mathieu Desnoyers]

----- On Jan 28, 2019, at 5:39 PM, paulmck paulmck@linux.ibm.com wrote:

> On Mon, Jan 28, 2019 at 05:07:07PM -0500, Mathieu Desnoyers wrote:
>> Jann Horn identified a racy access to p->mm in the global expedited
>> command of the membarrier system call.
>> 
>> The suggested fix is to hold the task_lock() around the accesses to
>> p->mm and to the mm_struct membarrier_state field to guarantee the
>> existence of the mm_struct.
>> 
>> Link:
>> https://lore.kernel.org/lkml/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Yu3cw@mail.gmail.com
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> Tested-by: Jann Horn <jannh@google.com>
>> CC: Jann Horn <jannh@google.com>
>> CC: Thomas Gleixner <tglx@linutronix.de>
>> CC: Peter Zijlstra (Intel) <peterz@infradead.org>
>> CC: Ingo Molnar <mingo@kernel.org>
>> CC: Andrea Parri <parri.andrea@gmail.com>
>> CC: Andy Lutomirski <luto@kernel.org>
>> CC: Avi Kivity <avi@scylladb.com>
>> CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>> CC: Boqun Feng <boqun.feng@gmail.com>
>> CC: Dave Watson <davejwatson@fb.com>
>> CC: David Sehr <sehr@google.com>
>> CC: H. Peter Anvin <hpa@zytor.com>
>> CC: Linus Torvalds <torvalds@linux-foundation.org>
>> CC: Maged Michael <maged.michael@gmail.com>
>> CC: Michael Ellerman <mpe@ellerman.id.au>
>> CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>> CC: Paul Mackerras <paulus@samba.org>
>> CC: Russell King <linux@armlinux.org.uk>
>> CC: Will Deacon <will.deacon@arm.com>
>> CC: stable@vger.kernel.org # v4.16+
>> CC: linux-api@vger.kernel.org
>> ---
>>  kernel/sched/membarrier.c | 27 +++++++++++++++++++++------
>>  1 file changed, 21 insertions(+), 6 deletions(-)
>> 
>> diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
>> index 76e0eaf4654e..305fdcc4c5f7 100644
>> --- a/kernel/sched/membarrier.c
>> +++ b/kernel/sched/membarrier.c
>> @@ -81,12 +81,27 @@ static int membarrier_global_expedited(void)
>> 
>>  		rcu_read_lock();
>>  		p = task_rcu_dereference(&cpu_rq(cpu)->curr);
>> -		if (p && p->mm && (atomic_read(&p->mm->membarrier_state) &
>> -				   MEMBARRIER_STATE_GLOBAL_EXPEDITED)) {
>> -			if (!fallback)
>> -				__cpumask_set_cpu(cpu, tmpmask);
>> -			else
>> -				smp_call_function_single(cpu, ipi_mb, NULL, 1);
>> +		/*
>> +		 * Skip this CPU if the runqueue's current task is NULL or if
>> +		 * it is a kernel thread.
>> +		 */
>> +		if (p && READ_ONCE(p->mm)) {
>> +			bool mm_match;
>> +
>> +			/*
>> +			 * Read p->mm and access membarrier_state while holding
>> +			 * the task lock to ensure existence of mm.
>> +			 */
>> +			task_lock(p);
>> +			mm_match = p->mm && (atomic_read(&p->mm->membarrier_state) &
> 
> Are we guaranteed that this p->mm will be the same as the one loaded via
> READ_ONCE() above?  Either way, wouldn't it be better to READ_ONCE() it a
> single time and use the same value everywhere?

The first "READ_ONCE()" above is _outside_ of the task_lock() critical section.
Those two accesses _can_ load two different pointers, and this is why we
need to re-read the p->mm pointer within the task_lock() critical section to
ensure existence of the mm_struct that we use.

If we move the READ_ONCE() into the task_lock(), we need to uselessly
take a lock before we can skip kernel threads.

If we lead the READ_ONCE() outside the task_lock(), then p->mm can be updated
between the READ_ONCE() and reference to the mm_struct content within the
task_lock(), which is racy and does not guarantee its existence.

Or am I missing your point ?

Thanks,

Mathieu


> 
>							Thanx, Paul
> 
>> +					     MEMBARRIER_STATE_GLOBAL_EXPEDITED);
>> +			task_unlock(p);
>> +			if (mm_match) {
>> +				if (!fallback)
>> +					__cpumask_set_cpu(cpu, tmpmask);
>> +				else
>> +					smp_call_function_single(cpu, ipi_mb, NULL, 1);
>> +			}
>>  		}
>>  		rcu_read_unlock();
>>  	}
>> --
>> 2.17.1

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Re: [PATCH] Fix: membarrier: racy access to p->mm in membarrier_global_expedited()

[Paul E. McKenney]

On Mon, Jan 28, 2019 at 11:45:32PM +0100, Jann Horn wrote:
> On Mon, Jan 28, 2019 at 11:39 PM Paul E. McKenney <paulmck@linux.ibm.com> wrote:
> > On Mon, Jan 28, 2019 at 05:07:07PM -0500, Mathieu Desnoyers wrote:
> > > Jann Horn identified a racy access to p->mm in the global expedited
> > > command of the membarrier system call.
> > >
> > > The suggested fix is to hold the task_lock() around the accesses to
> > > p->mm and to the mm_struct membarrier_state field to guarantee the
> > > existence of the mm_struct.
> > >
> > > Link: https://lore.kernel.org/lkml/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Yu3cw@mail.gmail.com
> > > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> [...]
> > > --- a/kernel/sched/membarrier.c
> > > +++ b/kernel/sched/membarrier.c
> > > @@ -81,12 +81,27 @@ static int membarrier_global_expedited(void)
> > >
> > >               rcu_read_lock();
> > >               p = task_rcu_dereference(&cpu_rq(cpu)->curr);
> > > -             if (p && p->mm && (atomic_read(&p->mm->membarrier_state) &
> > > -                                MEMBARRIER_STATE_GLOBAL_EXPEDITED)) {
> > > -                     if (!fallback)
> > > -                             __cpumask_set_cpu(cpu, tmpmask);
> > > -                     else
> > > -                             smp_call_function_single(cpu, ipi_mb, NULL, 1);
> > > +             /*
> > > +              * Skip this CPU if the runqueue's current task is NULL or if
> > > +              * it is a kernel thread.
> > > +              */
> > > +             if (p && READ_ONCE(p->mm)) {
> > > +                     bool mm_match;
> > > +
> > > +                     /*
> > > +                      * Read p->mm and access membarrier_state while holding
> > > +                      * the task lock to ensure existence of mm.
> > > +                      */
> > > +                     task_lock(p);
> > > +                     mm_match = p->mm && (atomic_read(&p->mm->membarrier_state) &
> >
> > Are we guaranteed that this p->mm will be the same as the one loaded via
> > READ_ONCE() above?
> 
> No; the way I read it, that's just an optimization and has no effect
> on correctness.
> 
> > Either way, wouldn't it be better to READ_ONCE() it a
> > single time and use the same value everywhere?
> 
> No; the first READ_ONCE() returns a pointer that you can't access
> because it wasn't read under a lock. You can only use it for a NULL
> check.

Ah, of course!  Thank you both!

							Thanx, Paul