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/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Y... 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(); }
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/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Y... 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);elsesmp_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);elsesmp_call_function_single(cpu, ipi_mb, NULL, 1);}-- 2.17.1
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/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Y... 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);elsesmp_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.
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/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Y... 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);elsesmp_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
----- 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/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Y... 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);elsesmp_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);elsesmp_call_function_single(cpu, ipi_mb, NULL, 1);}-- 2.17.1
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Jann Horn -
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