Re: [perf] more perf_fuzzer memory corruption

From: Peter Zijlstra
Date: Fri May 02 2014 - 12:14:42 EST

On Thu, May 01, 2014 at 02:49:01PM -0400, Vince Weaver wrote:
> It is a rance condition of sorts, because it's just a 10us or so
> interleaving of calls that causes the bug to happen or not.
>
> In the good trace:
>
> [parent] __perf_event_task_sched_out (and hence perf_swevent_del)
> [child] perf_release
>
> In the buggy trace:
>
> [child] perf_release
> [parent] __perf_event_task_sched_out (perf_swevent_del never happens)
>

Can you give this a spin?

---
Subject: perf: Fix race in removing an event
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Fri May 2 16:56:01 CEST 2014

When removing a (sibling) event we do:

raw_spin_lock_irq(&ctx->lock);
perf_group_detach(event);
raw_spin_unlock_irq(&ctx->lock);

<hole>

perf_remove_from_context(event);
raw_spin_lock_irq(&ctx->lock);
...
raw_spin_unlock_irq(&ctx->lock);

Now, assuming the event is a sibling, it will be 'unreachable' for
things like ctx_sched_out() because that iterates the
groups->siblings, and we just unhooked the sibling.

So, if during <hole> we get ctx_sched_out(), it will miss the event
and not call event_sched_out() on it, leaving it programmed on the
PMU.

The subsequent perf_remove_from_context() call will find the ctx is
inactive and only call list_del_event() to remove the event from all
other lists.

Hereafter we can proceed to free the event; while still programmed!

Close this hole by moving perf_group_detach() inside the same
ctx->lock region(s) perf_remove_from_context() has.

The condition on inherited events only in __perf_event_exit_task() is
likely complete crap because non-inherited events are part of groups
too and we're tearing down just the same. But leave that for another
patch.

Reported-by: Vince Weaver <vincent.weaver@xxxxxxxxx>
Much-staring-at-traces-by: Vince Weaver <vincent.weaver@xxxxxxxxx>
Much-staring-at-traces-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
kernel/events/core.c | 41 +++++++++++++++++++++++------------------
1 file changed, 23 insertions(+), 18 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1444,6 +1444,11 @@ group_sched_out(struct perf_event *group
cpuctx->exclusive = 0;
}

+struct remove_event {
+ struct perf_event *event;
+ bool detach_group;
+};
+
/*
* Cross CPU call to remove a performance event
*
@@ -1452,12 +1457,15 @@ group_sched_out(struct perf_event *group
*/
static int __perf_remove_from_context(void *info)
{
- struct perf_event *event = info;
+ struct remove_event *re = info;
+ struct perf_event *event = re->event;
struct perf_event_context *ctx = event->ctx;
struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);

raw_spin_lock(&ctx->lock);
event_sched_out(event, cpuctx, ctx);
+ if (re->detach_group)
+ perf_group_detach(event);
list_del_event(event, ctx);
if (!ctx->nr_events && cpuctx->task_ctx == ctx) {
ctx->is_active = 0;
@@ -1482,10 +1490,14 @@ static int __perf_remove_from_context(vo
* When called from perf_event_exit_task, it's OK because the
* context has been detached from its task.
*/
-static void perf_remove_from_context(struct perf_event *event)
+static void perf_remove_from_context(struct perf_event *event, bool detach_group)
{
struct perf_event_context *ctx = event->ctx;
struct task_struct *task = ctx->task;
+ struct remove_event re = {
+ .event = event,
+ .detach_group = detach_group,
+ };

lockdep_assert_held(&ctx->mutex);

@@ -1494,12 +1506,12 @@ static void perf_remove_from_context(str
* Per cpu events are removed via an smp call and
* the removal is always successful.
*/
- cpu_function_call(event->cpu, __perf_remove_from_context, event);
+ cpu_function_call(event->cpu, __perf_remove_from_context, &re);
return;
}

retry:
- if (!task_function_call(task, __perf_remove_from_context, event))
+ if (!task_function_call(task, __perf_remove_from_context, &re))
return;

raw_spin_lock_irq(&ctx->lock);
@@ -1516,6 +1528,8 @@ static void perf_remove_from_context(str
* Since the task isn't running, its safe to remove the event, us
* holding the ctx->lock ensures the task won't get scheduled in.
*/
+ if (detach_group)
+ perf_group_detach(event);
list_del_event(event, ctx);
raw_spin_unlock_irq(&ctx->lock);
}
@@ -3285,10 +3299,7 @@ int perf_event_release_kernel(struct per
* to trigger the AB-BA case.
*/
mutex_lock_nested(&ctx->mutex, SINGLE_DEPTH_NESTING);
- raw_spin_lock_irq(&ctx->lock);
- perf_group_detach(event);
- raw_spin_unlock_irq(&ctx->lock);
- perf_remove_from_context(event);
+ perf_remove_from_context(event, true);
mutex_unlock(&ctx->mutex);

free_event(event);
@@ -7180,7 +7191,7 @@ SYSCALL_DEFINE5(perf_event_open,
struct perf_event_context *gctx = group_leader->ctx;

mutex_lock(&gctx->mutex);
- perf_remove_from_context(group_leader);
+ perf_remove_from_context(group_leader, false);

/*
* Removing from the context ends up with disabled
@@ -7190,7 +7201,7 @@ SYSCALL_DEFINE5(perf_event_open,
perf_event__state_init(group_leader);
list_for_each_entry(sibling, &group_leader->sibling_list,
group_entry) {
- perf_remove_from_context(sibling);
+ perf_remove_from_context(sibling, false);
perf_event__state_init(sibling);
put_ctx(gctx);
}
@@ -7320,7 +7331,7 @@ void perf_pmu_migrate_context(struct pmu
mutex_lock(&src_ctx->mutex);
list_for_each_entry_safe(event, tmp, &src_ctx->event_list,
event_entry) {
- perf_remove_from_context(event);
+ perf_remove_from_context(event, false);
unaccount_event_cpu(event, src_cpu);
put_ctx(src_ctx);
list_add(&event->migrate_entry, &events);
@@ -7382,13 +7393,7 @@ __perf_event_exit_task(struct perf_event
struct perf_event_context *child_ctx,
struct task_struct *child)
{
- if (child_event->parent) {
- raw_spin_lock_irq(&child_ctx->lock);
- perf_group_detach(child_event);
- raw_spin_unlock_irq(&child_ctx->lock);
- }
-
- perf_remove_from_context(child_event);
+ perf_remove_from_context(child_event, !!child_event->parent);

/*
* It can happen that the parent exits first, and has events

Attachment: pgpTDqv9kjxOd.pgp
Description: PGP signature