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Date: Tue, 5 Jul 2022 15:07:26 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Yang Jihong <yangjihong1@...wei.com>
Cc: mingo@...hat.com, acme@...nel.org, mark.rutland@....com,
alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
namhyung@...nel.org, linux-perf-users@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] perf/core: Fix data race between
perf_event_set_output and perf_mmap_close
On Mon, Jul 04, 2022 at 05:26:04PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 04, 2022 at 08:00:06PM +0800, Yang Jihong wrote:
> > Data race exists between perf_event_set_output and perf_mmap_close.
> > The scenario is as follows:
> >
> > CPU1 CPU2
> > perf_mmap_close(event2)
> > if (atomic_dec_and_test(&event2->rb->mmap_count) // mmap_count 1 -> 0
> > detach_rest = true;
> > ioctl(event1, PERF_EVENT_IOC_SET_OUTPUT, event2)
> > perf_event_set_output(event1, event2)
> > if (!detach_rest)
> > goto out_put;
> > list_for_each_entry_rcu(event, &event2->rb->event_list, rb_entry)
> > ring_buffer_attach(event, NULL)
> > // because event1 has not been added to event2->rb->event_list,
> > // event1->rb is not set to NULL in these loops
> >
> > ring_buffer_attach(event1, event2->rb)
> > list_add_rcu(&event1->rb_entry, &event2->rb->event_list)
> >
> > The above data race causes a problem, that is, event1->rb is not NULL, but event1->rb->mmap_count is 0.
> > If the perf_mmap interface is invoked for the fd of event1, the kernel keeps in the perf_mmap infinite loop:
> >
> > again:
> > mutex_lock(&event->mmap_mutex);
> > if (event->rb) {
> > <SNIP>
> > if (!atomic_inc_not_zero(&event->rb->mmap_count)) {
> > /*
> > * Raced against perf_mmap_close() through
> > * perf_event_set_output(). Try again, hope for better
> > * luck.
> > */
> > mutex_unlock(&event->mmap_mutex);
> > goto again;
> > }
> > <SNIP>
>
> Too tired, must look again tomorrow, little feeback below.
With brain more awake I ended up with the below. Does that work?
---
kernel/events/core.c | 45 +++++++++++++++++++++++++++++++--------------
1 file changed, 31 insertions(+), 14 deletions(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4d8c335a07db..c9d32d4d2e20 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6262,10 +6262,10 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
if (!atomic_inc_not_zero(&event->rb->mmap_count)) {
/*
- * Raced against perf_mmap_close() through
- * perf_event_set_output(). Try again, hope for better
- * luck.
+ * Raced against perf_mmap_close(); remove the
+ * event and try again.
*/
+ ring_buffer_attach(event, NULL);
mutex_unlock(&event->mmap_mutex);
goto again;
}
@@ -11840,14 +11840,25 @@ static int perf_copy_attr(struct perf_event_attr __user *uattr,
goto out;
}
+static void mutex_lock_double(struct mutex *a, struct mutex *b)
+{
+ if (b < a)
+ swap(a, b);
+
+ mutex_lock(a);
+ mutex_lock_nested(b, SINGLE_DEPTH_NESTING);
+}
+
static int
perf_event_set_output(struct perf_event *event, struct perf_event *output_event)
{
struct perf_buffer *rb = NULL;
int ret = -EINVAL;
- if (!output_event)
+ if (!output_event) {
+ mutex_lock(&event->mmap_mutex);
goto set;
+ }
/* don't allow circular references */
if (event == output_event)
@@ -11885,8 +11896,15 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_event)
event->pmu != output_event->pmu)
goto out;
+ /*
+ * Hold both mmap_mutex to serialize against perf_mmap_close(). Since
+ * output_event is already on rb->event_list, and the list iteration
+ * restarts after every removal, it is guaranteed this new event is
+ * observed *OR* if output_event is already removed, it's guaranteed we
+ * observe !rb->mmap_count.
+ */
+ mutex_lock_double(&event->mmap_mutex, &output_event->mmap_mutex);
set:
- mutex_lock(&event->mmap_mutex);
/* Can't redirect output if we've got an active mmap() */
if (atomic_read(&event->mmap_count))
goto unlock;
@@ -11896,6 +11914,12 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_event)
rb = ring_buffer_get(output_event);
if (!rb)
goto unlock;
+
+ /* did we race against perf_mmap_close() */
+ if (!atomic_read(&rb->mmap_count)) {
+ ring_buffer_put(rb);
+ goto unlock;
+ }
}
ring_buffer_attach(event, rb);
@@ -11903,20 +11927,13 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_event)
ret = 0;
unlock:
mutex_unlock(&event->mmap_mutex);
+ if (output_event)
+ mutex_unlock(&output_event->mmap_mutex);
out:
return ret;
}
-static void mutex_lock_double(struct mutex *a, struct mutex *b)
-{
- if (b < a)
- swap(a, b);
-
- mutex_lock(a);
- mutex_lock_nested(b, SINGLE_DEPTH_NESTING);
-}
-
static int perf_event_set_clock(struct perf_event *event, clockid_t clk_id)
{
bool nmi_safe = false;
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