[PATCH v2] perf/ring_buffer: Fix exposing a temporarily decreased data_head.

[Yabin Cui <yabinc@google.com>]

In perf_output_put_handle(), an IRQ/NMI can happen in below location and
write records to the same ring buffer:
	...
	local_dec_and_test(&rb->nest)
	...                          <-- an IRQ/NMI can happen here
	rb->user_page->data_head = head;
	...

In this case, a value A is written to data_head in the IRQ, then a value
B is written to data_head after the IRQ. And A > B. As a result,
data_head is temporarily decreased from A to B. And a reader may see
data_head < data_tail if it read the buffer frequently enough, which
creates unexpected behaviors.

This can be fixed by moving dec(&rb->nest) to after updating data_head,
which prevents the IRQ/NMI above from updating data_head.

Signed-off-by: Yabin Cui <yabinc@google.com>
---

v1 -> v2: change rb->nest from local_t to unsigned int, and add barriers.

---
 kernel/events/internal.h    |  2 +-
 kernel/events/ring_buffer.c | 24 ++++++++++++++++++------
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 79c47076700a..0a8c003b9bcf 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -24,7 +24,7 @@ struct ring_buffer {
 	atomic_t			poll;		/* POLL_ for wakeups */
 
 	local_t				head;		/* write position    */
-	local_t				nest;		/* nested writers    */
+	unsigned int			nest;		/* nested writers    */
 	local_t				events;		/* event limit       */
 	local_t				wakeup;		/* wakeup stamp      */
 	local_t				lost;		/* nr records lost   */
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 674b35383491..c677beb01fb1 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -38,7 +38,8 @@ static void perf_output_get_handle(struct perf_output_handle *handle)
 	struct ring_buffer *rb = handle->rb;
 
 	preempt_disable();
-	local_inc(&rb->nest);
+	rb->nest++;
+	barrier();
 	handle->wakeup = local_read(&rb->wakeup);
 }
 
@@ -54,8 +55,10 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
 	 * IRQ/NMI can happen here, which means we can miss a head update.
 	 */
 
-	if (!local_dec_and_test(&rb->nest))
+	if (rb->nest > 1) {
+		rb->nest--;
 		goto out;
+	}
 
 	/*
 	 * Since the mmap() consumer (userspace) can run on a different CPU:
@@ -84,14 +87,23 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
 	 * See perf_output_begin().
 	 */
 	smp_wmb(); /* B, matches C */
-	rb->user_page->data_head = head;
+	WRITE_ONCE(rb->user_page->data_head, head);
+
+	/*
+	 * Clear rb->nest after updating data_head. This prevents IRQ/NMI from
+	 * updating data_head before us. If that happens, we will expose a
+	 * temporarily decreased data_head.
+	 */
+	WRITE_ONCE(rb->nest, 0);
 
 	/*
-	 * Now check if we missed an update -- rely on previous implied
-	 * compiler barriers to force a re-read.
+	 * Now check if we missed an update -- use barrier() to force a
+	 * re-read.
 	 */
+	barrier();
 	if (unlikely(head != local_read(&rb->head))) {
-		local_inc(&rb->nest);
+		rb->nest++;
+		barrier();
 		goto again;
 	}
 
-- 
2.21.0.1020.gf2820cf01a-goog