[PATCH] 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>
---
 kernel/events/ring_buffer.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 674b35383491..0b9aefe13b04 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -54,8 +54,10 @@ 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 (local_read(&rb->nest) > 1) {
+		local_dec(&rb->nest);
 		goto out;
+	}
 
 	/*
 	 * Since the mmap() consumer (userspace) can run on a different CPU:
@@ -86,6 +88,13 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
 	smp_wmb(); /* B, matches C */
 	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.
+	 */
+	local_set(&rb->nest, 0);
+
 	/*
 	 * Now check if we missed an update -- rely on previous implied
 	 * compiler barriers to force a re-read.
-- 
2.21.0.1020.gf2820cf01a-goog