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