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Subject[PATCH 1/4] perf/ring_buffer: Fix exposing a temporarily decreased data_head.
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.

Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: ef60777c9abd ("perf: Optimize the perf_output() path by removing IRQ-disables")
Signed-off-by: Yabin Cui <yabinc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20190516184010.167903-1-yabinc@google.com
---
kernel/events/ring_buffer.c | 24 ++++++++++++++++++++----
1 file changed, 20 insertions(+), 4 deletions(-)

--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -51,11 +51,18 @@ static void perf_output_put_handle(struc
head = local_read(&rb->head);

/*
- * IRQ/NMI can happen here, which means we can miss a head update.
+ * IRQ/NMI can happen here and advance @rb->head, causing our
+ * load above to be stale.
*/

- if (!local_dec_and_test(&rb->nest))
+ /*
+ * If this isn't the outermost nesting, we don't have to update
+ * @rb->user_page->data_head.
+ */
+ if (local_read(&rb->nest) > 1) {
+ local_dec(&rb->nest);
goto out;
+ }

/*
* Since the mmap() consumer (userspace) can run on a different CPU:
@@ -87,9 +94,18 @@ static void perf_output_put_handle(struc
rb->user_page->data_head = head;

/*
- * Now check if we missed an update -- rely on previous implied
- * compiler barriers to force a re-read.
+ * We must publish the head before decrementing the nest count,
+ * otherwise an IRQ/NMI can publish a more recent head value and our
+ * write will (temporarily) publish a stale value.
+ */
+ barrier();
+ local_set(&rb->nest, 0);
+
+ /*
+ * Ensure we decrement @rb->nest before we validate the @rb->head.
+ * Otherwise we cannot be sure we caught the 'last' nested update.
*/
+ barrier();
if (unlikely(head != local_read(&rb->head))) {
local_inc(&rb->nest);
goto again;

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