perf/ring_buffer: Fix exposing a temporarily decreased data_head [Linux 5.2]

perf/ring_buffer: Fix exposing a temporarily decreased data_head [Linux 5.2]

This Linux kernel change "perf/ring_buffer: Fix exposing a temporarily decreased data_head" is included in the Linux 5.2 release. This change is authored by Yabin Cui <yabinc [at] google.com> on Fri May 17 13:52:31 2019 +0200. The commit for this change in Linux stable tree is 1b038c6 (patch).

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.

[ Split up by peterz. ]

Signed-off-by: Yabin Cui <yabinc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: mark.rutland@arm.com
Fixes: ef60777c9abd ("perf: Optimize the perf_output() path by removing IRQ-disables")
Link: http://lkml.kernel.org/r/20190517115418.224478157@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

There are 24 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 kernel/events/ring_buffer.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 674b353..009467a 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -51,11 +51,18 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
    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(struct perf_output_handle *handle)
    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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