queue-4.19/perf-ring_buffer-fix-exposing-a-temporarily-decrease.patch

   1 From b7f7f5b54f5b1562d24115a9b2d98b2ce98e9074 Mon Sep 17 00:00:00 2001
   2 From: Yabin Cui <yabinc@google.com>
   3 Date: Fri, 17 May 2019 13:52:31 +0200
   4 Subject: perf/ring_buffer: Fix exposing a temporarily decreased data_head
   5
   6 [ Upstream commit 1b038c6e05ff70a1e66e3e571c2e6106bdb75f53 ]
   7
   8 In perf_output_put_handle(), an IRQ/NMI can happen in below location and
   9 write records to the same ring buffer:
  10
  11         ...
  12         local_dec_and_test(&rb->nest)
  13         ...                          <-- an IRQ/NMI can happen here
  14         rb->user_page->data_head = head;
  15         ...
  16
  17 In this case, a value A is written to data_head in the IRQ, then a value
  18 B is written to data_head after the IRQ. And A > B. As a result,
  19 data_head is temporarily decreased from A to B. And a reader may see
  20 data_head < data_tail if it read the buffer frequently enough, which
  21 creates unexpected behaviors.
  22
  23 This can be fixed by moving dec(&rb->nest) to after updating data_head,
  24 which prevents the IRQ/NMI above from updating data_head.
  25
  26 [ Split up by peterz. ]
  27
  28 Signed-off-by: Yabin Cui <yabinc@google.com>
  29 Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
  30 Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
  31 Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
  32 Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
  33 Cc: Jiri Olsa <jolsa@redhat.com>
  34 Cc: Linus Torvalds <torvalds@linux-foundation.org>
  35 Cc: Namhyung Kim <namhyung@kernel.org>
  36 Cc: Peter Zijlstra <peterz@infradead.org>
  37 Cc: Stephane Eranian <eranian@google.com>
  38 Cc: Thomas Gleixner <tglx@linutronix.de>
  39 Cc: Vince Weaver <vincent.weaver@maine.edu>
  40 Cc: mark.rutland@arm.com
  41 Fixes: ef60777c9abd ("perf: Optimize the perf_output() path by removing IRQ-disables")
  42 Link: http://lkml.kernel.org/r/20190517115418.224478157@infradead.org
  43 Signed-off-by: Ingo Molnar <mingo@kernel.org>
  44 Signed-off-by: Sasha Levin <sashal@kernel.org>
  45 ---
  46  kernel/events/ring_buffer.c | 24 ++++++++++++++++++++----
  47  1 file changed, 20 insertions(+), 4 deletions(-)
  48
  49 diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
  50 index 99c7f199f2d4..31edf1f39cca 100644
  51 --- a/kernel/events/ring_buffer.c
  52 +++ b/kernel/events/ring_buffer.c
  53 @@ -52,11 +52,18 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
  54         head = local_read(&rb->head);
  55
  56         /*
  57 -        * IRQ/NMI can happen here, which means we can miss a head update.
  58 +        * IRQ/NMI can happen here and advance @rb->head, causing our
  59 +        * load above to be stale.
  60          */
  61
  62 -       if (!local_dec_and_test(&rb->nest))
  63 +       /*
  64 +        * If this isn't the outermost nesting, we don't have to update
  65 +        * @rb->user_page->data_head.
  66 +        */
  67 +       if (local_read(&rb->nest) > 1) {
  68 +               local_dec(&rb->nest);
  69                 goto out;
  70 +       }
  71
  72         /*
  73          * Since the mmap() consumer (userspace) can run on a different CPU:
  74 @@ -88,9 +95,18 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
  75         rb->user_page->data_head = head;
  76
  77         /*
  78 -        * Now check if we missed an update -- rely on previous implied
  79 -        * compiler barriers to force a re-read.
  80 +        * We must publish the head before decrementing the nest count,
  81 +        * otherwise an IRQ/NMI can publish a more recent head value and our
  82 +        * write will (temporarily) publish a stale value.
  83 +        */
  84 +       barrier();
  85 +       local_set(&rb->nest, 0);
  86 +
  87 +       /*
  88 +        * Ensure we decrement @rb->nest before we validate the @rb->head.
  89 +        * Otherwise we cannot be sure we caught the 'last' nested update.
  90          */
  91 +       barrier();
  92         if (unlikely(head != local_read(&rb->head))) {
  93                 local_inc(&rb->nest);
  94                 goto again;
  95 --
  96 2.20.1
  97