Messages in this thread Patch in this message | | | Date | Thu, 16 May 2019 11:40:10 -0700 | Subject | [PATCH v2] perf/ring_buffer: Fix exposing a temporarily decreased data_head. | From | Yabin Cui <> |
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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> ---
v1 -> v2: change rb->nest from local_t to unsigned int, and add barriers.
--- kernel/events/internal.h | 2 +- kernel/events/ring_buffer.c | 24 ++++++++++++++++++------ 2 files changed, 19 insertions(+), 7 deletions(-)
diff --git a/kernel/events/internal.h b/kernel/events/internal.h index 79c47076700a..0a8c003b9bcf 100644 --- a/kernel/events/internal.h +++ b/kernel/events/internal.h @@ -24,7 +24,7 @@ struct ring_buffer { atomic_t poll; /* POLL_ for wakeups */ local_t head; /* write position */ - local_t nest; /* nested writers */ + unsigned int nest; /* nested writers */ local_t events; /* event limit */ local_t wakeup; /* wakeup stamp */ local_t lost; /* nr records lost */ diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c index 674b35383491..c677beb01fb1 100644 --- a/kernel/events/ring_buffer.c +++ b/kernel/events/ring_buffer.c @@ -38,7 +38,8 @@ static void perf_output_get_handle(struct perf_output_handle *handle) struct ring_buffer *rb = handle->rb; preempt_disable(); - local_inc(&rb->nest); + rb->nest++; + barrier(); handle->wakeup = local_read(&rb->wakeup); } @@ -54,8 +55,10 @@ 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 (rb->nest > 1) { + rb->nest--; goto out; + } /* * Since the mmap() consumer (userspace) can run on a different CPU: @@ -84,14 +87,23 @@ static void perf_output_put_handle(struct perf_output_handle *handle) * See perf_output_begin(). */ smp_wmb(); /* B, matches C */ - rb->user_page->data_head = head; + WRITE_ONCE(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. + */ + WRITE_ONCE(rb->nest, 0); /* - * Now check if we missed an update -- rely on previous implied - * compiler barriers to force a re-read. + * Now check if we missed an update -- use barrier() to force a + * re-read. */ + barrier(); if (unlikely(head != local_read(&rb->head))) { - local_inc(&rb->nest); + rb->nest++; + barrier(); goto again; } -- 2.21.0.1020.gf2820cf01a-goog
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