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SubjectRe: [PATCH] perf session: Remap buf if there is no space for event
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On 30/03/2022 04:11, Denis Nikitin wrote:
> If a perf event doesn't fit into remaining buffer space return NULL to
> remap buf and fetch the event again.
> Keep the logic to error out on inadequate input from fuzzing.
>
> This fixes perf failing on ChromeOS (with 32b userspace):
>
> $ perf report -v -i perf.data
> ...
> prefetch_event: head=0x1fffff8 event->header_size=0x30, mmap_size=0x2000000: fuzzed or compressed perf.data?
> Error:
> failed to process sample
>
> Fixes: 57fc032ad643 ("perf session: Avoid infinite loop when seeing invalid header.size")
> Signed-off-by: Denis Nikitin <denik@chromium.org>
> ---
> tools/perf/util/session.c | 15 ++++++++++++---
> 1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
> index 3b8dfe603e50..45a30040ec8d 100644
> --- a/tools/perf/util/session.c
> +++ b/tools/perf/util/session.c
> @@ -2095,6 +2095,7 @@ prefetch_event(char *buf, u64 head, size_t mmap_size,
> bool needs_swap, union perf_event *error)
> {
> union perf_event *event;
> + u16 event_size;
>
> /*
> * Ensure we have enough space remaining to read
> @@ -2107,15 +2108,23 @@ prefetch_event(char *buf, u64 head, size_t mmap_size,
> if (needs_swap)
> perf_event_header__bswap(&event->header);
>
> - if (head + event->header.size <= mmap_size)
> + event_size = event->header.size;
> + if (head + event_size <= mmap_size)
> return event;
>
> /* We're not fetching the event so swap back again */
> if (needs_swap)
> perf_event_header__bswap(&event->header);
>
> - pr_debug("%s: head=%#" PRIx64 " event->header_size=%#x, mmap_size=%#zx:"
> - " fuzzed or compressed perf.data?\n",__func__, head, event->header.size, mmap_size);
> + /* Check if the event fits into the next mmapped buf. */
> + if (event_size <= mmap_size - head % page_size) {
> + /* Remap buf and fetch again. */
> + return NULL;

Hi Denis,

I tested this and it does fix the issue with a 32bit build. One concern is that the calculation to
see if it will fit in the next map is dependent on the implementation of reader__mmap(). I think it
would be possible for that to change slightly and then you could still get an infinite loop.

But I can't really see a better way to do it, and it's unlikely for reader__mmap() to be modified
to map data in a way to waste part of the buffer so it's probably fine.

Maybe you could extract a function to calculate where the new offset would be in the buffer and share
it between here and reader__mmap(). That would also make it more obvious what the 'head % page_size'
bit is for.

Either way:

Reviewed-by: James Clark <james.clark@arm.com>

> + }
> +
> + /* Invalid input. Event size should never exceed mmap_size. */
> + pr_debug("%s: head=%#" PRIx64 " event->header.size=%#x, mmap_size=%#zx:"
> + " fuzzed or compressed perf.data?\n", __func__, head, event_size, mmap_size);
>
> return error;
> }

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