* [PATCH] perf core: apply calculated padding to PERF_SAMPLE_RAW output
@ 2020-05-19 13:26 Timo Beckers
2023-03-14 19:27 ` Peter Zijlstra
0 siblings, 1 reply; 3+ messages in thread
From: Timo Beckers @ 2020-05-19 13:26 UTC (permalink / raw)
Cc: timo, Peter Zijlstra, Ingo Molnar, Arnaldo Carvalho de Melo,
Mark Rutland, Alexander Shishkin, Jiri Olsa, Namhyung Kim,
open list:PERFORMANCE EVENTS SUBSYSTEM
Zero the amount of padding bytes determined in perf_prepare_sample().
This prevents garbage being read from the ring buffer after it has wrapped
the page boundary at least once.
Signed-off-by: Timo Beckers <timo@incline.eu>
---
kernel/events/core.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 80cf996a7f19..d4e0b003ece0 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -6807,8 +6807,16 @@ void perf_output_sample(struct perf_output_handle *handle,
break;
frag = frag->next;
} while (1);
- if (frag->pad)
- __output_skip(handle, NULL, frag->pad);
+ /*
+ * The padding value is determined in
+ * perf_prepare_sample() and is not
+ * expected to exceed u64.
+ */
+ if (frag->pad) {
+ u64 zero = 0;
+
+ __output_copy(handle, &zero, frag->pad);
+ }
} else {
struct {
u32 size;
--
2.26.2
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] perf core: apply calculated padding to PERF_SAMPLE_RAW output
2020-05-19 13:26 [PATCH] perf core: apply calculated padding to PERF_SAMPLE_RAW output Timo Beckers
@ 2023-03-14 19:27 ` Peter Zijlstra
2023-03-16 16:32 ` Timo Beckers
0 siblings, 1 reply; 3+ messages in thread
From: Peter Zijlstra @ 2023-03-14 19:27 UTC (permalink / raw)
To: Timo Beckers
Cc: Ingo Molnar, Arnaldo Carvalho de Melo, Mark Rutland,
Alexander Shishkin, Jiri Olsa, Namhyung Kim,
open list:PERFORMANCE EVENTS SUBSYSTEM
On Tue, May 19, 2020 at 03:26:16PM +0200, Timo Beckers wrote:
> Zero the amount of padding bytes determined in perf_prepare_sample().
> This prevents garbage being read from the ring buffer after it has wrapped
> the page boundary at least once.
But it's user garbage, right? And they should be unconsumed anyway.
> Signed-off-by: Timo Beckers <timo@incline.eu>
> ---
> kernel/events/core.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 80cf996a7f19..d4e0b003ece0 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6807,8 +6807,16 @@ void perf_output_sample(struct perf_output_handle *handle,
> break;
> frag = frag->next;
> } while (1);
> - if (frag->pad)
> - __output_skip(handle, NULL, frag->pad);
> + /*
> + * The padding value is determined in
> + * perf_prepare_sample() and is not
> + * expected to exceed u64.
> + */
> + if (frag->pad) {
> + u64 zero = 0;
> +
> + __output_copy(handle, &zero, frag->pad);
> + }
> } else {
> struct {
> u32 size;
> --
> 2.26.2
>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] perf core: apply calculated padding to PERF_SAMPLE_RAW output
2023-03-14 19:27 ` Peter Zijlstra
@ 2023-03-16 16:32 ` Timo Beckers
0 siblings, 0 replies; 3+ messages in thread
From: Timo Beckers @ 2023-03-16 16:32 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Ingo Molnar, Arnaldo Carvalho de Melo, Mark Rutland,
Alexander Shishkin, Jiri Olsa, Namhyung Kim,
open list:PERFORMANCE EVENTS SUBSYSTEM
On Tue, 14 Mar 2023 at 20:27, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, May 19, 2020 at 03:26:16PM +0200, Timo Beckers wrote:
> > Zero the amount of padding bytes determined in perf_prepare_sample().
> > This prevents garbage being read from the ring buffer after it has wrapped
> > the page boundary at least once.
>
> But it's user garbage, right?
Hey Peter, correct. Not a security issue, but rather a usability one. (IMO)
It would be nice if the receiver could verify if the trailing bytes
are all zeroes
after interpreting the payload. (I deal with Go interop; C<->Go struct
alignment behaviour differs subtly, so this helps debugging)
I know the ship has sailed and it's been like this for a long time, but getting
it fixed in a non-invasive way would be neat if the performance penalty is
not too steep. I think Jiri was playing around with some benchmarks.
> And they should be unconsumed anyway.
Well, not quite. perf_event_sample.size contains the size of .data including
padding, so the reader always needs to copy out the full event, which
potentially includes garbage. .data is completely opaque from a generic
perf reader POV, so it can't automatically trim it or choose not to read it.
Haven't looked at the kernel side in a while, but maybe setting .size to
the length of the input on the bpf side would be a better solution? Then no
zeroing needs to be done. I assume there's no strong need to increase
.size in 8-byte aligned steps, as I currently see values like 4, 12,
20, 28, etc.
Please correct me if I'm wrong.
Thanks,
Timo
>
> > Signed-off-by: Timo Beckers <timo@incline.eu>
> > ---
> > kernel/events/core.c | 12 ++++++++++--
> > 1 file changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 80cf996a7f19..d4e0b003ece0 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -6807,8 +6807,16 @@ void perf_output_sample(struct perf_output_handle *handle,
> > break;
> > frag = frag->next;
> > } while (1);
> > - if (frag->pad)
> > - __output_skip(handle, NULL, frag->pad);
> > + /*
> > + * The padding value is determined in
> > + * perf_prepare_sample() and is not
> > + * expected to exceed u64.
> > + */
> > + if (frag->pad) {
> > + u64 zero = 0;
> > +
> > + __output_copy(handle, &zero, frag->pad);
> > + }
> > } else {
> > struct {
> > u32 size;
> > --
> > 2.26.2
> >
^ permalink raw reply [flat|nested] 3+ messages in thread
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2020-05-19 13:26 [PATCH] perf core: apply calculated padding to PERF_SAMPLE_RAW output Timo Beckers
2023-03-14 19:27 ` Peter Zijlstra
2023-03-16 16:32 ` Timo Beckers
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