Re: [PATCH v2] perf/x86: Check data address for IBS software filter

Namhyung Kim
2025-03-19T20:30:06Z

On Wed, Mar 19, 2025 at 04:24:12PM +0530, Ravi Bangoria wrote:
> Hi Namhyung,
>
> >>> IBS software filter was to filter kernel samples for regular users in
> >>> PMI handler. It checks the instruction address in the IBS register to
> >>> determine if it was in the kernel more or not.
> >>>
> >>> But it turns out that it's possible to report a kernel data address even
> >>> if the instruction address belongs to the user space. Matteo Rizzo
> >>> found that when an instruction raises an exception, IBS can report some
> >>> kernel data address like IDT while holding the faulting instruction's
> >>> RIP. To prevent an information leak, it should double check if the data
> >>> address in PERF_SAMPLE_DATA is in the kernel space as well.
> >>
> >> PERF_SAMPLE_RAW can also leak kernel data address. How about:
> >
> > Thanks for your review.
> >
> > I think RAW is different as it requires perf_event_paranoid == -1.
>
> IBS allows PERF_SAMPLE_RAW irrespective of perf_event_paranoid. e.g.:
>
> $ cat /proc/sys/kernel/perf_event_paranoid
> 2
>
> $ ./perf record -e ibs_op/swfilt=1/u --raw-samples -- make
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 1.371 MB perf.data (3957 samples) ]
>
> $ ./perf script -D | egrep -A2 "LdOp 1.*DcLinAddrValid 1" | egrep "IbsDCLinAd:\s*f"
> IbsDCLinAd: fffffe00000000e8

Oh, I thought it was enforced in the core layer but it turns out that
it's checked only by tracepoint events.

>
> We have two options:
> 1) Restrict IBS + PERF_SAMPLE_RAW to privilege users.
> 2) Remove all sensitive information from raw register dump before
> passing it to userspace. (Kernel data addresses and all physical
> addresses are the only sensitive info I suppose?).
>
> 2 is better IMO since it will allow unprivileged user to use IBS
> with full potential. wdyt?

I'm slightly inclined to #1 for simplicity and safety, but #2 is fine to
me as well.

Thanks,
Namhyung