Re: [PATCH v1 2/2] perf/core: Fake regs for leaked kernel samples

[peterz]

On Wed, Aug 05, 2020 at 02:44:54PM +0200, peterz@xxxxxxxxxxxxx wrote:

> How's this?

Clearly I didn't even hold it near a compiler...

> ---
>  kernel/events/core.c | 38 +++++++++++++++++++++++++++++++++-----
>  1 file changed, 33 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 7c436d705fbd..3e4e328b521a 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6988,23 +6988,49 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs)
>  	return callchain ?: &__empty_callchain;
>  }
>  
> +/*
> + * Due to interrupt latency (skid), we may enter the kernel before taking the
> + * PMI, even if the PMU is configured to only count user events. To avoid
> + * leaking kernel addresses, use task_pt_regs(), when available.
> + */
> +static struct pt_regs *sanitize_sample_regs(struct perf_event *event, struct pt_regs *regs)
> +{
> +	struct pt_regs *sample_regs = regs;
> +
> +	/* user only */
> +	if (!event->attr.exclude_kernel || !event->attr.exclude_hv ||
> +	    !event->attr.exclude_host   || !event->attr.exclude_guest)
> +		return sample_regs;
> +
> +	if (sample_regs(regs))
> +		return sample_regs;

That wants to he:

	if (user_regs(regs))
		return sample_regs;

> +
> +	if (!(current->flags & PF_KTHREAD)) {

s/{//

> +		sample_regs = task_pt_regs(current);
> +	else
> +		instruction_pointer_set(regs, -1L);
> +
> +	return sample_regs;
> +}