Re: [PATCH v2 4/5] perf/amd/ibs: Avoid calling perf_allow_kernel() from the IBS NMI handler

[Michael Petlan <mpetlan@redhat.com>]

On Mon, 16 Feb 2026, Ravi Bangoria wrote:
> Calling perf_allow_kernel() from the NMI context is unsafe and could be
> fatal. Capture the permission at event-initialization time by storing it
> in event->hw.flags, and have the NMI handler rely on that cached flag
> instead of making the call directly.
>

Hi Ravi.

I have tested this fix on a machine (below), which without this fix can
be easily crashed by recording ibs_op/l3missonly=1/ event.

With this fix, it survived 4900+ iterations without any crash.

CPU details
===========
BIOS CPU family:      107
CPU family:            26
Model:                  2
Stepping:               1

Thanks for fixing.

Cheers,
Michael

> Reported-by: Sadasivan Shaiju <sadasivan.shaiju2@amd.com>
> Fixes: 50a53b60e141d ("perf/amd/ibs: Prevent leaking sensitive data to userspace")
> Signed-off-by: Ravi Bangoria <ravi.bangoria@amd.com>

Tested-by: Michael Petlan <mpetlan@redhat.com>

> ---
>  arch/x86/events/amd/ibs.c          | 5 ++++-
>  arch/x86/events/perf_event_flags.h | 1 +
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
> index 05b7c9f2ec33..004226b52ac7 100644
> --- a/arch/x86/events/amd/ibs.c
> +++ b/arch/x86/events/amd/ibs.c
> @@ -313,6 +313,9 @@ static int perf_ibs_init(struct perf_event *event)
>  	if (ret)
>  		return ret;
>  
> +	if (perf_allow_kernel())
> +		hwc->flags |= PERF_X86_EVENT_UNPRIVILEGED;
> +
>  	if (hwc->sample_period) {
>  		if (config & perf_ibs->cnt_mask)
>  			/* raw max_cnt may not be set */
> @@ -1349,7 +1352,7 @@ static int perf_ibs_handle_irq(struct perf_ibs *perf_ibs, struct pt_regs *iregs)
>  	 * unprivileged users.
>  	 */
>  	if ((event->attr.sample_type & PERF_SAMPLE_RAW) &&
> -	    perf_allow_kernel()) {
> +	    (hwc->flags & PERF_X86_EVENT_UNPRIVILEGED)) {
>  		perf_ibs_phyaddr_clear(perf_ibs, &ibs_data);
>  	}
>  
> diff --git a/arch/x86/events/perf_event_flags.h b/arch/x86/events/perf_event_flags.h
> index 70078334e4a3..47f84ee8f540 100644
> --- a/arch/x86/events/perf_event_flags.h
> +++ b/arch/x86/events/perf_event_flags.h
> @@ -23,3 +23,4 @@ PERF_ARCH(PEBS_LAT_HYBRID,	0x0020000) /* ld and st lat for hybrid */
>  PERF_ARCH(NEEDS_BRANCH_STACK,	0x0040000) /* require branch stack setup */
>  PERF_ARCH(BRANCH_COUNTERS,	0x0080000) /* logs the counters in the extra space of each branch */
>  PERF_ARCH(ACR,			0x0100000) /* Auto counter reload */
> +PERF_ARCH(UNPRIVILEGED,		0x0200000) /* Unprivileged event (wrt perf_allow_kernel()) */
> -- 
> 2.43.0
> 
> 
>