Re: [PATCH v2 3/3] powerpc/perf: Update prev_count of event to get accurate values

[Gautam Menghani <gautam@linux.ibm.com>]

On Thu, Aug 13, 2026 at 10:01:02AM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
> 
> New issues:
> - [Medium] Missing atomic cmpxchg loop in vpa_pmu_read() when updating prev_count.
> 
> Pre-existing issues:
> - [High] The VPA PMU `add` callback stores a raw pointer to `struct kvm_vcpu` in `event->pmu_private` without reference counting, leading to a Use-After-Free.
> --
> 
> --- Patch [3]: [PATCH v2 3/3] powerpc/perf: Update prev_count of event to get accurate values ---
> commit 7c392d724ba481a5282a90aed95397fac4533493
> Author: Gautam Menghani <gautam@linux.ibm.com>
> 
> powerpc/perf: Update prev_count of event to get accurate values
> 
> This patch updates the VPA PMU read function to keep event->hw.prev_count
> current. This prevents stale values from being returned on consecutive reads
> while a process is running on a CPU.
> 
> > diff --git a/arch/powerpc/perf/vpa-pmu.c b/arch/powerpc/perf/vpa-pmu.c
> > index e79d98447c743..efc39f71553bf 100644
> > --- a/arch/powerpc/perf/vpa-pmu.c
> > +++ b/arch/powerpc/perf/vpa-pmu.c
> > @@ -156,6 +156,7 @@ static void vpa_pmu_read(struct perf_event *event)
> >  	final_data = new_data - prev_data;
> >  
> >  	local64_add(final_data, &event->count);
> > +	local64_set(&event->hw.prev_count, new_data);
> 
> [Severity: Medium]
> Is it safe to use local64_set() here without a local64_cmpxchg() loop?
> 
> If vpa_pmu_read() is interrupted by an NMI (such as a hardware PMI) and a BPF
> program attached to the NMI reads the same event via bpf_perf_event_read(),
> the nested read will calculate its delta and update prev_count.
> 
> When the outer vpa_pmu_read() resumes, it could calculate its delta using the
> old prev_data (resulting in double-counting the inner delta) and then
> non-atomically overwrite prev_count.
> 
> Should this use a local64_cmpxchg() loop to safely update prev_count?

Yes, valid point.

> 
> >  }
> >  
> >  static void vpa_pmu_del(struct perf_event *event, int flags)
> 
> [Severity: High]
> This is a pre-existing issue, but does this code leave a stale vcpu pointer
> in event->pmu_private that can lead to a use-after-free?
> 
> Looking at vpa_pmu_add(), a raw pointer to struct kvm_vcpu is stored without
> taking a reference:
> 
> arch/powerpc/perf/vpa-pmu.c:vpa_pmu_add() {
>     ...
>     vcpu = local_paca->kvm_hstate.kvm_vcpu;
>     if (!vcpu)
>         goto out;
>     ...
>     event->pmu_private = vcpu;
> }
> 
> When the thread context-switches out, vpa_pmu_del() stops the event but
> does not clear event->pmu_private to NULL. When the thread is scheduled
> back in outside of KVM, vpa_pmu_add() sees the vcpu is NULL and returns
> early, leaving the event active with the stale vcpu pointer.
> 
> If the VM is then destroyed and the vcpu freed, would a subsequent thread
> exit or user read() calling vpa_pmu_read() trigger a use-after-free when
> dereferencing vcpu->arch.l1_to_l2_cs?
> 

Valid find, will fix in v3.

> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260813094535.10083-1-gautam@linux.ibm.com?part=3