Re: [PATCH v2 3/3] powerpc/perf: Update prev_count of event to get accurate values
From: Gautam Menghani <hidden>
Date: 2026-08-14 12:17:40
Also in:
kvm, sashiko-reviews
On Thu, Aug 13, 2026 at 10:01:02AM +0000, sashiko-bot@kernel.org wrote:
quoted hunk
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 [off-list ref] 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.quoted
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.
quoted
} 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