Description
In the Linux kernel, the following vulnerability has been resolved:

perf/x86: Move event pointer setup earlier in x86_pmu_enable()

A production AMD EPYC system crashed with a NULL pointer dereference
in the PMU NMI handler:

BUG: kernel NULL pointer dereference, address: 0000000000000198
RIP: x86_perf_event_update+0xc/0xa0
Call Trace:
<NMI>
amd_pmu_v2_handle_irq+0x1a6/0x390
perf_event_nmi_handler+0x24/0x40

The faulting instruction is `cmpq $0x0, 0x198(%rdi)` with RDI=0,
corresponding to the `if (unlikely(!hwc->event_base))` check in
x86_perf_event_update() where hwc = &event->hw and event is NULL.

drgn inspection of the vmcore on CPU 106 showed a mismatch between
cpuc->active_mask and cpuc->events[]:

active_mask: 0x1e (bits 1, 2, 3, 4)
events[1]: 0xff1100136cbd4f38 (valid)
events[2]: 0x0 (NULL, but active_mask bit 2 set)
events[3]: 0xff1100076fd2cf38 (valid)
events[4]: 0xff1100079e990a90 (valid)

The event that should occupy events[2] was found in event_list[2]
with hw.idx=2 and hw.state=0x0, confirming x86_pmu_start() had run
(which clears hw.state and sets active_mask) but events[2] was
never populated.

Another event (event_list[0]) had hw.state=0x7 (STOPPED|UPTODATE|ARCH),
showing it was stopped when the PMU rescheduled events, confirming the
throttle-then-reschedule sequence occurred.

The root cause is commit 7e772a93eb61 ("perf/x86: Fix NULL event access
and potential PEBS record loss") which moved the cpuc->events[idx]
assignment out of x86_pmu_start() and into step 2 of x86_pmu_enable(),
after the PERF_HES_ARCH check. This broke any path that calls
pmu->start() without going through x86_pmu_enable() -- specifically
the unthrottle path:

perf_adjust_freq_unthr_events()
-> perf_event_unthrottle_group()
-> perf_event_unthrottle()
-> event->pmu->start(event, 0)
-> x86_pmu_start() // sets active_mask but not events[]

The race sequence is:

1. A group of perf events overflows, triggering group throttle via
perf_event_throttle_group(). All events are stopped: active_mask
bits cleared, events[] preserved (x86_pmu_stop no longer clears
events[] after commit 7e772a93eb61).

2. While still throttled (PERF_HES_STOPPED), x86_pmu_enable() runs
due to other scheduling activity. Stopped events that need to
move counters get PERF_HES_ARCH set and events[old_idx] cleared.
In step 2 of x86_pmu_enable(), PERF_HES_ARCH causes these events
to be skipped -- events[new_idx] is never set.

3. The timer tick unthrottles the group via pmu->start(). Since
commit 7e772a93eb61 removed the events[] assignment from
x86_pmu_start(), active_mask[new_idx] is set but events[new_idx]
remains NULL.

4. A PMC overflow NMI fires. The handler iterates active counters,
finds active_mask[2] set, reads events[2] which is NULL, and
crashes dereferencing it.

Move the cpuc->events[hwc->idx] assignment in x86_pmu_enable() to
before the PERF_HES_ARCH check, so that events[] is populated even
for events that are not immediately started. This ensures the
unthrottle path via pmu->start() always finds a valid event pointer.
Published: 2026-04-03
Score: 5.5 Medium
EPSS: < 1% Very Low
KEV: No
Impact: Kernel crash (Denial of Service)
Action: Patch immediately
AI Analysis

Impact

A null pointer dereference occurs in the Linux kernel when the perf subsystem attempts to handle a PMU overflow after an unthrottle operation. The bug originates from moving the assignment of event pointers out of the routine that starts the PMU, leaving the events array with a NULL entry that the crash handler later dereferences. The result is a kernel panic, cutting system operation.

Affected Systems

The vulnerability affects the Linux kernel in all releases that contain the faulty commit before it was corrected. Both standard Linux distributions and custom builds that have not yet applied the patch are susceptible.

Risk and Exploitability

The flaw is a local kernel pointer dereference (CWE‑364) with exploitation requiring privileged or local code execution. An attacker could trigger a denial‑of‑service by inducing a perf event group overflow and then unthrottling, but the probability of successful exploitation is low (EPSS < 1 %) and the vulnerability is not listed in the CISA KEV catalog.

Generated by OpenCVE AI on April 7, 2026 at 09:50 UTC.

Remediation

No vendor fix or workaround currently provided.

OpenCVE Recommended Actions

  • Apply the latest kernel update that includes commit 7e772a93eb61.
  • Verify that the running kernel version reflects the applied fix.

Generated by OpenCVE AI on April 7, 2026 at 09:50 UTC.

Tracking

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Advisories

No advisories yet.

History

Thu, 23 Apr 2026 21:00:00 +0000

Type Values Removed Values Added
Weaknesses CWE-476
CPEs cpe:2.3:o:linux:linux_kernel:6.19:-:*:*:*:*:*:*
cpe:2.3:o:linux:linux_kernel:7.0:rc1:*:*:*:*:*:*
cpe:2.3:o:linux:linux_kernel:7.0:rc2:*:*:*:*:*:*
cpe:2.3:o:linux:linux_kernel:7.0:rc3:*:*:*:*:*:*
cpe:2.3:o:linux:linux_kernel:7.0:rc4:*:*:*:*:*:*
cpe:2.3:o:linux:linux_kernel:7.0:rc5:*:*:*:*:*:*
cpe:2.3:o:linux:linux_kernel:7.0:rc6:*:*:*:*:*:*
cpe:2.3:o:linux:linux_kernel:7.0:rc7:*:*:*:*:*:*
Metrics cvssV3_1

{'score': 5.5, 'vector': 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H'}

Tue, 07 Apr 2026 08:00:00 +0000

Type Values Removed Values Added
Weaknesses CWE-476
CWE-665

Sat, 04 Apr 2026 01:15:00 +0000

Type Values Removed Values Added
Weaknesses CWE-364
References

Fri, 03 Apr 2026 21:30:00 +0000

Type Values Removed Values Added
Weaknesses CWE-476
CWE-665

Fri, 03 Apr 2026 16:30:00 +0000

Type Values Removed Values Added
Description In the Linux kernel, the following vulnerability has been resolved: perf/x86: Move event pointer setup earlier in x86_pmu_enable() A production AMD EPYC system crashed with a NULL pointer dereference in the PMU NMI handler: BUG: kernel NULL pointer dereference, address: 0000000000000198 RIP: x86_perf_event_update+0xc/0xa0 Call Trace: <NMI> amd_pmu_v2_handle_irq+0x1a6/0x390 perf_event_nmi_handler+0x24/0x40 The faulting instruction is `cmpq $0x0, 0x198(%rdi)` with RDI=0, corresponding to the `if (unlikely(!hwc->event_base))` check in x86_perf_event_update() where hwc = &event->hw and event is NULL. drgn inspection of the vmcore on CPU 106 showed a mismatch between cpuc->active_mask and cpuc->events[]: active_mask: 0x1e (bits 1, 2, 3, 4) events[1]: 0xff1100136cbd4f38 (valid) events[2]: 0x0 (NULL, but active_mask bit 2 set) events[3]: 0xff1100076fd2cf38 (valid) events[4]: 0xff1100079e990a90 (valid) The event that should occupy events[2] was found in event_list[2] with hw.idx=2 and hw.state=0x0, confirming x86_pmu_start() had run (which clears hw.state and sets active_mask) but events[2] was never populated. Another event (event_list[0]) had hw.state=0x7 (STOPPED|UPTODATE|ARCH), showing it was stopped when the PMU rescheduled events, confirming the throttle-then-reschedule sequence occurred. The root cause is commit 7e772a93eb61 ("perf/x86: Fix NULL event access and potential PEBS record loss") which moved the cpuc->events[idx] assignment out of x86_pmu_start() and into step 2 of x86_pmu_enable(), after the PERF_HES_ARCH check. This broke any path that calls pmu->start() without going through x86_pmu_enable() -- specifically the unthrottle path: perf_adjust_freq_unthr_events() -> perf_event_unthrottle_group() -> perf_event_unthrottle() -> event->pmu->start(event, 0) -> x86_pmu_start() // sets active_mask but not events[] The race sequence is: 1. A group of perf events overflows, triggering group throttle via perf_event_throttle_group(). All events are stopped: active_mask bits cleared, events[] preserved (x86_pmu_stop no longer clears events[] after commit 7e772a93eb61). 2. While still throttled (PERF_HES_STOPPED), x86_pmu_enable() runs due to other scheduling activity. Stopped events that need to move counters get PERF_HES_ARCH set and events[old_idx] cleared. In step 2 of x86_pmu_enable(), PERF_HES_ARCH causes these events to be skipped -- events[new_idx] is never set. 3. The timer tick unthrottles the group via pmu->start(). Since commit 7e772a93eb61 removed the events[] assignment from x86_pmu_start(), active_mask[new_idx] is set but events[new_idx] remains NULL. 4. A PMC overflow NMI fires. The handler iterates active counters, finds active_mask[2] set, reads events[2] which is NULL, and crashes dereferencing it. Move the cpuc->events[hwc->idx] assignment in x86_pmu_enable() to before the PERF_HES_ARCH check, so that events[] is populated even for events that are not immediately started. This ensures the unthrottle path via pmu->start() always finds a valid event pointer.
Title perf/x86: Move event pointer setup earlier in x86_pmu_enable()
First Time appeared Linux
Linux linux Kernel
CPEs cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Vendors & Products Linux
Linux linux Kernel
References

Subscriptions

Linux Linux Kernel
cve-icon MITRE

Status: PUBLISHED

Assigner: Linux

Published:

Updated: 2026-04-13T06:07:23.984Z

Reserved: 2026-01-13T15:37:46.017Z

Link: CVE-2026-23435

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Analyzed

Published: 2026-04-03T16:16:25.083

Modified: 2026-04-23T20:59:39.733

Link: CVE-2026-23435

cve-icon Redhat

Severity :

Publid Date: 2026-04-03T00:00:00Z

Links: CVE-2026-23435 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2026-04-08T19:54:03Z

Weaknesses