{
  "affected": [
    {
      "ecosystem_specific": {
        "urgency": "not yet assigned"
      },
      "package": {
        "ecosystem": "Debian:14",
        "name": "linux"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "6.19.10-1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "details": "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:    \u003cNMI\u003e    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-\u003eevent_base))` check in x86_perf_event_update() where hwc = \u0026event-\u003ehw and event is NULL.  drgn inspection of the vmcore on CPU 106 showed a mismatch between cpuc-\u003eactive_mask and cpuc-\u003eevents[]:    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-\u003eevents[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-\u003estart() without going through x86_pmu_enable() -- specifically the unthrottle path:    perf_adjust_freq_unthr_events()     -\u003e perf_event_unthrottle_group()       -\u003e perf_event_unthrottle()         -\u003e event-\u003epmu-\u003estart(event, 0)           -\u003e 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-\u003estart(). 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-\u003eevents[hwc-\u003eidx] 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-\u003estart() always finds a valid event pointer.",
  "id": "DEBIAN-CVE-2026-23435",
  "modified": "2026-04-28T19:53:11.312923610Z",
  "published": "2026-04-03T16:16:25.083Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://security-tracker.debian.org/tracker/CVE-2026-23435"
    }
  ],
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "upstream": [
    "CVE-2026-23435"
  ]
}