CVE-2026-23435

Estado: AnalizadaMedia (5.5)—

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:

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[]:

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.

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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:

The race sequence is:

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.

Detalles técnicos trazas, registros y código del informe original

  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

  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)

  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[]

  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.

CVSS

Versión: 3.1
Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Puntuación base: 5.5

Probabilidad de explotación (EPSS)

Probabilidad de explotación en los próximos 30 días: 0.12%
Percentil entre todas las CVEs puntuadas: 2
Fecha de la puntuación: 6/10/2026

EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).

Tecnologías afectadas (1)

Linux — Linux Kernel

CWE

CWE-476

Referencias

JSON original (NVD)

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{
  "id": "CVE-2026-23435",
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        "source": "nvd@nist.gov",
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          "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
          "integrityImpact": "NONE",
          "userInteraction": "NONE",
          "attackComplexity": "LOW",
          "availabilityImpact": "HIGH",
          "privilegesRequired": "LOW",
          "confidentialityImpact": "NONE"
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  "published": "2026-04-03T16:16:25.083",
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    {
      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nperf/x86: Move event pointer setup earlier in x86_pmu_enable()\n\nA production AMD EPYC system crashed with a NULL pointer dereference\nin the PMU NMI handler:\n\n  BUG: kernel NULL pointer dereference, address: 0000000000000198\n  RIP: x86_perf_event_update+0xc/0xa0\n  Call Trace:\n   <NMI>\n   amd_pmu_v2_handle_irq+0x1a6/0x390\n   perf_event_nmi_handler+0x24/0x40\n\nThe faulting instruction is `cmpq $0x0, 0x198(%rdi)` with RDI=0,\ncorresponding to the `if (unlikely(!hwc->event_base))` check in\nx86_perf_event_update() where hwc = &event->hw and event is NULL.\n\ndrgn inspection of the vmcore on CPU 106 showed a mismatch between\ncpuc->active_mask and cpuc->events[]:\n\n  active_mask: 0x1e (bits 1, 2, 3, 4)\n  events[1]:   0xff1100136cbd4f38  (valid)\n  events[2]:   0x0                 (NULL, but active_mask bit 2 set)\n  events[3]:   0xff1100076fd2cf38  (valid)\n  events[4]:   0xff1100079e990a90  (valid)\n\nThe event that should occupy events[2] was found in event_list[2]\nwith hw.idx=2 and hw.state=0x0, confirming x86_pmu_start() had run\n(which clears hw.state and sets active_mask) but events[2] was\nnever populated.\n\nAnother event (event_list[0]) had hw.state=0x7 (STOPPED|UPTODATE|ARCH),\nshowing it was stopped when the PMU rescheduled events, confirming the\nthrottle-then-reschedule sequence occurred.\n\nThe root cause is commit 7e772a93eb61 (\"perf/x86: Fix NULL event access\nand potential PEBS record loss\") which moved the cpuc->events[idx]\nassignment out of x86_pmu_start() and into step 2 of x86_pmu_enable(),\nafter the PERF_HES_ARCH check. This broke any path that calls\npmu->start() without going through x86_pmu_enable() -- specifically\nthe unthrottle path:\n\n  perf_adjust_freq_unthr_events()\n    -> perf_event_unthrottle_group()\n      -> perf_event_unthrottle()\n        -> event->pmu->start(event, 0)\n          -> x86_pmu_start()     // sets active_mask but not events[]\n\nThe race sequence is:\n\n  1. A group of perf events overflows, triggering group throttle via\n     perf_event_throttle_group(). All events are stopped: active_mask\n     bits cleared, events[] preserved (x86_pmu_stop no longer clears\n     events[] after commit 7e772a93eb61).\n\n  2. While still throttled (PERF_HES_STOPPED), x86_pmu_enable() runs\n     due to other scheduling activity. Stopped events that need to\n     move counters get PERF_HES_ARCH set and events[old_idx] cleared.\n     In step 2 of x86_pmu_enable(), PERF_HES_ARCH causes these events\n     to be skipped -- events[new_idx] is never set.\n\n  3. The timer tick unthrottles the group via pmu->start(). Since\n     commit 7e772a93eb61 removed the events[] assignment from\n     x86_pmu_start(), active_mask[new_idx] is set but events[new_idx]\n     remains NULL.\n\n  4. A PMC overflow NMI fires. The handler iterates active counters,\n     finds active_mask[2] set, reads events[2] which is NULL, and\n     crashes dereferencing it.\n\nMove the cpuc->events[hwc->idx] assignment in x86_pmu_enable() to\nbefore the PERF_HES_ARCH check, so that events[] is populated even\nfor events that are not immediately started. This ensures the\nunthrottle path via pmu->start() always finds a valid event pointer."
    },
    {
      "lang": "es",
      "value": "En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\n\nperf/x86: Mover la configuración del puntero de evento antes en x86_pmu_enable()\n\nUn sistema AMD EPYC de producción falló con una desreferencia de puntero NULL en el gestor NMI de la PMU:\n\n  BUG: desreferencia de puntero NULL del kernel, dirección: 0000000000000198\n  RIP: x86_perf_event_update+0xc/0xa0\n  Traza de Llamada:\n   [NMI]\n   amd_pmu_v2_handle_irq+0x1a6/0x390\n   perf_event_nmi_handler+0x24/0x40\n\nLa instrucción que causó el fallo es 'cmpq $0x0, 0x198(%rdi)' con RDI=0, correspondiente a la comprobación 'if (unlikely(!hwc- event_base))' en x86_perf_event_update() donde hwc = &event- hw y event es NULL.\n\nLa inspección con drgn del vmcore en la CPU 106 mostró una discrepancia entre cpuc- active_mask y cpuc- events[]:\n\n  active_mask: 0x1e (bits 1, 2, 3, 4)\n  events[1]:   0xff1100136cbd4f38  (válido)\n  events[2]:   0x0                 (NULL, pero el bit 2 de active_mask está activado)\n  events[3]:   0xff1100076fd2cf38  (válido)\n  events[4]:   0xff1100079e990a90  (válido)\n\nEl evento que debería ocupar events[2] se encontró en event_list[2] con hw.idx=2 y hw.state=0x0, confirmando que x86_pmu_start() se había ejecutado (lo que borra hw.state y establece active_mask) pero events[2] nunca fue poblado.\n\nOtro evento (event_list[0]) tenía hw.state=0x7 (STOPPED|UPTODATE|ARCH), mostrando que se detuvo cuando la PMU reprogramó los eventos, confirmando que la secuencia de estrangulamiento y luego reprogramación ocurrió.\n\nLa causa raíz es el commit 7e772a93eb61 ( perf/x86: Solucionar acceso a evento NULL y posible pérdida de registro PEBS ) que movió la asignación de cpuc- events[idx] fuera de x86_pmu_start() y al paso 2 de x86_pmu_enable(), después de la comprobación PERF_HES_ARCH. Esto rompió cualquier ruta que llama a pmu- start() sin pasar por x86_pmu_enable() -- específicamente la ruta de desestrangulamiento:\n\n  perf_adjust_freq_unthr_events()\n    -  perf_event_unthrottle_group()\n      -  perf_event_unthrottle()\n        -  event- pmu- start(event, 0)\n          -  x86_pmu_start()     // establece active_mask pero no events[]\n\nLa secuencia de la condición de carrera es:\n\n  1. Un grupo de eventos perf desborda, lo que activa el estrangulamiento del grupo a través de perf_event_throttle_group(). Todos los eventos se detienen: los bits de active_mask se borran, events[] se preserva (x86_pmu_stop ya no borra events[] después del commit 7e772a93eb61).\n\n  2. Mientras aún está estrangulado (PERF_HES_STOPPED), x86_pmu_enable() se ejecuta debido a otra actividad de planificación. Los eventos detenidos que necesitan mover contadores obtienen PERF_HES_ARCH establecido y events[old_idx] borrado. En el paso 2 de x86_pmu_enable(), PERF_HES_ARCH hace que estos eventos se omitan -- events[new_idx] nunca se establece.\n\n  3. El tick del temporizador desestrángula el grupo a través de pmu- start(). Dado que el commit 7e772a93eb61 eliminó la asignación de events[] de x86_pmu_start(), active_mask[new_idx] se establece pero events[new_idx] permanece NULL.\n\n  4. Se dispara una NMI de desbordamiento de PMC. El gestor itera los contadores activos, encuentra active_mask[2] establecido, lee events[2] que es NULL, y falla al desreferenciarlo.\n\nMover la asignación de cpuc- events[hwc- idx] en x86_pmu_enable() a antes de la comprobación PERF_HES_ARCH, para que events[] se pueble incluso para eventos que no se inician inmediatamente. Esto asegura que la ruta de desestrangulamiento a través de pmu- start() siempre encuentre un puntero de evento válido."
    }
  ],
  "lastModified": "2026-07-21T19:10:00.107",
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