# cveboard — Historical CVE brief

## Instructions for the operator's AI
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Fix information unknown; no fixed version is derived here. Missing information does not prove that no fix exists.
Exploitation unknown; this catalogue does not assess exploitation.

## Source facts (JSON-quoted data)
CNA source text (English)
id: "CVE-2026-23435"
idYear: 2026
status: "PUBLISHED"
publishedAt: "2026-04-03T15:15:20.465Z"
updatedAt: "2026-05-23T16:04:50.015Z"
importedAt: "2026-10-24T01:58:02.025Z"
snapshot: {"id":"cve_2026-10-24_midnight_baseline","at":"2026-10-24T00:00:00.000Z","source":"https://github.com/CVEProject/cvelistV5/releases/tag/cve_2026-10-24_0100Z"}
url: "https://cveboard.org/cve/CVE-2026-23435"
json: "https://cveboard.org/cve/CVE-2026-23435.json"
sourceURL: "https://www.cve.org/CVERecord?id=CVE-2026-23435"
title: "perf/x86: Move event pointer setup earlier in x86_pmu_enable()"
summary: "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   \u003cNMI\u003e\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-\u003eevent_base))` check in\nx86_perf_event_update() where hwc = &event-\u003ehw and event is NULL.\n\ndrgn inspection of the vmcore on CPU 106 showed a mismatch between\ncpuc-\u003eactive_mask and cpuc-\u003eevents[]:\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-\u003eevents[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-\u003estart() without going through x86_pmu_enable() -- specifically\nthe unthrottle path:\n\n  perf_adjust_freq_unthr_events()\n    -\u003e perf_event_unthrottle_group()\n      -\u003e perf_event_unthrottle()\n        -\u003e event-\u003epmu-\u003estart(event, 0)\n          -\u003e 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-\u003estart(). 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-\u003eevents[hwc-\u003eidx] 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-\u003estart() always finds a valid event pointer."
affected: [{"versions":[{"version":"6b089028bff1f2ff9e0c62b8f1faca1a620e5d6e","status":"affected","versionType":"git","lessThan":"886fa869153917d902784098922defa20c3a2fe5"},{"version":"7e772a93eb61cb6265bdd1c5bde17d0f2718b452","status":"affected","versionType":"git","lessThan":"c1dd1e2b722d3f1f2e4977dad8d1be78fdfb30cb"},{"version":"7e772a93eb61cb6265bdd1c5bde17d0f2718b452","status":"affected","versionType":"git","lessThan":"8d5fae6011260de209aaf231120e8146b14bc8e0"},{"version":"cf69b99805c263117305ac6dffbc85aaf9259d32","status":"affected","versionType":"git"},{"version":"6.18.2","status":"affected","versionType":"semver","lessThan":"6.18.20"},{"version":"6.17.13","status":"affected","versionType":"semver","lessThan":"6.18"}],"vendor":"Linux","product":"Linux","defaultStatus":"unaffected"},{"versions":[{"version":"6.19","status":"affected"},{"version":"0","status":"unaffected","versionType":"semver","lessThan":"6.19"},{"version":"6.18.20","status":"unaffected","versionType":"semver","lessThanOrEqual":"6.18.*"},{"version":"6.19.10","status":"unaffected","versionType":"semver","lessThanOrEqual":"6.19.*"},{"version":"7.0","status":"unaffected","versionType":"original_commit_for_fix","lessThanOrEqual":"*"}],"vendor":"Linux","product":"Linux","defaultStatus":"affected"}]

## Source licences and attribution
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GitHub Advisory Database: https://github.com/advisories (CC BY 4.0)
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