{"id":"CVE-2026-23435","title":"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: ker…","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: ker…","severity":"medium","cvss":5.5,"cvssVector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","cwe":["CWE-476"],"vendor":"linux","product":"linux_kernel","affected":["linux_kernel >= 6.17.13, < 6.18","linux_kernel >= 6.18.2, < 6.18.20","linux_kernel >= 6.19.1, < 6.19.10","linux_kernel = 6.19","linux_kernel = 7.0"],"patched":["linux_kernel 6.19.10"],"published":"2026-04-03","updated":"2026-07-21","source":"NVD","sourceUrl":"https://nvd.nist.gov/vuln/detail/CVE-2026-23435","references":[{"url":"https://git.kernel.org/stable/c/886fa869153917d902784098922defa20c3a2fe5","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/8d5fae6011260de209aaf231120e8146b14bc8e0","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/c1dd1e2b722d3f1f2e4977dad8d1be78fdfb30cb","label":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"}],"tags":["nvd"],"epss":0.00121,"epssPercentile":0.02177,"ingestedAt":"2026-07-21T19:53:39.540Z","slug":"CVE-2026-23435","body":"## Overview\n\nIn 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.\n\n## Affected\n\n- `linux_kernel >= 6.17.13, < 6.18`\n- `linux_kernel >= 6.18.2, < 6.18.20`\n- `linux_kernel >= 6.19.1, < 6.19.10`\n- `linux_kernel = 6.19`\n- `linux_kernel = 7.0`\n\n## Remediation\n\nUpgrade past the affected range:\n\n- `linux_kernel 6.19.10`","depth":"sunlit","depthScore":30,"depthScoreParts":{"impact":30.3,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}