{"id":"DEBIAN-CVE-2026-23435","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 = &event-\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.","modified":"2026-10-01T01:02:26.596601622Z","published":"2026-04-03T16:16:25.083Z","upstream":["CVE-2026-23435"],"references":[{"type":"ADVISORY","url":"https://security-tracker.debian.org/tracker/CVE-2026-23435"}],"affected":[{"package":{"name":"linux","ecosystem":"Debian:14","purl":"pkg:deb/debian/linux?arch=source&distro=forky"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"6.19.10-1"}]}],"versions":["6.12.100-1","6.12.101-1","6.12.105-1","6.12.107-1","6.12.111-1","6.12.38-1","6.12.41-1","6.12.43-1","6.12.43-1~bpo12+1","6.12.48-1","6.12.57-1","6.12.57-1~bpo12+1","6.12.63-1","6.12.63-1~bpo12+1","6.12.69-1","6.12.69-1~bpo12+1","6.12.73-1","6.12.73-1~bpo12+1","6.12.74-1","6.12.74-2","6.12.74-2~bpo12+1","6.12.85-1","6.12.85-1~bpo12+1","6.12.86-1","6.12.86-1~bpo12+1","6.12.88-1","6.12.88-1~bpo12+1","6.12.90-1","6.12.90-1~bpo12+1","6.12.90-2","6.12.90-2~bpo12+1","6.12.94-1","6.12.94-1~bpo12+1","6.12.95-1","6.12.95-1~bpo12+1","6.12.96-1","6.13.10-1~exp1","6.13.11-1~exp1","6.13.2-1~exp1","6.13.3-1~exp1","6.13.4-1~exp1","6.13.5-1~exp1","6.13.6-1~exp1","6.13.7-1~exp1","6.13.8-1~exp1","6.13.9-1~exp1","6.13~rc6-1~exp1","6.13~rc7-1~exp1","6.14.3-1~exp1","6.14.5-1~exp1","6.14.6-1~exp1","6.15-1~exp1","6.15.1-1~exp1","6.15.2-1~exp1","6.15.3-1~exp1","6.15.4-1~exp1","6.15.5-1~exp1","6.15.6-1~exp1","6.15~rc7-1~exp1","6.16-1~exp1","6.16.1-1~exp1","6.16.10-1","6.16.11-1","6.16.12-1","6.16.12-1~bpo13+1","6.16.12-2","6.16.3-1","6.16.3-1~bpo13+1","6.16.5-1","6.16.6-1","6.16.7-1","6.16.8-1","6.16.9-1","6.16~rc7-1~exp1","6.17.10-1","6.17.11-1","6.17.12-1","6.17.13-1","6.17.13-1~bpo13+1","6.17.2-1~exp1","6.17.5-1~exp1","6.17.6-1","6.17.7-1","6.17.7-2","6.17.8-1","6.17.8-1~bpo13+1","6.17.9-1","6.18.1-1~exp1","6.18.10-1","6.18.12-1","6.18.12-1~bpo13+1","6.18.13-1","6.18.14-1","6.18.15-1","6.18.15-1~bpo13+1","6.18.2-1~exp1","6.18.3-1","6.18.5-1","6.18.5-1~bpo13+1","6.18.8-1","6.18.9-1","6.18.9-1~bpo13+1","6.18~rc4-1~exp1","6.18~rc4-1~exp2","6.18~rc5-1~exp1","6.18~rc6-1~exp1","6.18~rc7-1~exp1","6.19-1~exp1","6.19.10-1~bpo13+1","6.19.2-1~exp1","6.19.3-1~exp1","6.19.4-1~exp1","6.19.5-1~exp1","6.19.6-1","6.19.6-2","6.19.6-2~bpo13+1","6.19.8-1","6.19.8-1~bpo13+1","6.19~rc4-1~exp1","6.19~rc5-1~exp1","6.19~rc6-1~exp1","6.19~rc7-1~exp1","6.19~rc8-1~exp1"],"ecosystem_specific":{"urgency":"not yet assigned"},"database_specific":{"source":"https://storage.googleapis.com/debian-osv/debian-cve-osv/DEBIAN-CVE-2026-23435.json"}}],"schema_version":"1.9.0"}