{"id":"CVE-2025-39821","url":"https://www.cyber-defence.io/tools/cve/CVE-2025-39821","generatedAt":"2026-10-28T00:17:07.206Z","title":"In the Linux kernel, the following vulnerability has been resolved: perf: Avoid undefined behavior from stopping/starting inactive events Calling pmu->start()/stop() on perf events in PERF_EVENT_STATE_OFF can leave event->hw.idx at -1.","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nperf: Avoid undefined behavior from stopping/starting inactive events\n\nCalling pmu->start()/stop() on perf events in PERF_EVENT_STATE_OFF can\nleave event->hw.idx at -1. When PMU drivers later attempt to use this\nnegative index as a shift exponent in bitwise operations, it leads to UBSAN\nshift-out-of-bounds reports.\n\nThe issue is a logical flaw in how event groups handle throttling when some\nmembers are intentionally disabled. Based on the analysis and the\nreproducer provided by Mark Rutland (this issue on both arm64 and x86-64).\n\nThe scenario unfolds as follows:\n\n 1. A group leader event is configured with a very aggressive sampling\n    period (e.g., sample_period = 1). This causes frequent interrupts and\n    triggers the throttling mechanism.\n 2. A child event in the same group is created in a disabled state\n    (.disabled = 1). This event remains in PERF_EVENT_STATE_OFF.\n    Since it hasn't been scheduled onto the PMU, its event->hw.idx remains\n    initialized at -1.\n 3. When throttling occurs, perf_event_throttle_group() and later\n    perf_event_unthrottle_group() iterate through all siblings, including\n    the disabled child event.\n 4. perf_event_throttle()/unthrottle() are called on this inactive child\n    event, which then call event->pmu->start()/stop().\n 5. The PMU driver receives the event with hw.idx == -1 and attempts to\n    use it as a shift exponent. e.g., in macros like PMCNTENSET(idx),\n    leading to the UBSAN report.\n\nThe throttling mechanism attempts to start/stop events that are not\nactively scheduled on the hardware.\n\nMove the state check into perf_event_throttle()/perf_event_unthrottle() so\nthat inactive events are skipped entirely. This ensures only active events\nwith a valid hw.idx are processed, preventing undefined behavior and\nsilencing UBSAN warnings. The corrected check ensures true before\nproceeding with PMU operations.\n\nThe problem can be reproduced with the syzkaller reproducer:","published":"2025-09-16T13:15:59.000Z","lastModified":"2026-10-19T21:10:00.000Z","status":"Modified","sourceIdentifier":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","cvss":{"version":"3.1","score":7.8,"severity":"HIGH","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"},"cwe":["CWE-787"],"affected":["linux/linux kernel"],"epss":{"score":0.00164,"percentile":0.05527,"date":"2026-10-26","history":[]},"kev":{"listed":false},"exploits":{"count":0,"verified":false,"firstPublished":null,"source":"Exploit-DB (https://gitlab.com/exploit-database/exploitdb)","entries":[]},"verdict":{"level":"high","text":"High impact if exploited, but EPSS currently rates exploitation as unlikely (0.16%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit."},"changes":[],"references":[{"url":"https://git.kernel.org/stable/c/b64fdd422a85025b5e91ead794db9d3ef970e369","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"},{"url":"https://git.kernel.org/stable/c/d689135aa9c5e4e0eab5a92bbe35dab0c8d6677f","tags":["Patch"],"source":"416baaa9-dc9f-4396-8d5f-8c081fb06d67"}],"sources":{"nvd":"https://nvd.nist.gov/vuln/detail/CVE-2025-39821","epss":"https://www.first.org/epss/","kev":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","exploitdb":"https://www.exploit-db.com/"},"licence":"CC BY 4.0 — link back to the CVE Explorer if you publish the results; upstream data remains subject to its own terms."}