{"answers":{"affected":"It affects 1 product.","exploited":"There is no known exploitation of CVE-2025-39821 in the wild and no public exploit code.","fix":"Apply the vendor's patch.","what":"CVE-2025-39821 is a high-severity vulnerability in Linux kernel, published on 16 Sep 2025. It can only be exploited from the same machine or local network, with no action from the victim."},"fix_prompt":"Check this codebase for CVE-2025-39821 and fix it if it is affected.\n\nWhat it is: CVE-2025-39821 is a high-severity vulnerability in Linux kernel, published on 16 Sep 2025. It can only be exploited from the same machine or local network, with no action from the victim.\nExploitation: There is no known exploitation of CVE-2025-39821 in the wild and no public exploit code.\n\nAffected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.\n\nTask:\n1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.\n2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.\n3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.\n4. Run the tests and summarise what changed and why.\n5. If nothing in the project is affected, say so and change nothing.\n\nReference: https://insights.sbomify.com/CVE-2025-39821/ (Markdown: https://insights.sbomify.com/CVE-2025-39821/index.md)","id":"CVE-2025-39821","markdown":"https://insights.sbomify.com/CVE-2025-39821/index.md","name":"CVE-2025-39821","record":{"aliases":["BELL-CVE-2025-39821","DEBIAN-CVE-2025-39821","GHSA-cqwc-52gw-4x53","UBUNTU-CVE-2025-39821"],"analysis":{"component_type":"kernel","fix_type":"patch","impacts":["denial_of_service"],"network_reachable":"adjacent_or_local","non_default_config":false,"product_category":"Linux kernel","requires_auth":"low_privilege","summary":"A logic flaw in Linux kernel perf event throttling causes undefined behavior when processing inactive events, potentially leading to system instability or crashes.","user_interaction":"not_required","vuln_class":"integer_overflow"},"analysis_generated_by":"qwen3.8-flash-next","assigner":"Linux","cpe":[{"product":"linux_kernel","vendor":"linux"}],"cvss":{"score":7.8,"severity":"high","source":"cna","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","version":"3.1"},"cwes":[{"id":"CWE-787","name":"Out-of-bounds Write"}],"description":"In the Linux kernel, the following vulnerability has been resolved:\n\nperf: Avoid undefined behavior from stopping/starting inactive events\n\nCalling pmu-\u003estart()/stop() on perf events in PERF_EVENT_STATE_OFF can\nleave event-\u003ehw.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-\u003ehw.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-\u003epmu-\u003estart()/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:","ecosystems":[],"exploits":{"exploitdb":0,"first_seen":null,"github_poc":0,"max_poc_stars":null,"metasploit":0,"nuclei":0,"refs":[],"tier":"none_known"},"has_fix":true,"headline":"perf: Avoid undefined behavior from stopping/starting inactive events","id":"CVE-2025-39821","indexable":true,"kev":null,"kind":"cve","links":{"ecosystem":null,"product":"linux:linux_kernel","products":["linux:linux_kernel"],"type":"integer_overflow","year":"2025"},"n_packages":0,"n_references":2,"next":{"headline":"io_uring/kbuf: fix signedness in this_len calculation","id":"CVE-2025-39822"},"packages":[],"prev":{"headline":"drm/msm/dpu: Add a null ptr check for dpu_encoder_needs_modeset","id":"CVE-2025-39820"},"products":["Linux"],"published":"2025-09-16","related":[{"cvss_score":8.8,"headline":"Linux Kernel Out-of-Bounds Write","id":"CVE-2026-53266","kev_date_added":"2026-09-18","published":"2026-06-25"},{"cvss_score":8.8,"headline":"Linux Kernel Improper Input Validation","id":"CVE-2013-6282","kev_date_added":"2022-09-15","published":"2013-11-19"},{"cvss_score":8.4,"headline":"Linux Kernel Heap-Based Buffer Overflow","id":"CVE-2022-0185","kev_date_added":"2024-08-21","published":"2022-02-11"},{"cvss_score":8.4,"headline":"Linux Kernel Privilege Escalation","id":"CVE-2013-2094","kev_date_added":"2022-09-15","published":"2013-05-14"},{"cvss_score":7.8,"headline":"Linux Kernel Unspecified","id":"CVE-2026-53362","kev_date_added":"2026-08-27","published":"2026-07-04"},{"cvss_score":7.8,"headline":"Linux Kernel Missing Initialization of a Variable","id":"CVE-2026-46300","kev_date_added":"2026-10-28","published":"2026-05-23"},{"cvss_score":7.8,"headline":"Linux Kernel Race Condition","id":"CVE-2025-39964","kev_date_added":"2026-09-18","published":"2025-10-13"}],"sources":[{"class":"attribution","licence":"CVE Terms of Use (MITRE)","name":"CVE List","prefix":"CVE"},{"class":"attribution","licence":"US public information, NVD API terms","name":"NVD"},{"class":"attribution","licence":"CWE Terms of Use","name":"CWE (MITRE)"},{"class":"attribution","licence":"FIRST CVSS licence","name":"CVSS (FIRST)"}],"ssvc":{"automatable":"no","exploitation":"none","technical_impact":"total"},"state":"PUBLISHED","updated":"2026-08-05","vendors":["Linux"]},"summary":"A logic flaw in Linux kernel perf event throttling causes undefined behavior when processing inactive events, potentially leading to system instability or crashes.","url":"https://insights.sbomify.com/CVE-2025-39821/"}