一、 漏洞 CVE-2025-39821 基础信息
漏洞信息
# perf：停止/启动非活动事件的未定义行为防范
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神龙会尽力确保数据准确，但也请结合实际情况进行甄别与判断。
神龙祝您一切顺利！
漏洞标题
perf: Avoid undefined behavior from stopping/starting inactive events
来源：美国国家漏洞数据库 NVD
漏洞描述信息
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. When PMU drivers later attempt to use this
negative index as a shift exponent in bitwise operations, it leads to UBSAN
shift-out-of-bounds reports.
The issue is a logical flaw in how event groups handle throttling when some
members are intentionally disabled. Based on the analysis and the
reproducer provided by Mark Rutland (this issue on both arm64 and x86-64).
The scenario unfolds as follows:
1. A group leader event is configured with a very aggressive sampling
period (e.g., sample_period = 1). This causes frequent interrupts and
triggers the throttling mechanism.
2. A child event in the same group is created in a disabled state
(.disabled = 1). This event remains in PERF_EVENT_STATE_OFF.
Since it hasn't been scheduled onto the PMU, its event->hw.idx remains
initialized at -1.
3. When throttling occurs, perf_event_throttle_group() and later
perf_event_unthrottle_group() iterate through all siblings, including
the disabled child event.
4. perf_event_throttle()/unthrottle() are called on this inactive child
event, which then call event->pmu->start()/stop().
5. The PMU driver receives the event with hw.idx == -1 and attempts to
use it as a shift exponent. e.g., in macros like PMCNTENSET(idx),
leading to the UBSAN report.
The throttling mechanism attempts to start/stop events that are not
actively scheduled on the hardware.
Move the state check into perf_event_throttle()/perf_event_unthrottle() so
that inactive events are skipped entirely. This ensures only active events
with a valid hw.idx are processed, preventing undefined behavior and
silencing UBSAN warnings. The corrected check ensures true before
proceeding with PMU operations.
The problem can be reproduced with the syzkaller reproducer:
来源：美国国家漏洞数据库 NVD
CVSS信息
N/A
来源：美国国家漏洞数据库 NVD
漏洞类别
N/A
来源：美国国家漏洞数据库 NVD
漏洞标题
Linux kernel 安全漏洞
来源：中国国家信息安全漏洞库 CNNVD
漏洞描述信息
Linux kernel是美国Linux基金会的开源操作系统Linux所使用的内核。 Linux kernel存在安全漏洞，该漏洞源于对非活动事件执行停止或启动操作时未正确处理事件状态，可能导致未定义行为。
来源：中国国家信息安全漏洞库 CNNVD
CVSS信息
N/A
来源：中国国家信息安全漏洞库 CNNVD
漏洞类别
其他
来源：中国国家信息安全漏洞库 CNNVD
二、漏洞 CVE-2025-39821 的公开POC
| # | POC 描述 | 源链接 | 神龙链接 |
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三、漏洞 CVE-2025-39821 的情报信息
四、漏洞 CVE-2025-39821 的评论
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