CVE-2026-89791 (CVSS 7.8)

Description

In the Linux kernel, the following vulnerability has been resolved: perf: Fix use-after-free when perf mmap() revival races with the last munmap() perf_mmap_close() drops rb->mmap_count *without* holding event->mmap_mutex (the refcount_dec_and_test() right before the refcount_dec_and_mutex_lock() of event->mmap_count). A concurrent perf_mmap_rb() can slot its entire "revival" path into that window (perf_mmap holds event->mmap_mutex for its whole duration, including rb_alloc): munmap side (perf_mmap_close) mmap side (perf_mmap_rb) ----------------------------------- -------------------------------- rb->mmap_count 1 -> 0 (no lock) (holds event->mmap_mutex) inc_not_zero(rb->mmap_count) fails ring_buffer_attach(event, NULL) rb_alloc() + attach new rb refcount_set(&event->mmap_count, 1) lock; event->mmap_count 1 -> 0 ring_buffer_attach(event, NULL) ring_buffer_put() -> frees the *new* rb The revival's refcount_set(&event->mmap_count, 1) is an invisible 1 -> 1 write: the close frees the just-revived buffer although the other process still has it mapped -- a page-level use-after-free allowing local privilege escalation to root by any unprivileged user (default kernel.perf_event_paranoid=2). Swap the order of the two counter updates: event->mmap_count is dropped first via refcount_dec_and_mutex_lock(), so its 1 -> 0 transition and the ring_buffer_attach() stay serialized with perf_mmap(). rb->mmap_count == 0 then implies every event using the buffer is detached already, so the result of the rb->mmap_count drop can gate the remaining teardown directly and detach_rest is no longer needed. An earlier fix for this race from Kyle Zeng and David Lee takes event->mmap_mutex around both counter updates [0]; here the not-last close stays lockless.

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CVSS v3.1 Score

7.8

HIGH

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

EPSS — Exploit Prediction

0.0019

Probability of exploitation

0.09%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

References

Other References

https://git.kernel.org/stable/c/0c739f54f1c77f3a4643160cd2e031b6c2f2aab6 https://git.kernel.org/stable/c/58a8108bc73de0740d5b88150465d6690ea5f85f https://git.kernel.org/stable/c/929cb3b9dc818dd9fa89d510d4ff2b255e42badd

Frequently Asked Questions

What is CVE-2026-89791? +

In the Linux kernel, the following vulnerability has been resolved: perf: Fix use-after-free when perf mmap() revival races with the last munmap() perf_mmap_close() drops rb->mmap_count *without* holding event->mmap_mutex (the refcount_dec_and_test() right before the refcount_dec_and_mutex_lock() of event->mmap_count). A concurrent perf_mmap_rb() can slot its entire "revival" path into that window (perf_mmap holds event->mmap_mutex for its whole duration, including rb_alloc): munmap side (perf_mmap_close) mmap side (perf_mmap_rb) ----------------------------------- -------------------------------- rb->mmap_count 1 -> 0 (no lock) (holds event->mmap_mutex) inc_not_zero(rb->mmap_count) fails ring_buffer_attach(event, NULL) rb_alloc() + attach new rb refcount_set(&event->mmap_count, 1) lock; event->mmap_count 1 -> 0 ring_buffer_attach(event, NULL) ring_buffer_put() -> frees the *new* rb The revival's refcount_set(&event->mmap_count, 1) is an invisible 1 -> 1 write: the close frees the just-revived buffer although the other process still has it mapped -- a page-level use-after-free allowing local privilege escalation to root by any unprivileged user (default kernel.perf_event_paranoid=2). Swap the order of the two counter updates: event->mmap_count is dropped first via refcount_dec_and_mutex_lock(), so its 1 -> 0 transition and the ring_buffer_attach() stay serialized with perf_mmap(). rb->mmap_count == 0 then implies every event using the buffer is detached already, so the result of the rb->mmap_count drop can gate the remaining teardown directly and detach_rest is no longer needed. An earlier fix for this race from Kyle Zeng and David Lee takes event->mmap_mutex around both counter updates [0]; here the not-last close stays lockless. It has a CVSS v3.1 base score of 7.8 (HIGH).

How severe is CVE-2026-89791? +

CVE-2026-89791 has a CVSS v3.1 score of 7.8 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0019, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-89791? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

CVE-2026-89791