perf/core: Fix refcount bug and potential UAF in perf_mmap

Overview

In the Linux kernel, the following vulnerability has been resolved:

perf/core: Fix refcount bug and potential UAF in perf_mmap

Syzkaller reported a refcount_t: addition on 0; use-after-free warning in perf_mmap.

The issue is caused by a race condition between a failing mmap() setup and a concurrent mmap() on a dependent event (e.g., using output redirection).

In perf_mmap(), the ring_buffer (rb) is allocated and assigned to event->rb with the mmap_mutex held. The mutex is then released to perform map_range().

If map_range() fails, perf_mmap_close() is called to clean up. However, since the mutex was dropped, another thread attaching to this event (via inherited events or output redirection) can acquire the mutex, observe the valid event->rb pointer, and attempt to increment its reference count. If the cleanup path has already dropped the reference count to zero, this results in a use-after-free or refcount saturation warning.

Fix this by extending the scope of mmap_mutex to cover the map_range() call. This ensures that the ring buffer initialization and mapping (or cleanup on failure) happens atomically effectively, preventing other threads from accessing a half-initialized or dying ring buffer.

Affected

• Linux >= b709eb872e19a19607bbb6d2975bc264d59735cf < c27dea9f50ed525facb62ef647dddc4722456e07
• Linux >= b709eb872e19a19607bbb6d2975bc264d59735cf < ac7ecb65af170a7fc193e7bd8be15dac84ec6a56
• Linux >= b709eb872e19a19607bbb6d2975bc264d59735cf < 77de62ad3de3967818c3dbe656b7336ebee461d2
• Linux 6.14

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.