futex: Avoid private hash use-after-free on final put

Overview

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

futex: Avoid private hash use-after-free on final put

futex_private_hash_put() drops the reference to fph before evaluating fph->mm for wake_up_var(). futex_ref_put() enables preemption again before returning. If that put drops the final reference and the task is preempted, another task can pivot to the replacement hash and free the old hash after an RCU grace period. The first task then reads fph->mm from the freed allocation when it resumes.

KASAN reports a slab-use-after-free in futex_private_hash_put(), with the read at offset 24 in a freed kmalloc-512 allocation. The allocation and free stacks point to futex_hash_allocate() and the RCU free path, respectively.

Load the mm pointer while the fph reference is still held and pass the saved value to wake_up_var(). wake_up_var() uses the pointer as a waitqueue key and does not dereference the mm through it.

Affected

• Linux >= bd54df5ea7cadac520e346d5f0fe5d58e635b6ba < dba60d26e9dda3e157ad6b0934d4232f6c6904ee
• Linux >= bd54df5ea7cadac520e346d5f0fe5d58e635b6ba < 12cd315a7b6a7d7db3f69d98bfe3909275e9451d
• Linux >= bd54df5ea7cadac520e346d5f0fe5d58e635b6ba < 25408c62ed4ebcd491d0e9e4b4a5b512547512e4
• Linux >= bd54df5ea7cadac520e346d5f0fe5d58e635b6ba < 1c7efabfbaf796f11000a46094a69955a01ec6cc
• Linux 6.16

Remediation

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