CVE-2026-52973 — futex: Drop CLONE_THREAD requirement for private default hash alloc

Affected: Linux, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 9, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8

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

futex: Drop CLONE_THREAD requirement for private default hash alloc

Currently need_futex_hash_allocate_default() depends on strict pthread semantics, abusing CLONE_THREAD. This breaks the non-concurrency assumptions when doing the mm->futex_ref pcpu allocations, leading to bugs[0] when sharing the mm in other ways; ie:

BUG: KASAN: slab-use-after-free in futex_hash_put

... where the +1 bias can end up on a percpu counter that mm->futex_ref no longer points at.

Loosen the check to cover any CLONE_VM clone, except vfork(). Excluding vfork keeps the existing paths untouched (no overhead), and we can't race in the first place: either the parent is suspended and the child runs alone, or mm->futex_ref is already allocated from an earlier CLONE_VM.

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CVE-2026-52973 — futex: Drop CLONE_THREAD requirement for private default hash alloc