High [CVE-2026-52973] Drop CLONE_THREAD requirement for private default hash alloc

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.

The need_futex_hash_allocate_default() function incorrectly relies on CLONE_THREAD semantics, which can lead to non-concurrency issues when memory allocations (mm->futex_ref pcpu allocations) are shared across CLONE_VM clones, excluding vfork(). This can result in a use-after-free vulnerability in futex_hash_put, a form of memory corruption.

Such vulnerabilities can potentially lead to system instability or other security impacts. This vulnerability was introduced in Red Hat Enterprise Linux 9.8 and 10.2; therefore, earlier versions of RHEL are unaffected.