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

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

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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 Linux kernel CVE team has assigned CVE-2026-52973 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.17 with commit d9b05321e21e4b218de4ce8a590bf375f58b6346 and fixed in 6.18.33 with commit 1dcd36420af2da5bd59306dba9caf78e3d248b1d
	Issue introduced in 6.17 with commit d9b05321e21e4b218de4ce8a590bf375f58b6346 and fixed in 7.0.10 with commit 974ac49a9a068b0591a59f65c63eb06579a13091
	Issue introduced in 6.17 with commit d9b05321e21e4b218de4ce8a590bf375f58b6346 and fixed in 7.1 with commit ee9dce44362b2d8132c32964656ab6dff7dfbc6a

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-52973
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	kernel/fork.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/1dcd36420af2da5bd59306dba9caf78e3d248b1d
	https://git.kernel.org/stable/c/974ac49a9a068b0591a59f65c63eb06579a13091
	https://git.kernel.org/stable/c/ee9dce44362b2d8132c32964656ab6dff7dfbc6a