Commit ee9dce44 authored May 01, 2026 by Davidlohr Bueso Committed by Linus Torvalds May 01, 2026

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.

Link: https://lore.kernel.org/all/CAL_bE8LsmCQ-FAtYDuwbJhOkt9p2wwYQwAbMh=PifC=VsiBM6A@mail.gmail.com/

 [0]
Fixes: d9b05321 ("futex: Move futex_hash_free() back to __mmput()")
Reported-by: Yiming Qian <yimingqian591@gmail.com>
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent bb1d73f2

kernel/fork.c

+5 −7

Original line number	Diff line number	Diff line
		@@ -1951,9 +1951,11 @@ static void rv_task_fork(struct task_struct *p)

		static bool need_futex_hash_allocate_default(u64 clone_flags)
		{
		if ((clone_flags & (CLONE_THREAD \| CLONE_VM)) != (CLONE_THREAD \| CLONE_VM))
		return false;
		return true;
		/*
		* Allocate a default futex hash for any sibling that will
		* share the parent's mm, except vfork.
		*/
		return (clone_flags & (CLONE_VM \| CLONE_VFORK)) == CLONE_VM;
		}

		/*
		@@ -2380,10 +2382,6 @@ __latent_entropy struct task_struct *copy_process(
		if (retval)
		goto bad_fork_cancel_cgroup;

		/*
		* Allocate a default futex hash for the user process once the first
		* thread spawns.
		*/
		if (need_futex_hash_allocate_default(clone_flags)) {
		retval = futex_hash_allocate_default();
		if (retval)