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: d9b0532 ("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>

Author: Davidlohr Bueso

kernel/fork.c

@@ -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)