From ee9dce44362b2d8132c32964656ab6dff7dfbc6a Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Fri, 1 May 2026 12:41:23 -0700 Subject: [PATCH] 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: d9b05321e21e ("futex: Move futex_hash_free() back to __mmput()") Reported-by: Yiming Qian Signed-off-by: Davidlohr Bueso Signed-off-by: Linus Torvalds --- kernel/fork.c | 12 +++++------- 1 file changed, 5 insertions(+), 7 deletions(-) diff --git a/kernel/fork.c b/kernel/fork.c index f1ad69c6dc2d..5f3fdfdb14c7 100644 --- a/kernel/fork.c +++ b/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)