[PATCH] futex: Drop CLONE_THREAD requirement for private default hash alloc

[PATCH] futex: Drop CLONE_THREAD requirement for private default hash alloc

[Mohamed Ayman]

From: Davidlohr Bueso <dave@stgolabs.net>

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 <yimingqian591@gmail.com>
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
 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)
-- 
2.34.1

Re: [PATCH] futex: Drop CLONE_THREAD requirement for private default hash alloc

[Andrew Morton]

On Sat,  2 May 2026 18:57:29 +0300 Mohamed Ayman <mohamedaymanworkspace@gmail.com> wrote:

> From: Davidlohr Bueso <dave@stgolabs.net>
> 
> 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 <yimingqian591@gmail.com>
> Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

I hate to get all bureaucratic, but....

Given what you've sent and from my reading of the security@ and
linux-kernel@ lists,

- I can't find a patch from Davidlohr which looks like this one - I
  can't figure out where you got that from.

- I can't find any code from Linus which would lead to the addition of
  his Signed-off-by:.  I don't know where you got that from either.

- Let's cc the author of the Fixes target, Sebastian?

I'd like to have this metadata tightened up, please - so a reader of
this patch can understand who did & said what and when and why.

Thanks.

>  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)
> -- 
> 2.34.1

Re: [PATCH] futex: Drop CLONE_THREAD requirement for private default hash alloc

[Linus Torvalds]

On Sat, 2 May 2026 at 10:24, Andrew Morton <akpm@linux-foundation.org> wrote:
>
> - I can't find a patch from Davidlohr which looks like this one - I
>   can't figure out where you got that from.
>
> - I can't find any code from Linus which would lead to the addition of
>   his Signed-off-by:.  I don't know where you got that from either.

It looks to be just a re-send from the kernel git tree (commit
ee9dce44362b: "futex: Drop CLONE_THREAD requirement for private
default hash alloc").

People who use git-send-email by mistalke often end up re-sending
other peoples patches.

And that commit came from

    https://lore.kernel.org/all/20260501194123.948643-1-dave@stgolabs.net/

except I fixed the "from an earlier CLONE_THREAD" in the commit
message to be about CLONE_VM when I applied it (as per the thread
there)

              Linus

[PATCH] futex: Drop CLONE_THREAD requirement for private default hash alloc

[Davidlohr Bueso]

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_THREAD.

[0] https://lore.kernel.org/all/CAL_bE8LsmCQ-FAtYDuwbJhOkt9p2wwYQwAbMh=PifC=VsiBM6A@mail.gmail.com/
Fixes: d9b05321e21e ("futex: Move futex_hash_free() back to __mmput()")
Reported-by: Yiming Qian <yimingqian591@gmail.com>
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
---
 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)
-- 
2.39.5

Re: [PATCH] futex: Drop CLONE_THREAD requirement for private default hash alloc

[Linus Torvalds]

On Fri, 1 May 2026 at 12:41, Davidlohr Bueso <dave@stgolabs.net> wrote:
>
> 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_THREAD.

That "earlier CLONE_THREAD" should presumably be "earlier CLONE_VM", no?

         Linus

Re: [PATCH] futex: Drop CLONE_THREAD requirement for private default hash alloc

[Davidlohr Bueso]

On Fri, 01 May 2026, Linus Torvalds wrote:

>On Fri, 1 May 2026 at 12:41, Davidlohr Bueso <dave@stgolabs.net> wrote:
>>
>> 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_THREAD.
>
>That "earlier CLONE_THREAD" should presumably be "earlier CLONE_VM", no?

yes :)

Re: [PATCH] futex: Drop CLONE_THREAD requirement for private default hash alloc

[Linus Torvalds]

On Fri, 1 May 2026 at 13:05, Davidlohr Bueso <dave@stgolabs.net> wrote:
>
> On Fri, 01 May 2026, Linus Torvalds wrote:
> >
> >That "earlier CLONE_THREAD" should presumably be "earlier CLONE_VM", no?
>
> yes :)

I ended up applying this directly since I was looking at it anyway and
it felt like an obvious fix.

Famous last words.

             Linus