Re: [PATCH v2] futex: Use-after-free between futex_key_to_node_opt and vma_replace_policy

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/25/26 16:22, Peter Zijlstra wrote:
> On Wed, Mar 25, 2026 at 08:19:24AM -0700, Eric Dumazet wrote:
>> On Wed, Mar 25, 2026 at 8:14 AM Peter Zijlstra <peterz@infradead.org> wrote:
>>>
>>>
>>> Bah, at that point we might as well RCU the thing like so, I suppose.
>>>
>>> --- a/mm/mempolicy.c
>>> +++ b/mm/mempolicy.c
>>> @@ -1026,7 +1026,7 @@ static int vma_replace_policy(struct vm_
>>>         }
>>>
>>>         old = vma->vm_policy;
>>> -       vma->vm_policy = new; /* protected by mmap_lock */
>>> +       rcu_assign_pointer(vma->vm_policy, new); /* protected by mmap_lock */
>>>         mpol_put(old);
>>>
>>>         return 0;
>>> diff --git a/kernel/futex/core.c b/kernel/futex/core.c
>>> index 4bacf5565368..6336a80e3dca 100644
>>> --- a/kernel/futex/core.c
>>> +++ b/kernel/futex/core.c
>>> @@ -342,7 +342,7 @@ static int __futex_key_to_node(struct mm_struct *mm, unsigned long addr)
>>>         if (!vma)
>>>                 return FUTEX_NO_NODE;
>>>
>>> -       mpol = vma_policy(vma);
>>> +       mpol = rcu_dereference_raw(vma->vm_policy);
>>>         if (!mpol)
>>>                 return FUTEX_NO_NODE;
>>
>> Yes, but sparse will bite :)
> 
> Oh gawd, yes, and then people will go 'fix' it and it'll turn into an
> unholy mess.
> 
>> READ_ONCE()/WRITE_ONCE() on these two locations seems acceptable.
> 
> Fair enough. Like so then..
> 
> --- a/kernel/futex/core.c
> +++ b/kernel/futex/core.c
> @@ -342,7 +342,7 @@ static int __futex_key_to_node(struct mm
>  	if (!vma)
>  		return FUTEX_NO_NODE;
>  
> -	mpol = vma_policy(vma);
> +	mpol = READ_ONCE(vma->vm_policy);
>  	if (!mpol)
>  		return FUTEX_NO_NODE;
>  
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1026,7 +1026,7 @@ static int vma_replace_policy(struct vm_
>  	}
>  
>  	old = vma->vm_policy;
> -	vma->vm_policy = new; /* protected by mmap_lock */
> +	WRITE_ONCE(vma->vm_policy, new); /* protected by mmap_lock */
>  	mpol_put(old);
>  
>  	return 0;

LGTM, feel free to add my

Acked-by: David Hildenbrand (Arm) <david@kernel.org>


-- 
Cheers,

David