Re: [PATCH 5/6] mempolicy: fix a memory corruption by refcountimbalance in alloc_pages_vma()

[Ben Hutchings]

On Mon, 2012-06-11 at 05:17 -0400, kosaki.motohiro@xxxxxxxxx wrote:
> From: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> 
> commit cc9a6c8776 (cpuset: mm: reduce large amounts of memory barrier related
> damage v3) introduced a memory corruption.
> 
> shmem_alloc_page() passes pseudo vma and it has one significant unique
> combination, vma->vm_ops=NULL and (vma->policy->flags & MPOL_F_SHARED).
> 
> Now, get_vma_policy() does NOT increase a policy ref when vma->vm_ops=NULL
> and mpol_cond_put() DOES decrease a policy ref when a policy has MPOL_F_SHARED.
> Therefore, when alloc_pages_vma() goes 'goto retry_cpuset' path, a policy
> refcount will be decreased too much and therefore it will make a memory corruption.
[...]
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1544,18 +1544,29 @@ struct mempolicy *get_vma_policy(struct task_struct *task,
>  		struct vm_area_struct *vma, unsigned long addr)
>  {
>  	struct mempolicy *pol = task->mempolicy;
> +	int got_ref;

= 0

And this should really be a bool.

>  	if (vma) {
>  		if (vma->vm_ops && vma->vm_ops->get_policy) {
>  			struct mempolicy *vpol = vma->vm_ops->get_policy(vma,
>  									addr);
> -			if (vpol)
> +			if (vpol) {
>  				pol = vpol;
> +				got_ref = 1;
> +			}
>  		} else if (vma->vm_policy)
>  			pol = vma->vm_policy;
>  	}
>  	if (!pol)
>  		pol = &default_policy;
> +
> +	/*
> +	 * shmem_alloc_page() passes MPOL_F_SHARED policy with vma->vm_ops=NULL.
> +	 * Thus, we need to take additional ref for avoiding refcount imbalance.
> +	 */
> +	if (!got_ref && mpol_needs_cond_ref(pol))
> +		mpol_get(pol);
> +
>  	return pol;
>  }
>  
[...]

-- 
Ben Hutchings
Computers are not intelligent.	They only think they are.

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