Re: [PATCH 5/5] mempolicy: fix a memory corruption by refcount imbalancein alloc_pages_vma()

[KOSAKI Motohiro]

> mempolicy: fix a memory corruption by refcount imbalance in alloc_pages_vma()
> 
> [cc9a6c87: cpuset: mm: reduce large amounts of memory barrier related damage
> v3] introduced a potential memory corruption. shmem_alloc_page() uses a
> pseudo vma and it has one significant unique combination, vma->vm_ops=NULL
> and vma->policy->flags & MPOL_F_SHARED.
> 
> 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 a cpuset update race occurs, alloc_pages_vma() falls in 'goto
> retry_cpuset' path, decrements the reference count and frees the policy
> prematurely.
> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> ---
>  mm/mempolicy.c |   12 +++++++++++-
>  1 files changed, 11 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 45f9825..9842ef5 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1552,8 +1552,18 @@ struct mempolicy *get_vma_policy(struct task_struct *task,
>  									addr);
>  			if (vpol)
>  				pol = vpol;
> -		} else if (vma->vm_policy)
> +		} else if (vma->vm_policy) {
>  			pol = vma->vm_policy;
> +
> +			/*
> +			 * shmem_alloc_page() passes MPOL_F_SHARED policy with
> +			 * a pseudo vma whose vma->vm_ops=NULL. Take a reference
> +			 * count on these policies which will be dropped by
> +			 * mpol_cond_put() later
> +			 */
> +			if (mpol_needs_cond_ref(pol))
> +				mpol_get(pol);
> +		}

Ok, looks sene change. thank you.


Acked-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>

>  	}
>  	if (!pol)
>  		pol = &default_policy;
> 

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