Re: [PATCH] mm/mempolicy.c: do not put mempolicy before using its nodemask

[Michal Hocko]

On Wed 18-01-17 15:11:24, Vlastimil Babka wrote:
> Since commit be97a41b291e ("mm/mempolicy.c: merge alloc_hugepage_vma to
> alloc_pages_vma") alloc_pages_vma() can potentially free a mempolicy by
> mpol_cond_put() before accessing the embedded nodemask by
> __alloc_pages_nodemask(). The commit log says it's so "we can use a single
> exit path within the function" but that's clearly wrong. We can still do that
> when doing mpol_cond_put() after the allocation attempt.
> 
> Make sure the mempolicy is not freed prematurely, otherwise
> __alloc_pages_nodemask() can end up using a bogus nodemask, which could lead
> e.g. to premature OOM.
> 
> Fixes: be97a41b291e ("mm/mempolicy.c: merge alloc_hugepage_vma to alloc_pages_vma")
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  mm/mempolicy.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 2e346645eb80..1e7873e40c9a 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2017,8 +2017,8 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
>  
>  	nmask = policy_nodemask(gfp, pol);
>  	zl = policy_zonelist(gfp, pol, node);
> -	mpol_cond_put(pol);
>  	page = __alloc_pages_nodemask(gfp, order, zl, nmask);
> +	mpol_cond_put(pol);
>  out:
>  	if (unlikely(!page && read_mems_allowed_retry(cpuset_mems_cookie)))
>  		goto retry_cpuset;
> -- 
> 2.11.0
> 
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-- 
Michal Hocko
SUSE Labs