linux-kernel - Re: [PATCH] mm/mempolicy: fix potential mpol_new leak in shared_policy

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Message-ID: <Yi9w7TCYbj+OLGXJ@dhcp22.suse.cz>
Date:   Mon, 14 Mar 2022 17:44:29 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, kosaki.motohiro@...fujitsu.com,
        mgorman@...e.de, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mempolicy: fix potential mpol_new leak in
 shared_policy_replace

On Fri 11-03-22 17:36:24, Miaohe Lin wrote:
> If mpol_new is allocated but not used in restart loop, mpol_new will be
> freed via mpol_put before returning to the caller. But refcnt is not
> initialized yet, so mpol_put could not do the right things and might
> leak the unused mpol_new.

The code is really hideous but is there really any bug there? AFAICS the
new policy is only allocated in if (n->end > end) branch and that one
will set the reference count on the retry. Or am I missing something?

> Fixes: 42288fe366c4 ("mm: mempolicy: Convert shared_policy mutex to spinlock")
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>  mm/mempolicy.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 34d2b29c96ad..f19f19d3558b 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -2733,6 +2733,7 @@ static int shared_policy_replace(struct shared_policy *sp, unsigned long start,
>  	mpol_new = kmem_cache_alloc(policy_cache, GFP_KERNEL);
>  	if (!mpol_new)
>  		goto err_out;
> +	refcount_set(&mpol_new->refcnt, 1);
>  	goto restart;
>  }
>  
> -- 
> 2.23.0

-- 
Michal Hocko
SUSE Labs