linux-kernel - Re: [PATCH v2] mm/mempolicy: fix mpol_new leak in shared_policy

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Date:   Mon, 21 Mar 2022 13:12:00 +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 v2] mm/mempolicy: fix mpol_new leak in
 shared_policy_replace

On Tue 22-03-22 16:34:56, 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.

I would just add:

This would happen if mempolicy was updated on the shared shmem file
while the sp->lock has been dropped during the memory allocation.

> This issue could be triggered easily with the below
> code snippet if there're many processes doing the below work at the same
> time:
> 
>   shmid = shmget((key_t)5566, 1024 * PAGE_SIZE, 0666|IPC_CREAT);
>   shm = shmat(shmid, 0, 0);
>   loop many times {
>     mbind(shm, 1024 * PAGE_SIZE, MPOL_LOCAL, mask, maxnode, 0);
>     mbind(shm + 128 * PAGE_SIZE, 128 * PAGE_SIZE, MPOL_DEFAULT, mask,
>           maxnode, 0);
>   }
> 
> Fixes: 42288fe366c4 ("mm: mempolicy: Convert shared_policy mutex to spinlock")
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> Cc: <stable@...r.kernel.org> # 3.8

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks a lot!
> ---
> v1->v2:
>   Add reproducer snippet and Cc stable.
>   Thanks Michal Hocko for review and comment!
> ---
>  mm/mempolicy.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index a2516d31db6c..4cdd425b2752 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