Re: [PATCH v3] mm/mempolicy: fix mpol_new leak in shared_policy_replace

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 22 Mar 2022 18:43:45 +0800 Miaohe Lin <linmiaohe@huawei.com> 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. 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
> are 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);
>   }
> 
> ...
>
> --- 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;
>  }

Two other sites in this file do

	atomic_set(&policy->refcnt, 1);


Could we please instead have a little helper function which does the
kmem_cache_alloc()+refcount_set()?