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

[Miaohe Lin]

On 2022/3/26 8:29, Andrew Morton wrote:
> On Tue, 22 Mar 2022 18:43:45 +0800 Miaohe Lin <linmiaohe@xxxxxxxxxx> 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()?> .

There are usecases like below:

	struct mempolicy *new = kmem_cache_alloc(policy_cache, GFP_KERNEL);
	*new = *old;
	^^^^^^^^^^^^
	refcount_set(&new->refcnt, 1);

If we use helper function to do kmem_cache_alloc()+refcount_set() above, separate
refcount_set(&new->refcnt, 1) is still needed as old is copied to new and overwrites
the refcnt field. So that little helper function might not work. Or am I miss something?

Many thanks.

>