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

[Miaohe Lin]

On 2022/3/15 0:44, Michal Hocko wrote:
> 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?
> 

Many thanks for your comment.
IIUC, new policy is allocated via the below code:

shared_policy_replace:
	alloc_new:
		write_unlock(&sp->lock);
		ret = -ENOMEM;
		n_new = kmem_cache_alloc(sn_cache, GFP_KERNEL);
		if (!n_new)
			goto err_out;
		mpol_new = kmem_cache_alloc(policy_cache, GFP_KERNEL);
		if (!mpol_new)
			goto err_out;
		goto restart;

And mpol_new' reference count will be set before used in n->end > end case. But
if that is "not" the case, i.e. mpol_new is not inserted into the rb_tree, mpol_new
will be freed via mpol_put before return:

shared_policy_replace:
	err_out:
		if (mpol_new)
			mpol_put(mpol_new);
		if (n_new)
			kmem_cache_free(sn_cache, n_new);

But mpol_new' reference count is not set yet, we have trouble now.
Does this makes sense for you? Or am I miss something?

Thanks.

>> Fixes: 42288fe366c4 ("mm: mempolicy: Convert shared_policy mutex to spinlock")
>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>> ---
>>  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
>