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

[Miaohe Lin]

On 2022/3/16 17:56, Michal Hocko wrote:
> On Wed 16-03-22 14:39:37, Miaohe Lin wrote:
>> On 2022/3/15 23:27, Michal Hocko wrote:
>>> On Tue 15-03-22 21:42:29, Miaohe Lin wrote:
>>>> 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:
>>>
>>> One thing I have missed previously is that the lock is dropped during
>>> the allocation so I guess the memory policy could have been changed
>>> during that time. Is this possible? Have you explored this possibility?
>>> Is this a theoretical problem or it can be triggered intentionally.
>>>
>>
>> This is found via code investigation. I think this could be triggered if there
>> are many concurrent mpol_set_shared_policy in place. But the user-visible effect
>> might be obscure as only sizeof(struct mempolicy) bytes leaks possiblely every time.
>>
>>> These details would be really interesting for the changelog so that we
>>> can judge how important this would be.
>>
>> This might not be that important as this issue should have been well-concealed for
>> almost ten years (since commit 42288fe366c4 ("mm: mempolicy: Convert shared_policy mutex to spinlock")).
> 
> I think it is really worth to drill down to the bottom of the issue.
> While theoretically possible can be a good enough to justify the change
> it is usually preferred to describe the underlying problem for future
> maintainability. 

This issue mainly causes mpol_new memory leaks and this is pointed out in the commit log.
Am I supposed to do something more to move forward this patch ? Could you point that out
for me?

Many thanks!

>