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

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Message-ID: <207bbd69-6678-5120-3760-e2bcd9803a14@huawei.com>
Date:   Thu, 17 Mar 2022 17:34:32 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Michal Hocko <mhocko@...e.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] mm/mempolicy: fix potential mpol_new leak in
 shared_policy_replace

On 2022/3/17 17:03, Michal Hocko wrote:
> On Thu 17-03-22 10:05:08, Miaohe Lin wrote:
>> 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?
> 
> Sorry if I was not really clear. My main request is to have a clear
> insight whether this is a theretical issue or the leak could be really
> triggered. If the later we need to mark it properly and backport to
> older kernels because memory leaks can lead to DoS when they are
> reasonably easy to trigger.
> 
> Is this more clear now?

I see. Many thanks. I would have a try to trigger this. :)

>