linux-kernel - Re: [PATCH v4 1/4] mm/mlock: return EINVAL if len overflows for mlock/munlock

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Message-ID: <3600eca8-1924-0a8e-ea9a-ab362c9e2967@redhat.com>
Date:   Wed, 22 Mar 2023 12:27:40 +0100
From:   David Hildenbrand <david@...hat.com>
To:     mawupeng <mawupeng1@...wei.com>, akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kuleshovmail@...il.com, aneesh.kumar@...ux.ibm.com
Subject: Re: [PATCH v4 1/4] mm/mlock: return EINVAL if len overflows for
 mlock/munlock

On 22.03.23 10:20, mawupeng wrote:
> 
> 
> On 2023/3/22 17:01, David Hildenbrand wrote:
>> On 22.03.23 09:54, David Hildenbrand wrote:
>>> On 22.03.23 03:14, mawupeng wrote:
>>>>
>>>>
>>>> On 2023/3/21 22:19, David Hildenbrand wrote:
>>>>> On 21.03.23 08:44, mawupeng wrote:
>>>>>>
>>>>>>
>>>>>> On 2023/3/20 18:54, David Hildenbrand wrote:
>>>>>>> On 20.03.23 03:47, Wupeng Ma wrote:
>>>>>>>> From: Ma Wupeng <mawupeng1@...wei.com>
>>>>>>>>
>>>>>>>> While testing mlock, we have a problem if the len of mlock is ULONG_MAX.
>>>>>>>> The return value of mlock is zero. But nothing will be locked since the
>>>>>>>> len in do_mlock overflows to zero due to the following code in mlock:
>>>>>>>>
>>>>>>>>        len = PAGE_ALIGN(len + (offset_in_page(start)));
>>>>>>>>
>>>>>>>> The same problem happens in munlock.
>>>>>>>>
>>>>>>>> Add new check and return -EINVAL to fix this overflowing scenarios since
>>>>>>>> they are absolutely wrong.
>>>>>>>
>>>>>>> Thinking again, wouldn't we reject mlock(0, ULONG_MAX) now as well?
>>>>>>
>>>>>> mlock will return 0 if len is zero which is the same w/o this patchset.
>>>>>> Here is the calltrace if len is zero.
>>>>>>
>>>>>> mlock(len == 0)
>>>>>>        do_mlock(len == 0)
>>>>>>            if (!len)
>>>>>>                return 0
>>>>>>
>>>>>
>>>>> I was asking about addr=0, len=ULONG_MAX.
>>>>>
>>>>> IIUC, that used to work but could now fail? I haven't played with it, though.
>>>>
>>>> Thanks for reviewing.
>>>>
>>>> Previous for add = 0 and len == ULONG_MAX, mlock will return ok(0) since len overflows to zero.
>>>> IFAICT, this is not right since mlock do noting(lock nothing) and return ok(0).
>>>>
>>>> With this patch, for the same situation, mlock can return EINVAL as expected.
>>>
>>> Quoting the man page:
>>>
>>> "EINVAL (mlock(),  mlock2(),  and  munlock()) The result of the addition
>>> addr+len was less than addr (e.g., the addition may have resulted in an
>>> overflow)."
>>>
>>> ULONG_MAX+0 = ULONG_MAX
>>>
>>> There is no overflow expected. The proper way to implement it would be
>>> to handle that case and not fail with EINVAL.
>>>
>>> At least that would be expected when reading the man page.
>>>
>>
>> As a side note, I agree that failing with EINVAL is better than doing noting (mlocking nothing). But I am not sure what we are expected to do in that case ... the man page is a bit vague on that.
> 
> Thanks for you reviewing.
> 
> Can we try to expand the man page since overflow is considered in man page?

I guess we could spell out that Linux aligns the length up to the next 
page boundary, and that overflow checks are performed on this aligned 
length.

-- 
Thanks,

David / dhildenb