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

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Message-ID: <9b531543-9f97-178f-8a97-494322410806@redhat.com>
Date:   Wed, 22 Mar 2023 10:01:16 +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 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,

David / dhildenb