linux-kernel - Re: [PATCH v3 1/4] mm/mlock: return EINVAL if len overflows for mlock/munlock

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Date:   Wed, 8 Feb 2023 14:51:08 +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 v3 1/4] mm/mlock: return EINVAL if len overflows for
 mlock/munlock

On 07.02.23 02:24, mawupeng wrote:
> 
> 
> On 2023/2/7 1:05, David Hildenbrand wrote:
>> On 06.02.23 01:48, mawupeng wrote:
>>>
>>>
>>> On 2023/2/4 1:14, David Hildenbrand wrote:
>>>> On 28.01.23 07:32, 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.
>>>>>
>>>>> Return 0 early to avoid burn a bunch of cpu cycles if len == 0.
>>>>>
>>>>> Signed-off-by: Ma Wupeng <mawupeng1@...wei.com>
>>>>> ---
>>>>>     mm/mlock.c | 14 ++++++++++++--
>>>>>     1 file changed, 12 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/mm/mlock.c b/mm/mlock.c
>>>>> index 7032f6dd0ce1..eb09968ba27f 100644
>>>>> --- a/mm/mlock.c
>>>>> +++ b/mm/mlock.c
>>>>> @@ -478,8 +478,6 @@ static int apply_vma_lock_flags(unsigned long start, size_t len,
>>>>>         end = start + len;
>>>>>         if (end < start)
>>>>>             return -EINVAL;
>>>>> -    if (end == start)
>>>>> -        return 0;
>>>>>         vma = mas_walk(&mas);
>>>>>         if (!vma)
>>>>>             return -ENOMEM;
>>>>> @@ -575,7 +573,13 @@ static __must_check int do_mlock(unsigned long start, size_t len, vm_flags_t fla
>>>>>         if (!can_do_mlock())
>>>>>             return -EPERM;
>>>>>     +    if (!len)
>>>>> +        return 0;
>>>>> +
>>>>>         len = PAGE_ALIGN(len + (offset_in_page(start)));
>>>>> +    if (!len)
>>>>> +        return -EINVAL;
>>>>> +
>>>>>         start &= PAGE_MASK;
>>>>
>>>> The "ordinary" overflows are detected in apply_vma_lock_flags(), correct?
>>>
>>> Overflow is not checked anywhere however the ordinary return early if len == 0 is detected in apply_vma_lock_flags().
>>>
>>
>> I meant the
>>
>> end = start + len;
>> if (end < start)
>>      return -EINVAL;
>>
>> Essentially, what I wanted to double-check is that with your changes, we catch all kinds of overflows as documented in the man page, correct?
> 
> Oh i see. You are right, The "ordinary" overflows are detected for mlock/munlock in apply_vma_lock_flags().
> 
> Yes, we may need to update the man page for all these four syscalls.

E.g., mlock() already documents "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)."

Just to rephrase my question what I wanted to double-check: are we now 
identifying all such overflows or are you aware of other corner cases?

-- 
Thanks,

David / dhildenb