Re: [PATCH] mm: don't clobber partially overlapping VMA with MAP_FIXED_NOREPLACE

[John Hubbard]

On 10/10/18 10:38 AM, Michal Hocko wrote:
> On Wed 10-10-18 19:26:50, Jann Horn wrote:
> [...]
>> As you can see, the first page of the mapping at 0x10001000 was clobbered.
>>
>>>> diff --git a/mm/mmap.c b/mm/mmap.c
>>>> index 5f2b2b184c60..f7cd9cb966c0 100644
>>>> --- a/mm/mmap.c
>>>> +++ b/mm/mmap.c
>>>> @@ -1410,7 +1410,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>>>>       if (flags & MAP_FIXED_NOREPLACE) {
>>>>               struct vm_area_struct *vma = find_vma(mm, addr);
>>>>
>>>> -             if (vma && vma->vm_start <= addr)
>>>> +             if (vma && vma->vm_start < addr + len)
>>>
>>> find_vma is documented to - Look up the first VMA which satisfies addr <
>>> vm_end, NULL if none.
>>> This means that the above check guanratees that
>>>         vm_start <= addr < vm_end
>>> so an overlap is guanrateed. Why should we care how much we overlap?
>>
>> "an overlap is guaranteed"? I have no idea what you're trying to say.
> 
> I have misread your changelog and the patch. Sorry about that. I thought
> you meant a false possitive but you in fact meant false negative. Now it
> makes complete sense.
> 
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> 
> And thanks a lot for catching that!
> 

This also looks good to me. 

thanks,
-- 
John Hubbard
NVIDIA