Re: [PATCH 2/2] mm: refuse wrapped vm_brk requests

[Kees Cook]

On Mon, Jul 11, 2016 at 8:28 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> I think both patches are fine, just a question.
>
> On 07/08, Kees Cook wrote:
>>
>> -static int do_brk(unsigned long addr, unsigned long len)
>> +static int do_brk(unsigned long addr, unsigned long request)
>>  {
>>       struct mm_struct *mm = current->mm;
>>       struct vm_area_struct *vma, *prev;
>> -     unsigned long flags;
>> +     unsigned long flags, len;
>>       struct rb_node **rb_link, *rb_parent;
>>       pgoff_t pgoff = addr >> PAGE_SHIFT;
>>       int error;
>>
>> -     len = PAGE_ALIGN(len);
>> +     len = PAGE_ALIGN(request);
>> +     if (len < request)
>> +             return -ENOMEM;
>
> So iiuc "len < request" is only possible if len == 0, right?

Oh, hrm, good point.

>
>>       if (!len)
>>               return 0;
>
> and thus this patch fixes the error code returned by do_brk() in case
> of overflow, now it returns -ENOMEM rather than zero. Perhaps
>
>         if (!len)
>                 return 0;
>         len = PAGE_ALIGN(len);
>         if (!len)
>                 return -ENOMEM;
>
> would be more clear but this is subjective.

I'm fine either way.

> I am wondering if we should shift this overflow check to the caller(s).
> Say, sys_brk() does find_vma_intersection(mm, oldbrk, newbrk+PAGE_SIZE)
> before do_brk(), and in case of overflow find_vma_intersection() can
> wrongly return NULL.
>
> Then do_brk() will be called with len = -oldbrk, this can overflow or
> not but in any case this doesn't look right too.
>
> Or I am totally confused?

I think the callers shouldn't request a negative value, sure, but
vm_brk should notice and refuse it.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security