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
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