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

[Kees Cook]

On Tue, Jul 12, 2016 at 9:39 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 07/11, Kees Cook wrote:
>>
>> On Mon, Jul 11, 2016 at 8:28 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>> >
>> > 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.
>
> Me too, so feel free to ignore,
>
>> > 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.
>
> Not sure I understand...
>
> I tried to say that, with or without this change, sys_brk() should check
> for overflow too, otherwise it looks buggy.

Hmm, it's not clear to me the right way to fix sys_brk(), but it looks
like my change to do_brk() would catch the problem?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security