Re: Re: [PATCH v3 06/13] fork: Add generic vmalloced stack support

Andy Lutomirski
2016-06-21T19:43:56Z

On Tue, Jun 21, 2016 at 12:44 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> On Tuesday, June 21, 2016 2:32:28 PM CEST Rik van Riel wrote:
>> On Tue, 2016-06-21 at 10:13 -0700, Kees Cook wrote:
>> > On Tue, Jun 21, 2016 at 9:59 AM, Andy Lutomirski <luto@amacapital.net
>> > > wrote:
>> > >
>> > > I'm tempted to explicitly disallow VM_NO_GUARD in the vmalloc
>> > > range.
>> > > It has no in-tree users for non-fixed addresses right now.
>> > What about the lack of pre-range guard page? That seems like a
>> > critical feature for this.
>>
>> If VM_NO_GUARD is disallowed, and every vmalloc area has
>> a guard area behind it, then every subsequent vmalloc area
>> will have a guard page ahead of it.
>>
>> I think disallowing VM_NO_GUARD will be all that is required.
>>
>> The only thing we may want to verify on the architectures that
>> we care about is that there is nothing mapped immediately before
>> the start of the vmalloc range, otherwise the first vmalloced
>> area will not have a guard page below it.
>
> FWIW, ARM has an 8MB guard area between the linear mapping of
> physical memory and the start of the vmalloc area. I have not
> checked any of the other architectures though.

If we start banning VM_NO_GUARD in the vmalloc area, we could also
explicitly prevent use of the bottom page of the vmalloc area.

>
> Arnd

--
Andy Lutomirski
AMA Capital Management, LLC