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

Arnd Bergmann
2016-06-21T19:44:13Z

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.

Arnd