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

[Rik van Riel <riel@redhat.com>]

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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.

I suspect all the 64 bit architectures are fine in that regard,
with enormous gaps between kernel memory ranges.

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