Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and pmdp_set_wrprotect for _PAGE_DIRTY_SW

[Jann Horn]

On Thu, Aug 30, 2018 at 10:25 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>
> On Thu, 2018-08-30 at 19:59 +0200, Jann Horn wrote:
> > On Thu, Aug 30, 2018 at 7:58 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > wrote:
> > >
> > >
> > > On Thu, 2018-08-30 at 10:33 -0700, Dave Hansen wrote:
> > > >
> > > > On 08/30/2018 10:26 AM, Yu-cheng Yu wrote:
> > > > >
> > > > >
> > > > > We don't have the guard page now, but there is a shadow stack
> > > > > token
> > > > > there, which cannot be used as a return address.
> > > > The overall concern is that we could overflow into a page that
> > > > we
> > > > did
> > > > not intend.  Either another actual shadow stack or something
> > > > that a
> > > > page
> > > > that the attacker constructed, like the transient scenario Jann
> > > > described.
> > > >
> > > A task could go beyond the bottom of its shadow stack by doing
> > > either
> > > 'ret' or 'incssp'.  If it is the 'ret' case, the token prevents
> > > it.
> > >  If it is the 'incssp' case, a guard page cannot prevent it
> > > entirely,
> > > right?
> > I mean the other direction, on "call".
>
> In the flow you described, if C writes to the overflow page before B
> gets in with a 'call', the return address is still correct for B.  To
> make an attack, C needs to write again before the TLB flush.  I agree
> that is possible.
>
> Assume we have a guard page, can someone in the short window do
> recursive calls in B, move ssp to the end of the guard page, and
> trigger the same again?  He can simply take the incssp route.

I don't understand what you're saying. If the shadow stack is between
guard pages, you should never be able to move SSP past that area's
guard pages without an appropriate shadow stack token (not even with
INCSSP, since that has a maximum range of PAGE_SIZE/2), and therefore,
it shouldn't matter whether memory outside that range is incorrectly
marked as shadow stack. Am I missing something?