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 10:57 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>
> On Thu, 2018-08-30 at 22:44 +0200, Jann Horn wrote:
> > On Thu, Aug 30, 2018 at 10:25 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > wrote:
> ...
> > > 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?
>
> INCSSP has a range of 256, but we can do multiple of that.
> But I realize the key is not to have the transient SHSTK page at all.
> The guard page is !pte_write() and even we have flaws in
> ptep_set_wrprotect(), there will not be any transient SHSTK pages. I
> will add guard pages to both ends.
>
> Still thinking how to fix ptep_set_wrprotect().

cmpxchg loop? Or is that slow?