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Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and
 pmdp_set_wrprotect for _PAGE_DIRTY_SW
From:   Yu-cheng Yu <yu-cheng.yu@intel.com>
To:     Jann Horn <jannh@google.com>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        the arch/x86 maintainers <x86@kernel.org>,
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Date:   Thu, 30 Aug 2018 13:52:46 -0700
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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@intel.com>
> 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().

Yu-cheng