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Date:   Tue, 17 Aug 2021 22:24:47 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     "Yu, Yu-cheng" <yu-cheng.yu@intel.com>, x86@kernel.org,
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        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        Rick P Edgecombe <rick.p.edgecombe@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v28 09/32] x86/mm: Introduce _PAGE_COW
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 <1A27F5DF-477B-45B7-AD33-CC68D9B7CB89@amacapital.net>
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On Tue, Aug 17, 2021 at 01:13:09PM -0700, Andy Lutomirski wrote:
> > If special kernel code using shadow stack management insns needs
> > to modify a shadow stack, then it can check whether a page is
> > pte/pmd_shstk() but that code is special anyway.
> > 
> > Hell, a shadow stack page is (Write=0, Dirty=1) so calling it writable
> >                  ^^^^^^^
> > is simply wrong.
> 
> But it *is* writable using WRUSS, and it’s also writable by CALL,

Well, if we have to be precise, CALL doesn't write it directly - it
causes for shadow stack to be written as part of CALL's execution. Yeah
yeah, potato potato.

> WRSS, etc.

Thus the "special kernel code" thing above. I've left it in instead of
snipping it.

> Now if the mm code tries to write protect it and expects sensible
> semantics, the results could be interesting. At the very least,
> someone would need to validate that RET reading a read only shadow
> stack page does the right thing.

Huh?

A shadow stack page is RO (W=0).

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette