linux-kernel - Re: [PATCH v28 09/32] x86/mm: Introduce _PAGE

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Message-ID: <YRwbD1hCYFXlYysI@zn.tnic>
Date:   Tue, 17 Aug 2021 22:24:47 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>,
        Rick P Edgecombe <rick.p.edgecombe@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v28 09/32] x86/mm: Introduce _PAGE_COW

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