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

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Message-Id: <490345b6-3e3d-4692-8162-85dcb71434c9@www.fastmail.com>
Date:   Tue, 17 Aug 2021 13:51:52 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Borislav Petkov" <bp@...en8.de>,
        "luto@...capital.net" <luto@...capital.net>
Cc:     "Yu-cheng Yu" <yu-cheng.yu@...el.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org,
        "Linux API" <linux-api@...r.kernel.org>,
        "Arnd Bergmann" <arnd@...db.de>,
        "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 (Intel)" <peterz@...radead.org>,
        "Randy Dunlap" <rdunlap@...radead.org>,
        "Shankar, Ravi V" <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 1:24 PM, Borislav Petkov wrote:
> 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.

Potahto.

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

WRSS can be used from user mode depending on the configuration.

> > 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).

Double-you shmouble-you.  You can't write it with MOV, but you can write it from user code and from kernel code.  As far as the mm is concerned, I think it should be considered writable.

Although... anyone who tries to copy_to_user() it is going to be a bit surprised.  Hmm.

> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette
>