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

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Message-ID: <YRwT7XX36fQ2GWXn@zn.tnic>
Date:   Tue, 17 Aug 2021 21:54:21 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Cc:     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 11:24:29AM -0700, Yu, Yu-cheng wrote:
> Indeed, this can be looked at in a few ways.  We can visualize pte_write()
> as 'CPU can write to it with MOV' or 'CPU can write to it with any opcodes'.
> Depending on whatever pte_write() is, copy-on-write code can be adjusted
> accordingly.

Can be?

I think you should exclude shadow stack pages from being writable
and treat them as read-only. How the CPU writes them is immaterial -
pte/pmd_write() is used by normal kernel code to query whether the page
is writable or not by any instruction - not by the CPU.

And since normal kernel code cannot write shadow stack pages, then for
that code those pages are read-only.

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.

Thx.

-- 
Regards/Gruss,
    Boris.

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