linux-kernel - RE: [PATCH 20/35] mm: Update can_follow_write

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Date:   Thu, 10 Feb 2022 22:45:00 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Dave Hansen' <dave.hansen@...el.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        "x86@...nel.org" <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-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        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>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        "joao.moreira@...el.com" <joao.moreira@...el.com>,
        John Allen <john.allen@....com>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "eranian@...gle.com" <eranian@...gle.com>
CC:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: RE: [PATCH 20/35] mm: Update can_follow_write_pte() for shadow stack

From: Dave Hansen
> Sent: 09 February 2022 22:52
> 
> On 1/30/22 13:18, Rick Edgecombe wrote:
> > Like a writable data page, a shadow stack page is writable, and becomes
> > read-only during copy-on-write, but it is always dirty.
> 
> One other thing...
> 
> The language in these changelogs is a bit sloppy.  For instance, what
> does "always dirty" mean here?  pte_dirty()?  Or strictly _PAGE_DIRTY?
> 
> In other words, logically dirty, or literally "has *the* dirty bit set"?

Doesn't COW have to set it readonly - so that the access faults.
And then set the fault code set it readonly+dirty (without write)
to allow the shadow stack accesses to not-fault.

Or am I mis-guessing what the docs actually say?

	David

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