linux-kernel - Re: [PATCH v2 10/39] x86/mm: Introduce _PAGE

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Message-ID: <Y0kusvhDIRQxB2+h@hirez.programming.kicks-ass.net>
Date:   Fri, 14 Oct 2022 11:41:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rick Edgecombe <rick.p.edgecombe@...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>,
        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>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        joao.moreira@...el.com, John Allen <john.allen@....com>,
        kcc@...gle.com, eranian@...gle.com, rppt@...nel.org,
        jamorris@...ux.microsoft.com, dethoma@...rosoft.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v2 10/39] x86/mm: Introduce _PAGE_COW

On Thu, Sep 29, 2022 at 03:29:07PM -0700, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
> 
> There is essentially no room left in the x86 hardware PTEs on some OSes
> (not Linux). That left the hardware architects looking for a way to
> represent a new memory type (shadow stack) within the existing bits.
> They chose to repurpose a lightly-used state: Write=0,Dirty=1.
> 
> The reason it's lightly used is that Dirty=1 is normally set _before_ a
> write. A write with a Write=0 PTE would typically only generate a fault,
> not set Dirty=1. Hardware can (rarely) both set Write=1 *and* generate the

s/Write/Dirty/

> fault, resulting in a Dirty=0,Write=1 PTE. Hardware which supports shadow

s/Dirty=0,Write=1/Write=0,Dirty=1/

> stacks will no longer exhibit this oddity.
> 
> The kernel should avoid inadvertently creating shadow stack memory because
> it is security sensitive. So given the above, all it needs to do is avoid
> manually crating Write=0,Dirty=1 PTEs in software.

Whichever way around you choose, please be consistent.

> In places where Linux normally creates Write=0,Dirty=1, it can use the
> software-defined _PAGE_COW in place of the hardware _PAGE_DIRTY. In other
> words, whenever Linux needs to create Write=0,Dirty=1, it instead creates
> Write=0,Cow=1 except for shadow stack, which is Write=0,Dirty=1. This
> clearly separates shadow stack from other data, and results in the
> following:
> 
> (a) (Write=0,Cow=1,Dirty=0) A modified, copy-on-write (COW) page.
>     Previously when a typical anonymous writable mapping was made COW via
>     fork(), the kernel would mark it Write=0,Dirty=1. Now it will instead
>     use the Cow bit.
> (b) (Write=0,Cow=1,Dirty=0) A R/O page that has been COW'ed. The user page
>     is in a R/O VMA, and get_user_pages() needs a writable copy. The page
>     fault handler creates a copy of the page and sets the new copy's PTE
>     as Write=0 and Cow=1.
> (c) (Write=0,Cow=0,Dirty=1) A shadow stack PTE.
> (d) (Write=0,Cow=1,Dirty=0) A shared shadow stack PTE. When a shadow stack
>     page is being shared among processes (this happens at fork()), its PTE
>     is made Dirty=0, so the next shadow stack access causes a fault, and
>     the page is duplicated and Dirty=1 is set again. This is the COW
>     equivalent for shadow stack pages, even though it's copy-on-access
>     rather than copy-on-write.
> (e) (Write=0,Cow=0,Dirty=1) A Cow PTE created when a processor without
>     shadow stack support set Dirty=1.

Please restureture this (and the comment) something like:


  (Write=0,Dirty=0,Cow=1):

	- copy_present_pte(): A modified copy-on-write page.
	- ...


  (Write=0,Dirty=1,Cow=0):

	- FEATURE_CET:  Shadow Stack entry
	- !FEATURE_CET: see the above Cow=1 cases