linux-kernel - Re: [PATCH v4 10/39] x86/mm: Introduce _PAGE

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Date:   Fri, 2 Dec 2022 18:31:12 -0800
From:   Kees Cook <keescook@...omium.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>,
        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>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Andrew.Cooper3@...rix.com, christina.schimpe@...el.com,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v4 10/39] x86/mm: Introduce _PAGE_COW

On Fri, Dec 02, 2022 at 04:35:37PM -0800, Rick Edgecombe wrote:
> Some OSes have a greater dependence on software available bits in PTEs than
> 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. So in order to support
> shadow stack memory, Linux should avoid creating memory with this PTE bit
> combination unless it intends for it to be shadow stack.
> 
> The reason it's lightly used is that Dirty=1 is normally set by HW
> _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 Dirty=1 *and*
> generate the fault, resulting in a Write=0,Dirty=1 PTE. Hardware which
> supports shadow stacks will no longer exhibit this oddity.
> 
> So that leaves Write=0,Dirty=1 PTEs created in software. To achieve this,
> 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.
> Further differentiated by VMA flags, these PTE bit combinations would be
> set as follows for various types of memory:
> 
> (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. This
>    happens in copy_present_pte().
>  - A R/O page that has been COW'ed. The user page is in a R/O VMA,
>    and get_user_pages(FOLL_FORCE) 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.
>  - 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.
> 
> (Write=0,Cow=0,Dirty=1):
>  - A shadow stack PTE.
>  - A Cow PTE created when a processor without shadow stack support set
>    Dirty=1.
> 
> There are six bits left available to software in the 64-bit PTE after
> consuming a bit for _PAGE_COW. No space is consumed in 32-bit kernels
> because shadow stacks are not enabled there.
> 
> This is a prepratory patch. Changes to actually start marking _PAGE_COW
> will follow once other pieces are in place.
> 
> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> Tested-by: John Allen <john.allen@....com>
> Co-developed-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook