linux-kernel - Re: [PATCH v23 09/28] x86/mm: Introduce _PAGE

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Message-ID: <20210322112951.6mqjgxmkafmiavpb@box>
Date:   Mon, 22 Mar 2021 14:29:51 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Yu-cheng Yu <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>,
        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>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...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>
Subject: Re: [PATCH v23 09/28] x86/mm: Introduce _PAGE_COW

On Tue, Mar 16, 2021 at 08:10:35AM -0700, Yu-cheng Yu wrote:
> 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 by hardware
> and cannot normally be set by hardware on a Write=0 PTE.  Software must
> normally be involved to create one of these PTEs, so software can simply
> opt to not create them.
> 
> 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) A modified, copy-on-write (COW) page: (Write=0, Cow=1)
> (b) A R/O page that has been COW'ed: (Write=0, Cow=1)
>     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) A shadow stack PTE: (Write=0, Dirty=1)
> (d) A shared shadow stack PTE: (Write=0, Cow=1)
>     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) A page where the processor observed a Write=1 PTE, started a write, set
>     Dirty=1, but then observed a Write=0 PTE.  That's possible today, but
>     will not happen on processors that support shadow stack.
> 
> Define _PAGE_COW and update pte_*() helpers and apply the same changes to
> pmd and pud.
> 
> After this, there are six free bits left in the 64-bit PTE, and no more
> free bits in the 32-bit PTE (except for PAE) and Shadow Stack is not
> implemented for the 32-bit kernel.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov