linux-kernel - Re: [PATCH v5 08/39] x86/mm: Remove _PAGE

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Message-ID: <202301191652.57B10DA48@keescook>
Date:   Thu, 19 Jan 2023 16:52:45 -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>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v5 08/39] x86/mm: Remove _PAGE_DIRTY from kernel RO pages

On Thu, Jan 19, 2023 at 01:22:46PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@...el.com>
> 
> New processors that support Shadow Stack regard Write=0,Dirty=1 PTEs as
> shadow stack pages.
> 
> In normal cases, it can be helpful to create Write=1 PTEs as also Dirty=1
> if HW dirty tracking is not needed, because if the Dirty bit is not already
> set the CPU has to set Dirty=1 when the memory gets written to. This
> creates additional work for the CPU. So traditional wisdom was to simply
> set the Dirty bit whenever you didn't care about it. However, it was never
> really very helpful for read-only kernel memory.
> 
> When CR4.CET=1 and IA32_S_CET.SH_STK_EN=1, some instructions can write to
> such supervisor memory. The kernel does not set IA32_S_CET.SH_STK_EN, so
> avoiding kernel Write=0,Dirty=1 memory is not strictly needed for any
> functional reason. But having Write=0,Dirty=1 kernel memory doesn't have
> any functional benefit either, so to reduce ambiguity between shadow stack
> and regular Write=0 pages, remove Dirty=1 from any kernel Write=0 PTEs.
> 
> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> Tested-by: John Allen <john.allen@....com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>

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

-- 
Kees Cook