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Date:   Fri, 2 Dec 2022 18:29:05 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc:     x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Arnd Bergmann <arnd@arndb.de>,
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        Andrew.Cooper3@citrix.com, christina.schimpe@intel.com,
        Yu-cheng Yu <yu-cheng.yu@intel.com>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v4 08/39] x86/mm: Remove _PAGE_DIRTY from kernel RO pages
Message-ID: <202212021829.6BC87357@keescook>
References: <20221203003606.6838-1-rick.p.edgecombe@intel.com>
 <20221203003606.6838-9-rick.p.edgecombe@intel.com>
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On Fri, Dec 02, 2022 at 04:35:35PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.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 it the memory gets written to. This
> creates addiontal work for the CPU. So tradional 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, removed Dirty=1 from any kernel Write=0 PTEs.
> 
> Tested-by: Pengfei Xu <pengfei.xu@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

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

-- 
Kees Cook