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Date:   Thu, 4 Feb 2021 12:10:15 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc:     x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v19 07/25] x86/mm: Remove _PAGE_DIRTY from kernel RO pages
Message-ID: <202102041209.B1A0FF3D@keescook>
References: <20210203225547.32221-1-yu-cheng.yu@intel.com>
 <20210203225547.32221-8-yu-cheng.yu@intel.com>
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In-Reply-To: <20210203225547.32221-8-yu-cheng.yu@intel.com>
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On Wed, Feb 03, 2021 at 02:55:29PM -0800, Yu-cheng Yu wrote:
> The x86 family of processors do not directly create read-only and Dirty
> PTEs.  These PTEs are created by software.  One such case is that kernel
> read-only pages are historically setup as Dirty.
> 
> New processors that support Shadow Stack regard read-only and Dirty PTEs as
> shadow stack pages.  This results in ambiguity between shadow stack and
> kernel read-only pages.  To resolve this, removed Dirty from kernel read-
> only pages.

Does this have any other side-effects?

Otherwise, sure:

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

-Kees

> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> ---
>  arch/x86/include/asm/pgtable_types.h | 6 +++---
>  arch/x86/mm/pat/set_memory.c         | 2 +-
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 394757ee030a..b8b79d618bbc 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -194,10 +194,10 @@ enum page_cache_mode {
>  #define _KERNPG_TABLE		 (__PP|__RW|   0|___A|   0|___D|   0|   0| _ENC)
>  #define _PAGE_TABLE_NOENC	 (__PP|__RW|_USR|___A|   0|___D|   0|   0)
>  #define _PAGE_TABLE		 (__PP|__RW|_USR|___A|   0|___D|   0|   0| _ENC)
> -#define __PAGE_KERNEL_RO	 (__PP|   0|   0|___A|__NX|___D|   0|___G)
> -#define __PAGE_KERNEL_ROX	 (__PP|   0|   0|___A|   0|___D|   0|___G)
> +#define __PAGE_KERNEL_RO	 (__PP|   0|   0|___A|__NX|   0|   0|___G)
> +#define __PAGE_KERNEL_ROX	 (__PP|   0|   0|___A|   0|   0|   0|___G)
>  #define __PAGE_KERNEL_NOCACHE	 (__PP|__RW|   0|___A|__NX|___D|   0|___G| __NC)
> -#define __PAGE_KERNEL_VVAR	 (__PP|   0|_USR|___A|__NX|___D|   0|___G)
> +#define __PAGE_KERNEL_VVAR	 (__PP|   0|_USR|___A|__NX|   0|   0|___G)
>  #define __PAGE_KERNEL_LARGE	 (__PP|__RW|   0|___A|__NX|___D|_PSE|___G)
>  #define __PAGE_KERNEL_LARGE_EXEC (__PP|__RW|   0|___A|   0|___D|_PSE|___G)
>  #define __PAGE_KERNEL_WP	 (__PP|__RW|   0|___A|__NX|___D|   0|___G| __WP)
> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 16f878c26667..6bebb95a6988 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -1932,7 +1932,7 @@ int set_memory_nx(unsigned long addr, int numpages)
>  
>  int set_memory_ro(unsigned long addr, int numpages)
>  {
> -	return change_page_attr_clear(&addr, numpages, __pgprot(_PAGE_RW), 0);
> +	return change_page_attr_clear(&addr, numpages, __pgprot(_PAGE_RW | _PAGE_DIRTY), 0);
>  }
>  
>  int set_memory_rw(unsigned long addr, int numpages)
> -- 
> 2.21.0
> 

-- 
Kees Cook