Re: [PATCH v19 07/25] x86/mm: Remove _PAGE_DIRTY from kernel RO pages

Kees Cook
2021-02-04T20:10:34Z

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