linux-kernel - Re: [PATCH 2/3] x86/mm: introduce __PAGE_KERNEL

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Message-ID: <0f8abc68-1092-1bae-d244-1adbbee455f9@linux.intel.com>
Date:   Fri, 16 Feb 2018 11:06:30 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org, luto@...nel.org,
        torvalds@...ux-foundation.org, keescook@...gle.com,
        hughd@...gle.com, jgross@...e.com, x86@...nel.org
Subject: Re: [PATCH 2/3] x86/mm: introduce __PAGE_KERNEL_GLOBAL

On 02/16/2018 10:25 AM, Nadav Amit wrote:
>> +#ifdef CONFIG_PAGE_TABLE_ISOLATION
>> +#define __PAGE_KERNEL_GLOBAL		0
>> +#else
>> +#define __PAGE_KERNEL_GLOBAL		_PAGE_GLOBAL
>> +#endif
> ...
>> --- a/arch/x86/mm/pageattr.c~kpti-no-global-for-kernel-mappings	2018-02-13 15:17:56.148210060 -0800
>> +++ b/arch/x86/mm/pageattr.c	2018-02-13 15:17:56.153210060 -0800
>> @@ -593,7 +593,8 @@ try_preserve_large_page(pte_t *kpte, uns
>> 	 * different bit positions in the two formats.
>> 	 */
>> 	req_prot = pgprot_4k_2_large(req_prot);
>> -	req_prot = pgprot_set_on_present(req_prot, _PAGE_GLOBAL | _PAGE_PSE);
>> +	req_prot = pgprot_set_on_present(req_prot,
>> +			__PAGE_KERNEL_GLOBAL | _PAGE_PSE);
>> 	req_prot = canon_pgprot(req_prot);
> From these chunks, it seems to me as req_prot will not have the global bit
> on when “nopti” parameter is provided. What am I missing?

That's a good point.  The current patch does not allow the use of
_PAGE_GLOBAL via _PAGE_KERNEL_GLOBAL when CONFIG_PAGE_TABLE_ISOLATION=y,
but booted with nopti.  It's a simple enough fix.  Logically:

#ifdef CONFIG_PAGE_TABLE_ISOLATION
#define __PAGE_KERNEL_GLOBAL	static_cpu_has(X86_FEATURE_PTI) ?
					0 : _PAGE_GLOBAL
#else
#define __PAGE_KERNEL_GLOBAL	_PAGE_GLOBAL
#endif

But I don't really want to hide that gunk in a macro like that.  It
might make more sense as a static inline.  I'll give that a shot and resent.