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

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Date:   Fri, 16 Feb 2018 11:54:03 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        keescook@...gle.com, Hugh Dickins <hughd@...gle.com>,
        Juergen Gross <jgross@...e.com>, x86@...nel.org
Subject: Re: [PATCH 2/3] x86/mm: introduce __PAGE_KERNEL_GLOBAL

Dave Hansen <dave.hansen@...ux.intel.com> wrote:

> 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.

Since determining whether PTI is on is done in several places in the kernel,
maybe there should a single function to determine whether PTI is on,
something like:

static inline bool is_pti_on(void)
{
	return IS_ENABLED(CONFIG_PAGE_TABLE_ISOLATION) && 
		static_cpu_has(X86_FEATURE_PTI);
}