linux-kernel - Re: [patch 36/60] x86/mm/kpti: Add functions to clone kernel PMDs

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Date:   Wed, 6 Dec 2017 16:39:12 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirsky <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>, keescook@...gle.com,
        hughd@...gle.com, Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>, daniel.gruss@...k.tugraz.at
Subject: Re: [patch 36/60] x86/mm/kpti: Add functions to clone kernel PMDs

On Mon, Dec 04, 2017 at 03:07:42PM +0100, Thomas Gleixner wrote:
> From: Andy Lutomirski <luto@...nel.org>
> 
> Provide infrastructure to:
> 
>  - find a kernel PMD for a mapping which must be visible to user space for
>    the entry/exit code to work.
> 
>  - walk an address range and share the kernel PMD with it.
> 
> This reuses a small part of the original KAISER patches to populate the
> user space page table.
> 
> [ tglx: Made it universally usable so it can be used for any kind of shared
>   	mapping. Add a mechanism to clear specific bits in the user space
> 	visible PMD entry. ]
> 
> Originally-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> 
> ---
>  arch/x86/mm/kpti.c |  102 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 102 insertions(+)
> 
> --- a/arch/x86/mm/kpti.c
> +++ b/arch/x86/mm/kpti.c
> @@ -65,6 +65,108 @@ void __init kpti_check_boottime_disable(
>  }
>  
>  /*
> + * Walk the user copy of the page tables (optionally) trying to allocate
> + * page table pages on the way down.
> + *
> + * Returns a pointer to a PMD on success, or NULL on failure.
> + */
> +static pmd_t *kpti_user_pagetable_walk_pmd(unsigned long address)
> +{
> +	pgd_t *pgd = kernel_to_user_pgdp(pgd_offset_k(address));
> +	gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
> +	pud_t *pud;
> +	p4d_t *p4d;
> +
> +	if (address < PAGE_OFFSET) {
> +		WARN_ONCE(1, "attempt to walk user address\n");
> +		return NULL;
> +	}
> +
> +	if (pgd_none(*pgd)) {
> +		WARN_ONCE(1, "All user pgds should have been populated\n");
> +		return NULL;
> +	}
> +	BUILD_BUG_ON(pgd_large(*pgd) != 0);

Must be some 5LEVEL thing? Because it currently does:

static inline int pgd_large(pgd_t pgd) { return 0; }

> +
> +	p4d = p4d_offset(pgd, address);
> +	BUILD_BUG_ON(p4d_large(*p4d) != 0);

That too.

> +	if (p4d_none(*p4d)) {
> +		unsigned long new_pud_page = __get_free_page(gfp);
> +		if (!new_pud_page)
> +			return NULL;
> +
> +		if (p4d_none(*p4d)) {

We already tested that above or does __get_free_page() have side-effects?

> +			set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
> +			new_pud_page = 0;
> +		}
> +		if (new_pud_page)
> +			free_page(new_pud_page);
> +	}
> +
> +	pud = pud_offset(p4d, address);
> +	/* The user page tables do not use large mappings: */
> +	if (pud_large(*pud)) {
> +		WARN_ON(1);
> +		return NULL;
> +	}
> +	if (pud_none(*pud)) {
> +		unsigned long new_pmd_page = __get_free_page(gfp);
> +		if (!new_pmd_page)
> +			return NULL;
> +
> +		if (pud_none(*pud)) {

Ditto.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--