linux-kernel - Re: [patch 31/60] x86/mm/kpti: Add mapping helper functions

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Date:   Tue, 5 Dec 2017 17:01:34 +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,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [patch 31/60] x86/mm/kpti: Add mapping helper functions

On Mon, Dec 04, 2017 at 03:07:37PM +0100, Thomas Gleixner wrote:
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> Add the pagetable helper functions do manage the separate user space page
> tables.
> 
> [ tglx: Split out from the big combo kaiser patch ]
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> 
> ---
>  arch/x86/include/asm/pgtable_64.h |  139 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 139 insertions(+)

...

> +/*
> + * Take a PGD location (pgdp) and a pgd value that needs to be set there.
> + * Populates the user and returns the resulting PGD that must be set in
> + * the kernel copy of the page tables.
> + */
> +static inline pgd_t kpti_set_user_pgd(pgd_t *pgdp, pgd_t pgd)
> +{

Btw, do we want to inline a relatively big function like that? I see at
least 20-ish callsites of set_pgd() only.

> +#ifdef CONFIG_KERNEL_PAGE_TABLE_ISOLATION
> +	if (!static_cpu_has_bug(X86_BUG_CPU_SECURE_MODE_KPTI))
> +		return pgd;
> +
> +	if (pgd_userspace_access(pgd)) {
> +		if (pgdp_maps_userspace(pgdp)) {
> +			/*
> +			 * The user page tables get the full PGD,
> +			 * accessible from userspace:
> +			 */
> +			kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd;
> +			/*
> +			 * For the copy of the pgd that the kernel uses,
> +			 * make it unusable to userspace.  This ensures on
> +			 * in case that a return to userspace with the
> +			 * kernel CR3 value, userspace will crash instead
> +			 * of running.
> +			 *
> +			 * Note: NX might be not available or disabled.
> +			 */
> +			if (__supported_pte_mask & _PAGE_NX)
> +				pgd.pgd |= _PAGE_NX;
> +		}
> +	} else if (pgd_userspace_access(*pgdp)) {
> +		/*
> +		 * We are clearing a _PAGE_USER PGD for which we presumably
> +		 * populated the user PGD.  We must now clear the user PGD
> +		 * entry.
> +		 */
> +		if (pgdp_maps_userspace(pgdp)) {
> +			kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd;
> +		} else {
> +			/*
> +			 * Attempted to clear a _PAGE_USER PGD which is in
> +			 * the kernel porttion of the address space.  PGDs

"portion"

> +			 * are pre-populated and we never clear them.
> +			 */
> +			WARN_ON_ONCE(1);
> +		}
> +	} else {
> +		/*
> +		 * _PAGE_USER was not set in either the PGD being set or
> +		 * cleared.  All kernel PGDs should be pre-populated so
> +		 * this should never happen after boot.
> +		 */
> +		WARN_ON_ONCE(system_state == SYSTEM_RUNNING);
> +	}

Btw, we could keep the warning and have a separate path kernel users
like kernel_ident_mapping_init() (i.e., kexec, hibernation, et al) call
to bypass the warning vs all the remaining users which call the default
set_pgd().

> +#endif
> +	/* return the copy of the PGD we want the kernel to use: */
> +	return pgd;
> +}
> +
> +
>  static inline void native_set_p4d(p4d_t *p4dp, p4d_t p4d)
>  {
> +#if defined(CONFIG_KERNEL_PAGE_TABLE_ISOLATION) && !defined(CONFIG_X86_5LEVEL)
> +	p4dp->pgd = kpti_set_user_pgd(&p4dp->pgd, p4d.pgd);
> +#else
>  	*p4dp = p4d;
> +#endif
>  }
>  
>  static inline void native_p4d_clear(p4d_t *p4d)
> @@ -147,7 +282,11 @@ static inline void native_p4d_clear(p4d_
>  
>  static inline void native_set_pgd(pgd_t *pgdp, pgd_t pgd)
>  {
> +#ifdef CONFIG_KERNEL_PAGE_TABLE_ISOLATION
> +	*pgdp = kpti_set_user_pgd(pgdp, pgd);
> +#else
>  	*pgdp = pgd;
> +#endif

I guess that ifdef is not needed as kpti_set_user_pgd() already has it
so we can do simply:

static inline void native_set_pgd(pgd_t *pgdp, pgd_t pgd)
{
        *pgdp = kpti_set_user_pgd(pgdp, pgd);
}

AFAICT.

-- 
Regards/Gruss,
    Boris.

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