linux-kernel - Re: [PATCH 09/30] x86, kaiser: only populate shadow page tables for userspace

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Date:   Mon, 20 Nov 2017 21:12:40 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        moritz.lipp@...k.tugraz.at, daniel.gruss@...k.tugraz.at,
        michael.schwarz@...k.tugraz.at, richard.fellner@...dent.tugraz.at,
        luto@...nel.org, torvalds@...ux-foundation.org,
        keescook@...gle.com, hughd@...gle.com, x86@...nel.org
Subject: Re: [PATCH 09/30] x86, kaiser: only populate shadow page tables for
 userspace

On Fri, 10 Nov 2017, Dave Hansen wrote:

This should be folded into the previous patch.

>  b/arch/x86/include/asm/pgtable_64.h |   94 +++++++++++++++++++++++-------------
>  1 file changed, 61 insertions(+), 33 deletions(-)
> 
> diff -puN arch/x86/include/asm/pgtable_64.h~kaiser-set-pgd-careful-plus-NX arch/x86/include/asm/pgtable_64.h
> --- a/arch/x86/include/asm/pgtable_64.h~kaiser-set-pgd-careful-plus-NX	2017-11-10 11:22:09.932244947 -0800
> +++ b/arch/x86/include/asm/pgtable_64.h	2017-11-10 11:22:09.935244947 -0800
> @@ -177,38 +177,76 @@ static inline p4d_t *native_get_normal_p
>  /*
>   * Page table pages are page-aligned.  The lower half of the top
>   * level is used for userspace and the top half for the kernel.
> - * This returns true for user pages that need to get copied into
> - * both the user and kernel copies of the page tables, and false
> - * for kernel pages that should only be in the kernel copy.
> + *
> + * Returns true for parts of the PGD that map userspace and
> + * false for the parts that map the kernel.
>   */
> -static inline bool is_userspace_pgd(void *__ptr)
> +static inline bool pgdp_maps_userspace(void *__ptr)
>  {
>  	unsigned long ptr = (unsigned long)__ptr;
>  
>  	return ((ptr % PAGE_SIZE) < (PAGE_SIZE / 2));
>  }
>  
> +/*
> + * Does this PGD allow access via userspace?

s/via/from/

> + */
> +static inline bool pgd_userspace_access(pgd_t pgd)
> +{
> +	return (pgd.pgd & _PAGE_USER);
> +}
> +
> +/*
> + * Returns the pgd_t that the kernel should use in its page tables.

Should? Can the caller still decide to put something different there? I
doubt that.

> +static inline pgd_t kaiser_set_shadow_pgd(pgd_t *pgdp, pgd_t pgd)
> +{
> +#ifdef CONFIG_KAISER
> +	if (pgd_userspace_access(pgd)) {
> +		if (pgdp_maps_userspace(pgdp)) {
> +			/*
> +			 * The user/shadow page tables get the full
> +			 * PGD, accessible to userspace:

s/to/from/

> +			 */
> +			native_get_shadow_pgd(pgdp)->pgd = pgd.pgd;
> +			/*
> +			 * For the copy of the pgd that the kernel
> +			 * uses, make it unusable to userspace.  This
> +			 * ensures if we get out to userspace with the
> +			 * wrong CR3 value, userspace will crash
> +			 * instead of running.
> +			 */
> +			pgd.pgd |= _PAGE_NX;
> +		}
> +	} else if (!pgd.pgd) {
> +		/*
> +		 * We are clearing the PGD and can not check  _PAGE_USER
> +		 * in the zero'd PGD.

Just the argument cannot be checked because it's clearing the entry. The
pgd entry itself is not yet modified, so it could be checked.

  		 * We never do this on the
> +		 * pre-populated kernel PGDs, except for pgd_bad().
> +		 */
> +		if (pgdp_maps_userspace(pgdp)) {
> +			native_get_shadow_pgd(pgdp)->pgd = pgd.pgd;
> +		} else {
> +			/*
> +			 * Uh, we are very confused.  We have been
> +			 * asked to clear a PGD that is in the kernel
> +			 * part of the address space.  We preallocated
> +			 * all the KAISER PGDs, so this should never
> +			 * happen.
> +			 */
> +			WARN_ON_ONCE(1);
> +		}
> +	}

Thanks,

	tglx