Re: [PATCH 25/43] x86/mm/kaiser: Unmap kernel from userspace page tables (core patch), noexec=off

[Borislav Petkov]

On Fri, Nov 24, 2017 at 06:23:53PM +0100, Ingo Molnar wrote:
> + * Take a PGD location (pgdp) and a pgd value that needs
> + * to be set there.  Populates the shadow and returns
> + * the resulting PGD that must be set in the kernel copy
> + * of the page tables.
> + */
> +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 from userspace:
> +			 */
> +			kernel_to_shadow_pgdp(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;

Lemme hold this down here so that we don't forget (and tglx is working on it
already... ):

So we need to handle the case where we boot with "noexec=off" and thus
clear _PAGE_NX from __supported_pte_mask. I'd vouch for a conservative
solution where we warn if _PAGE_NX is not set in __supported_pte_mask
and thus at least tell the user that she shouldn't do noexec kernels and
expect kaiser protection...

-- 
Regards/Gruss,
    Boris.

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