Re: [PATCH 12/16] x86/mm/pae: Populate the user page-table with user pgd's

[Dave Hansen]

On 01/16/2018 08:36 AM, Joerg Roedel wrote:
> +#ifdef CONFIG_X86_64
>  	/*
>  	 * If this is normal user memory, make it NX in the kernel
>  	 * pagetables so that, if we somehow screw up and return to
> @@ -134,10 +135,16 @@ pgd_t __pti_set_user_pgd(pgd_t *pgdp, pgd_t pgd)
>  	 *     may execute from it
>  	 *  - we don't have NX support
>  	 *  - we're clearing the PGD (i.e. the new pgd is not present).
> +	 *  - We run on a 32 bit kernel. 2-level paging doesn't support NX at
> +	 *    all and PAE paging does not support it on the PGD level. We can
> +	 *    set it in the PMD level there in the future, but that means we
> +	 *    need to unshare the PMDs between the kernel and the user
> +	 *    page-tables.
>  	 */
>  	if ((pgd.pgd & (_PAGE_USER|_PAGE_PRESENT)) == (_PAGE_USER|_PAGE_PRESENT) &&
>  	    (__supported_pte_mask & _PAGE_NX))
>  		pgd.pgd |= _PAGE_NX;
> +#endif

Ugh.  The ghosts of PAE have come back to haunt us.

Could we do:

static inline bool pgd_supports_nx(unsigned long)
{
#ifdef CONFIG_X86_64
	return (__supported_pte_mask & _PAGE_NX);
#else
	/* No 32-bit page tables support NX at PGD level */
	return 0;
#endif
}

Nobody will ever spot the #ifdef the way you laid it out.