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

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