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SubjectRe: [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@linux.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@linux.intel.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.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)
--

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