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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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