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    SubjectRe: [patch 36/60] x86/mm/kpti: Add functions to clone kernel PMDs
    On Mon, Dec 04, 2017 at 03:07:42PM +0100, Thomas Gleixner wrote:
    > From: Andy Lutomirski <luto@kernel.org>
    >
    > Provide infrastructure to:
    >
    > - find a kernel PMD for a mapping which must be visible to user space for
    > the entry/exit code to work.
    >
    > - walk an address range and share the kernel PMD with it.
    >
    > This reuses a small part of the original KAISER patches to populate the
    > user space page table.
    >
    > [ tglx: Made it universally usable so it can be used for any kind of shared
    > mapping. Add a mechanism to clear specific bits in the user space
    > visible PMD entry. ]
    >
    > Originally-by: Dave Hansen <dave.hansen@linux.intel.com>
    > Signed-off-by: Andy Lutomirski <luto@kernel.org>
    > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    >
    > ---
    > arch/x86/mm/kpti.c | 102 +++++++++++++++++++++++++++++++++++++++++++++++++++++
    > 1 file changed, 102 insertions(+)
    >
    > --- a/arch/x86/mm/kpti.c
    > +++ b/arch/x86/mm/kpti.c
    > @@ -65,6 +65,108 @@ void __init kpti_check_boottime_disable(
    > }
    >
    > /*
    > + * Walk the user copy of the page tables (optionally) trying to allocate
    > + * page table pages on the way down.
    > + *
    > + * Returns a pointer to a PMD on success, or NULL on failure.
    > + */
    > +static pmd_t *kpti_user_pagetable_walk_pmd(unsigned long address)
    > +{
    > + pgd_t *pgd = kernel_to_user_pgdp(pgd_offset_k(address));
    > + gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
    > + pud_t *pud;
    > + p4d_t *p4d;
    > +
    > + if (address < PAGE_OFFSET) {
    > + WARN_ONCE(1, "attempt to walk user address\n");
    > + return NULL;
    > + }
    > +
    > + if (pgd_none(*pgd)) {
    > + WARN_ONCE(1, "All user pgds should have been populated\n");
    > + return NULL;
    > + }
    > + BUILD_BUG_ON(pgd_large(*pgd) != 0);

    Must be some 5LEVEL thing? Because it currently does:

    static inline int pgd_large(pgd_t pgd) { return 0; }

    > +
    > + p4d = p4d_offset(pgd, address);
    > + BUILD_BUG_ON(p4d_large(*p4d) != 0);

    That too.

    > + if (p4d_none(*p4d)) {
    > + unsigned long new_pud_page = __get_free_page(gfp);
    > + if (!new_pud_page)
    > + return NULL;
    > +
    > + if (p4d_none(*p4d)) {

    We already tested that above or does __get_free_page() have side-effects?

    > + set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
    > + new_pud_page = 0;
    > + }
    > + if (new_pud_page)
    > + free_page(new_pud_page);
    > + }
    > +
    > + pud = pud_offset(p4d, address);
    > + /* The user page tables do not use large mappings: */
    > + if (pud_large(*pud)) {
    > + WARN_ON(1);
    > + return NULL;
    > + }
    > + if (pud_none(*pud)) {
    > + unsigned long new_pmd_page = __get_free_page(gfp);
    > + if (!new_pmd_page)
    > + return NULL;
    > +
    > + if (pud_none(*pud)) {

    Ditto.

    --
    Regards/Gruss,
    Boris.

    SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
    --

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