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SubjectRe: [PATCH 09/30] x86, kaiser: only populate shadow page tables for userspace
On Fri, 10 Nov 2017, Dave Hansen wrote:

This should be folded into the previous patch.

> b/arch/x86/include/asm/pgtable_64.h | 94 +++++++++++++++++++++++-------------
> 1 file changed, 61 insertions(+), 33 deletions(-)
>
> diff -puN arch/x86/include/asm/pgtable_64.h~kaiser-set-pgd-careful-plus-NX arch/x86/include/asm/pgtable_64.h
> --- a/arch/x86/include/asm/pgtable_64.h~kaiser-set-pgd-careful-plus-NX 2017-11-10 11:22:09.932244947 -0800
> +++ b/arch/x86/include/asm/pgtable_64.h 2017-11-10 11:22:09.935244947 -0800
> @@ -177,38 +177,76 @@ static inline p4d_t *native_get_normal_p
> /*
> * Page table pages are page-aligned. The lower half of the top
> * level is used for userspace and the top half for the kernel.
> - * This returns true for user pages that need to get copied into
> - * both the user and kernel copies of the page tables, and false
> - * for kernel pages that should only be in the kernel copy.
> + *
> + * Returns true for parts of the PGD that map userspace and
> + * false for the parts that map the kernel.
> */
> -static inline bool is_userspace_pgd(void *__ptr)
> +static inline bool pgdp_maps_userspace(void *__ptr)
> {
> unsigned long ptr = (unsigned long)__ptr;
>
> return ((ptr % PAGE_SIZE) < (PAGE_SIZE / 2));
> }
>
> +/*
> + * Does this PGD allow access via userspace?

s/via/from/

> + */
> +static inline bool pgd_userspace_access(pgd_t pgd)
> +{
> + return (pgd.pgd & _PAGE_USER);
> +}
> +
> +/*
> + * Returns the pgd_t that the kernel should use in its page tables.

Should? Can the caller still decide to put something different there? I
doubt that.

> +static inline pgd_t kaiser_set_shadow_pgd(pgd_t *pgdp, pgd_t pgd)
> +{
> +#ifdef CONFIG_KAISER
> + if (pgd_userspace_access(pgd)) {
> + if (pgdp_maps_userspace(pgdp)) {
> + /*
> + * The user/shadow page tables get the full
> + * PGD, accessible to userspace:

s/to/from/

> + */
> + native_get_shadow_pgd(pgdp)->pgd = pgd.pgd;
> + /*
> + * For the copy of the pgd that the kernel
> + * uses, make it unusable to userspace. This
> + * ensures if we get out to userspace with the
> + * wrong CR3 value, userspace will crash
> + * instead of running.
> + */
> + pgd.pgd |= _PAGE_NX;
> + }
> + } else if (!pgd.pgd) {
> + /*
> + * We are clearing the PGD and can not check _PAGE_USER
> + * in the zero'd PGD.

Just the argument cannot be checked because it's clearing the entry. The
pgd entry itself is not yet modified, so it could be checked.

* We never do this on the
> + * pre-populated kernel PGDs, except for pgd_bad().
> + */
> + if (pgdp_maps_userspace(pgdp)) {
> + native_get_shadow_pgd(pgdp)->pgd = pgd.pgd;
> + } else {
> + /*
> + * Uh, we are very confused. We have been
> + * asked to clear a PGD that is in the kernel
> + * part of the address space. We preallocated
> + * all the KAISER PGDs, so this should never
> + * happen.
> + */
> + WARN_ON_ONCE(1);
> + }
> + }

Thanks,

tglx

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