Re: [PATCH 21/23] x86, kaiser: un-poison PGDs at runtime

Eduardo Valentin
2017-11-25T01:18:05Z

On Wed, Nov 22, 2017 at 04:35:21PM -0800, Dave Hansen wrote:
>
> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> With KAISER Kernel PGDs that map userspace are "poisoned" with
> the NX bit. This ensures that if a kernel->user CR3 switch is
> missed, userspace crashes instead of running in an unhardened
> state.
>
> This code will be needed in a moment when KAISER is turned
> on and off at runtime.
>
> Note that an __ASSEMBLY__ #ifdef is now required since kaiser.h
> is indirectly included into assembly.
>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Moritz Lipp <moritz.lipp@iaik.tugraz.at>
> Cc: Daniel Gruss <daniel.gruss@iaik.tugraz.at>
> Cc: Michael Schwarz <michael.schwarz@iaik.tugraz.at>
> Cc: Richard Fellner <richard.fellner@student.tugraz.at>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Kees Cook <keescook@google.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: x86@kernel.org
> ---
>
> b/arch/x86/include/asm/pgtable_64.h | 16 ++++++++++++++-
> b/arch/x86/mm/kaiser.c | 38 ++++++++++++++++++++++++++++++++++++
> b/include/linux/kaiser.h | 3 +-
> 3 files changed, 55 insertions(+), 2 deletions(-)
>
> diff -puN arch/x86/include/asm/pgtable_64.h~kaiser-dynamic-unpoison-pgd arch/x86/include/asm/pgtable_64.h
> --- a/arch/x86/include/asm/pgtable_64.h~kaiser-dynamic-unpoison-pgd 2017-11-22 15:45:55.818619722 -0800
> +++ b/arch/x86/include/asm/pgtable_64.h 2017-11-22 15:45:55.824619722 -0800
> @@ -3,6 +3,7 @@
> #define _ASM_X86_PGTABLE_64_H
>
> #include <linux/const.h>
> +#include <linux/kaiser.h>
> #include <asm/pgtable_64_types.h>
>
> #ifndef __ASSEMBLY__
> @@ -199,6 +200,18 @@ static inline bool pgd_userspace_access(
> return pgd.pgd & _PAGE_USER;
> }
>
> +static inline void kaiser_poison_pgd(pgd_t *pgd)
> +{
> + if (pgd->pgd & _PAGE_PRESENT)
> + pgd->pgd |= _PAGE_NX;
> +}
> +
> +static inline void kaiser_unpoison_pgd(pgd_t *pgd)
> +{
> + if (pgd->pgd & _PAGE_PRESENT)
> + pgd->pgd &= ~_PAGE_NX;
> +}
> +
> /*
> * Take a PGD location (pgdp) and a pgd value that needs
> * to be set there. Populates the shadow and returns
> @@ -222,7 +235,8 @@ static inline pgd_t kaiser_set_shadow_pg
> * wrong CR3 value, userspace will crash
> * instead of running.
> */
> - pgd.pgd |= _PAGE_NX;
> + if (kaiser_active())
> + kaiser_poison_pgd(&pgd);
> }
> } else if (pgd_userspace_access(*pgdp)) {
> /*
> diff -puN arch/x86/mm/kaiser.c~kaiser-dynamic-unpoison-pgd arch/x86/mm/kaiser.c
> --- a/arch/x86/mm/kaiser.c~kaiser-dynamic-unpoison-pgd 2017-11-22 15:45:55.819619722 -0800
> +++ b/arch/x86/mm/kaiser.c 2017-11-22 15:45:55.825619722 -0800
> @@ -501,6 +501,9 @@ static ssize_t kaiser_enabled_write_file
> if (enable > 1)
> return -EINVAL;
>
> + if (kaiser_enabled == enable)
> + return count;
> +
> WRITE_ONCE(kaiser_enabled, enable);
> return count;
> }

Shouldn't the above hunk be part of the patch that adds the debugfs entry?

> @@ -518,3 +521,38 @@ static int __init create_kaiser_enabled(
> return 0;
> }
> late_initcall(create_kaiser_enabled);
> +
> +enum poison {
> + KAISER_POISON,
> + KAISER_UNPOISON
> +};
> +void kaiser_poison_pgd_page(pgd_t *pgd_page, enum poison do_poison)
> +{
> + int i = 0;
> +
> + for (i = 0; i < PTRS_PER_PGD; i++) {
> + pgd_t *pgd = &pgd_page[i];
> +
> + /* Stop once we hit kernel addresses: */
> + if (!pgdp_maps_userspace(pgd))
> + break;
> +
> + if (do_poison == KAISER_POISON)
> + kaiser_poison_pgd(pgd);
> + else
> + kaiser_unpoison_pgd(pgd);
> + }
> +
> +}
> +
> +void kaiser_poison_pgds(enum poison do_poison)
> +{
> + struct page *page;
> +
> + spin_lock(&pgd_lock);
> + list_for_each_entry(page, &pgd_list, lru) {
> + pgd_t *pgd = (pgd_t *)page_address(page);
> + kaiser_poison_pgd_page(pgd, do_poison);
> + }
> + spin_unlock(&pgd_lock);
> +}
> diff -puN include/linux/kaiser.h~kaiser-dynamic-unpoison-pgd include/linux/kaiser.h
> --- a/include/linux/kaiser.h~kaiser-dynamic-unpoison-pgd 2017-11-22 15:45:55.821619722 -0800
> +++ b/include/linux/kaiser.h 2017-11-22 15:45:55.826619722 -0800
> @@ -4,7 +4,7 @@
> #ifdef CONFIG_KAISER
> #include <asm/kaiser.h>
> #else
> -
> +#ifndef __ASSEMBLY__
> /*
> * These stubs are used whenever CONFIG_KAISER is off, which
> * includes architectures that support KAISER, but have it
> @@ -33,5 +33,6 @@ static inline bool kaiser_active(void)
> {
> return 0;
> }
> +#endif /* __ASSEMBLY__ */
> #endif /* !CONFIG_KAISER */
> #endif /* _INCLUDE_KAISER_H */
> _
>

--
All the best,
Eduardo Valentin