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

Dave Hansen
2017-11-23T00:36:30Z

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;
}
@@ -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 */
_