From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
We poison kernel PGDs that map userspace with the NX bit. This
ensures that if we miss a kernel->user CR3 switch, userspace
crashes instead of running in an unhardened state.
We will need this code in a moment when we turn kaiser on and off
at runtime.
Note that we now need an __ASSEMBLY__ #ifdef since we are now
indirectly including kaiser.h into assembly.
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Moritz Lipp <moritz.lipp@xxxxxxxxxxxxxx>
Cc: Daniel Gruss <daniel.gruss@xxxxxxxxxxxxxx>
Cc: Michael Schwarz <michael.schwarz@xxxxxxxxxxxxxx>
Cc: Richard Fellner <richard.fellner@xxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
---
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 2017-11-08 10:45:40.781681366 -0800
+++ b/arch/x86/include/asm/pgtable_64.h 2017-11-08 10:45:40.788681366 -0800
@@ -2,6 +2,7 @@
#define _ASM_X86_PGTABLE_64_H
#include <linux/const.h>
+#include <linux/kaiser.h>
#include <asm/pgtable_64_types.h>
#ifndef __ASSEMBLY__
@@ -196,6 +197,18 @@ 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;
+}
+
/*
* Returns the pgd_t that the kernel should use in its page tables.
*/
@@ -216,7 +229,8 @@ 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.pgd) {
/*
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 2017-11-08 10:45:40.783681366 -0800
+++ b/arch/x86/mm/kaiser.c 2017-11-08 10:45:40.788681366 -0800
@@ -477,6 +477,9 @@ 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;
}
@@ -494,3 +497,38 @@ 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 2017-11-08 10:45:40.784681366 -0800
+++ b/include/linux/kaiser.h 2017-11-08 10:45:40.788681366 -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
@@ -29,5 +29,6 @@ static inline bool kaiser_active(void)
{
return 0;
}
+#endif /* __ASSEMBLY__ */
#endif /* !CONFIG_KAISER */
#endif /* _INCLUDE_KAISER_H */
_
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