[PATCH v2 02/10] x86-64: Give vvars their own page

Andy Lutomirski
2011-05-30T03:49:26Z

Move vvars out of the vsyscall page into their own page and mark it
NX.

Without this patch, an attacker who can force a daemon to call some
fixed address could wait until the time contains, say, 0xCD80, and
then execute the current time.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
---
arch/x86/include/asm/fixmap.h | 1 +
arch/x86/include/asm/pgtable_types.h | 2 ++
arch/x86/include/asm/vvar.h | 22 ++++++++++------------
arch/x86/kernel/vmlinux.lds.S | 27 ++++++++++++++++-----------
arch/x86/kernel/vsyscall_64.c | 5 +++++
5 files changed, 34 insertions(+), 23 deletions(-)

diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
index 4729b2b..460c74e 100644
--- a/arch/x86/include/asm/fixmap.h
+++ b/arch/x86/include/asm/fixmap.h
@@ -78,6 +78,7 @@ enum fixed_addresses {
VSYSCALL_LAST_PAGE,
VSYSCALL_FIRST_PAGE = VSYSCALL_LAST_PAGE
+ ((VSYSCALL_END-VSYSCALL_START) >> PAGE_SHIFT) - 1,
+ VVAR_PAGE,
VSYSCALL_HPET,
#endif
FIX_DBGP_BASE,
diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
index d56187c..6a29aed6 100644
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -108,6 +108,7 @@
#define __PAGE_KERNEL_UC_MINUS (__PAGE_KERNEL | _PAGE_PCD)
#define __PAGE_KERNEL_VSYSCALL (__PAGE_KERNEL_RX | _PAGE_USER)
#define __PAGE_KERNEL_VSYSCALL_NOCACHE (__PAGE_KERNEL_VSYSCALL | _PAGE_PCD | _PAGE_PWT)
+#define __PAGE_KERNEL_VVAR (__PAGE_KERNEL_RO | _PAGE_USER)
#define __PAGE_KERNEL_LARGE (__PAGE_KERNEL | _PAGE_PSE)
#define __PAGE_KERNEL_LARGE_NOCACHE (__PAGE_KERNEL | _PAGE_CACHE_UC | _PAGE_PSE)
#define __PAGE_KERNEL_LARGE_EXEC (__PAGE_KERNEL_EXEC | _PAGE_PSE)
@@ -130,6 +131,7 @@
#define PAGE_KERNEL_LARGE_EXEC __pgprot(__PAGE_KERNEL_LARGE_EXEC)
#define PAGE_KERNEL_VSYSCALL __pgprot(__PAGE_KERNEL_VSYSCALL)
#define PAGE_KERNEL_VSYSCALL_NOCACHE __pgprot(__PAGE_KERNEL_VSYSCALL_NOCACHE)
+#define PAGE_KERNEL_VVAR __pgprot(__PAGE_KERNEL_VVAR)

#define PAGE_KERNEL_IO __pgprot(__PAGE_KERNEL_IO)
#define PAGE_KERNEL_IO_NOCACHE __pgprot(__PAGE_KERNEL_IO_NOCACHE)
diff --git a/arch/x86/include/asm/vvar.h b/arch/x86/include/asm/vvar.h
index a4eaca4..de656ac 100644
--- a/arch/x86/include/asm/vvar.h
+++ b/arch/x86/include/asm/vvar.h
@@ -10,15 +10,14 @@
* In normal kernel code, they are used like any other variable.
* In user code, they are accessed through the VVAR macro.
*
- * Each of these variables lives in the vsyscall page, and each
- * one needs a unique offset within the little piece of the page
- * reserved for vvars. Specify that offset in DECLARE_VVAR.
- * (There are 896 bytes available. If you mess up, the linker will
- * catch it.)
+ * These variables live in a page of kernel data that has an extra RO
+ * mapping for userspace. Each variable needs a unique offset within
+ * that page; specify that offset with the DECLARE_VVAR macro. (If
+ * you mess up, the linker will catch it.)
*/

-/* Offset of vars within vsyscall page */
-#define VSYSCALL_VARS_OFFSET (3072 + 128)
+/* Base address of vvars. This is not ABI. */
+#define VVAR_ADDRESS (-10*1024*1024 - 4096)

#if defined(__VVAR_KERNEL_LDS)

@@ -26,17 +25,17 @@
* right place.
*/
#define DECLARE_VVAR(offset, type, name) \
- EMIT_VVAR(name, VSYSCALL_VARS_OFFSET + offset)
+ EMIT_VVAR(name, offset)

#else

#define DECLARE_VVAR(offset, type, name) \
static type const * const vvaraddr_ ## name = \
- (void *)(VSYSCALL_START + VSYSCALL_VARS_OFFSET + (offset));
+ (void *)(VVAR_ADDRESS + (offset));

#define DEFINE_VVAR(type, name) \
- type __vvar_ ## name \
- __attribute__((section(".vsyscall_var_" #name), aligned(16)))
+ type name \
+ __attribute__((section(".vvar_" #name), aligned(16)))

#define VVAR(name) (*vvaraddr_ ## name)

@@ -49,4 +48,3 @@ DECLARE_VVAR(16, int, vgetcpu_mode)
DECLARE_VVAR(128, struct vsyscall_gtod_data, vsyscall_gtod_data)

#undef DECLARE_VVAR
-#undef VSYSCALL_VARS_OFFSET
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 1432bd4..3c1ec1c 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -161,12 +161,6 @@ SECTIONS

#define VVIRT_OFFSET (VSYSCALL_ADDR - __vsyscall_0)
#define VVIRT(x) (ADDR(x) - VVIRT_OFFSET)
-#define EMIT_VVAR(x, offset) .vsyscall_var_ ## x \
- ADDR(.vsyscall_0) + offset \
- : AT(VLOAD(.vsyscall_var_ ## x)) { \
- *(.vsyscall_var_ ## x) \
- } \
- x = VVIRT(.vsyscall_var_ ## x);

. = ALIGN(4096);
__vsyscall_0 = .;
@@ -192,17 +186,28 @@ SECTIONS
*(.vsyscall_3)
}

-#define __VVAR_KERNEL_LDS
-#include <asm/vvar.h>
-#undef __VVAR_KERNEL_LDS
-
- . = __vsyscall_0 + PAGE_SIZE;
+ . = ALIGN(__vsyscall_0 + PAGE_SIZE, PAGE_SIZE);

#undef VSYSCALL_ADDR
#undef VLOAD_OFFSET
#undef VLOAD
#undef VVIRT_OFFSET
#undef VVIRT
+
+ __vvar_page = .;
+
+#define EMIT_VVAR(name, offset) .vvar_ ## name \
+ (__vvar_page + offset) : \
+ AT(ADDR(.vvar_ ## name) - LOAD_OFFSET) { \
+ *(.vvar_ ## x) \
+ } :data
+
+#define __VVAR_KERNEL_LDS
+#include <asm/vvar.h>
+#undef __VVAR_KERNEL_LDS
+
+ . = ALIGN(__vvar_page + PAGE_SIZE, PAGE_SIZE);
+
#undef EMIT_VVAR

#endif /* CONFIG_X86_64 */
diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index 5f6ad03..ee22180 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -284,9 +284,14 @@ void __init map_vsyscall(void)
{
extern char __vsyscall_0;
unsigned long physaddr_page0 = __pa_symbol(&__vsyscall_0);
+ extern char __vvar_page;
+ unsigned long physaddr_vvar_page = __pa_symbol(&__vvar_page);

/* Note that VSYSCALL_MAPPED_PAGES must agree with the code below. */
__set_fixmap(VSYSCALL_FIRST_PAGE, physaddr_page0, PAGE_KERNEL_VSYSCALL);
+ __set_fixmap(VVAR_PAGE, physaddr_vvar_page, PAGE_KERNEL_VVAR);
+ BUILD_BUG_ON((unsigned long)__fix_to_virt(VVAR_PAGE) !=
+ (unsigned long)VVAR_ADDRESS);
}

static int __init vsyscall_init(void)
--
1.7.5.1