[PATCH v2 09/10] x86-64: Randomize int 0xcc magic al values at boot

Andy Lutomirski
2011-05-30T03:49:54Z

This is not a security feature. It's to prevent the int 0xcc
sequence from becoming ABI.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
---
arch/x86/kernel/vsyscall_64.c | 65 ++++++++++++++++++++++++++++++++++++++--
1 files changed, 61 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index 5b3d62a..f53f490 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -34,6 +34,8 @@
#include <linux/notifier.h>
#include <linux/syscalls.h>
#include <linux/ratelimit.h>
+#include <linux/random.h>
+#include <linux/highmem.h>

#include <asm/vsyscall.h>
#include <asm/pgtable.h>
@@ -54,6 +56,19 @@ DEFINE_VVAR(struct vsyscall_gtod_data, vsyscall_gtod_data) =
.lock = SEQLOCK_UNLOCKED,
};

+static u8 vsyscall_nr_offset; /* Cyclic permutation of al. */
+
+static inline u8 mangle_al(u8 al)
+{
+ /* Permute among 0xcc, 0xce, and 0xf0. */
+ return (al - 0xcc + 2*vsyscall_nr_offset) % 6 + 0xcc;
+}
+
+static inline u8 demangle_vsyscall_nr(u8 nr)
+{
+ return (nr + 3 - vsyscall_nr_offset) % 3;
+}
+
void update_vsyscall_tz(void)
{
unsigned long flags;
@@ -94,11 +109,12 @@ static void warn_bad_vsyscall(struct pt_regs *regs, bool is_warning,

tsk = current;

- printk("%s%s[%d] %s ip:%lx sp:%lx ax:%lx si:%lx di:%lx",
+ printk("%s%s[%d] %s ip:%lx sp:%lx ax:%lx offset:%d si:%lx di:%lx",
is_warning ? KERN_WARNING : KERN_INFO,
tsk->comm, task_pid_nr(tsk),
message,
- regs->ip - 2, regs->sp, regs->ax, regs->si, regs->di);
+ regs->ip - 2, regs->sp, regs->ax, (int)vsyscall_nr_offset,
+ regs->si, regs->di);
if (!in_vsyscall_page(regs->ip - 2))
print_vma_addr(" in ", regs->ip - 2);
printk("\n");
@@ -125,7 +141,8 @@ static __always_inline int fallback_gettimeofday(struct timeval *tv)
* Invoke do_emulate_vsyscall. Intentionally incompatible with
* the CONFIG_UNSAFE_VSYSCALLS=n case.
*/
- asm volatile("mov $0xce, %%al;\n\t"
+ asm volatile("fallback_gtod_movb:\n\t"
+ "movb $0xce, %%al;\n\t"
"int %[vec]"
: "=a" (ret)
: "D" (tv), [vec] "i" (VSYSCALL_EMU_VECTOR));
@@ -250,7 +267,7 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)

local_irq_enable();

- if ((regs->ax & 0xFF) != 0xce) {
+ if ((regs->ax & 0xFF) != mangle_al(0xce)) {
warn_bad_vsyscall(regs, false, "illegal int 0xcc "
"(exploit attempt?)");
force_sig(SIGSEGV, current);
@@ -316,6 +333,7 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
force_sig(SIGSEGV, current);
goto out;
}
+ vsyscall_nr = demangle_vsyscall_nr(vsyscall_nr);

if (regs->ip - 2 != vsyscall_intcc_addr(vsyscall_nr)) {
if (in_vsyscall_page(regs->ip - 2)) {
@@ -438,15 +456,54 @@ void __init map_vsyscall(void)
(unsigned long)VVAR_ADDRESS);
}

+static void __init mangle_vsyscall_movb(void *mapping,
+ unsigned long movb_addr, u8 initial)
+{
+ u8 *imm8;
+ BUG_ON(movb_addr >= 4095);
+
+ imm8 = (char*)(mapping) + movb_addr + 1;
+
+ BUG_ON(*imm8 != initial);
+ *imm8 = mangle_al(*imm8);
+}
+
static int __init vsyscall_init(void)
{
+ struct page *vsyscall_page;
+ extern char __vsyscall_0;
+ void *mapping;
+
#ifdef CONFIG_UNSAFE_VSYSCALLS
+ extern char fallback_gtod_movb;
+
BUG_ON(((unsigned long) &vgettimeofday !=
VSYSCALL_ADDR(__NR_vgettimeofday)));
BUG_ON((unsigned long) &vtime != VSYSCALL_ADDR(__NR_vtime));
BUG_ON((VSYSCALL_ADDR(0) != __fix_to_virt(VSYSCALL_FIRST_PAGE)));
BUG_ON((unsigned long) &vgetcpu != VSYSCALL_ADDR(__NR_vgetcpu));
#endif
+
+ /*
+ * Randomize the magic al values for int 0xcc invocation. This
+ * isn't really a security feature; it's to make sure that
+ * dynamic binary instrumentation tools don't start to think
+ * that the int 0xcc magic incantation is ABI.
+ */
+ vsyscall_nr_offset = get_random_int() % 3;
+ vsyscall_page = pfn_to_page(__pa_symbol(&__vsyscall_0) >> PAGE_SHIFT);
+ mapping = kmap_atomic(vsyscall_page);
+#ifdef CONFIG_UNSAFE_VSYSCALLS
+ mangle_vsyscall_movb(mapping,
+ &fallback_gtod_movb - (char*)&vgettimeofday, 0xce);
+#else
+ /* It's easier to hardcode the addresses -- they're ABI. */
+ mangle_vsyscall_movb(mapping, 0, 0xcc);
+ mangle_vsyscall_movb(mapping, 1024, 0xce);
+ mangle_vsyscall_movb(mapping, 2048, 0xf0);
+#endif
+ kunmap_atomic(mapping);
+
on_each_cpu(cpu_vsyscall_init, NULL, 1);
/* notifier priority > KVM */
hotcpu_notifier(cpu_vsyscall_notifier, 30);
--
1.7.5.1