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

[Andy Lutomirski]

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

Signed-off-by: Andy Lutomirski <luto@xxxxxxx>
---
 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

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