kprobes/x86: Fix kprobes to skip prefixes correctly

Fix resume_execution() and is_IF_modifier() to skip x86
instruction prefixes correctly by using x86 instruction
attribute.

Without this fix, resume_execution() can't handle instructions
which have non-REX prefixes (REX prefixes are skipped). This
will cause unexpected kernel panic by hitting bad address when a
kprobe hits on two-byte ret (e.g. "repz ret" generated for
Athlon/K8 optimization), because it just checks "repz" and can't
recognize the "ret" instruction.

These prefixes can be found easily with x86 instruction
attribute. This patch introduces skip_prefixes() and uses it in
resume_execution() and is_IF_modifier() to skip prefixes.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
LKML-Reference: <4C298A6E.8070609@hitachi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/kprobes.c

@@ -126,16 +126,22 @@ static void __kprobes synthesize_reljump(void *from, void *to)
 }
 
 /*
- * Check for the REX prefix which can only exist on X86_64
- * X86_32 always returns 0
+ * Skip the prefixes of the instruction.
  */
-static int __kprobes is_REX_prefix(kprobe_opcode_t *insn)
+static kprobe_opcode_t *__kprobes skip_prefixes(kprobe_opcode_t *insn)
 {
+	insn_attr_t attr;
+
+	attr = inat_get_opcode_attribute((insn_byte_t)*insn);
+	while (inat_is_legacy_prefix(attr)) {
+		insn++;
+		attr = inat_get_opcode_attribute((insn_byte_t)*insn);
+	}
 #ifdef CONFIG_X86_64
-	if ((*insn & 0xf0) == 0x40)
-		return 1;
+	if (inat_is_rex_prefix(attr))
+		insn++;
 #endif
-	return 0;
+	return insn;
 }
 
 /*
@@ -272,6 +278,9 @@ static int __kprobes can_probe(unsigned long paddr)
  */
 static int __kprobes is_IF_modifier(kprobe_opcode_t *insn)
 {
+	/* Skip prefixes */
+	insn = skip_prefixes(insn);
+
 	switch (*insn) {
 	case 0xfa:		/* cli */
 	case 0xfb:		/* sti */
@@ -280,13 +289,6 @@ static int __kprobes is_IF_modifier(kprobe_opcode_t *insn)
 		return 1;
 	}
 
-	/*
-	 * on X86_64, 0x40-0x4f are REX prefixes so we need to look
-	 * at the next byte instead.. but of course not recurse infinitely
-	 */
-	if (is_REX_prefix(insn))
-		return is_IF_modifier(++insn);
-
 	return 0;
 }
 
@@ -803,9 +805,8 @@ static void __kprobes resume_execution(struct kprobe *p,
 	unsigned long orig_ip = (unsigned long)p->addr;
 	kprobe_opcode_t *insn = p->ainsn.insn;
 
-	/*skip the REX prefix*/
-	if (is_REX_prefix(insn))
-		insn++;
+	/* Skip prefixes */
+	insn = skip_prefixes(insn);
 
 	regs->flags &= ~X86_EFLAGS_TF;
 	switch (*insn) {