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>

1 files changed, 17 insertions, 16 deletions

diff --git a/arch/x86/kernel/kprobes.c b/arch/x86/kernel/kprobes.c
index 345a4b1fe144..175f85ceace3 100644
--- a/arch/x86/kernel/kprobes.c
+++ b/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) {