x86/insn-eval: Use new for_each_insn_prefix() macro to loop over prefixes bytes

Since insn.prefixes.nbytes can be bigger than the size of
insn.prefixes.bytes[] when a prefix is repeated, the proper check must
be

  insn.prefixes.bytes[i] != 0 and i < 4

instead of using insn.prefixes.nbytes. Use the new
for_each_insn_prefix() macro which does it correctly.

Debugged by Kees Cook <keescook@chromium.org>.

 [ bp: Massage commit message. ]

Fixes: 32d0b95300db ("x86/insn-eval: Add utility functions to get segment selector")
Reported-by: syzbot+9b64b619f10f19d19a7c@syzkaller.appspotmail.com
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/160697104969.3146288.16329307586428270032.stgit@devnote2

diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index 58f7fb9..4229950 100644 (file)
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -63,13 +63,12 @@ static bool is_string_insn(struct insn *insn)
  */
 bool insn_has_rep_prefix(struct insn *insn)
 {
+       insn_byte_t p;
        int i;
 
        insn_get_prefixes(insn);
 
-       for (i = 0; i < insn->prefixes.nbytes; i++) {
-               insn_byte_t p = insn->prefixes.bytes[i];
-
+       for_each_insn_prefix(insn, i, p) {
                if (p == 0xf2 || p == 0xf3)
                        return true;
        }
@@ -95,14 +94,15 @@ static int get_seg_reg_override_idx(struct insn *insn)
 {
        int idx = INAT_SEG_REG_DEFAULT;
        int num_overrides = 0, i;
+       insn_byte_t p;
 
        insn_get_prefixes(insn);
 
        /* Look for any segment override prefixes. */
-       for (i = 0; i < insn->prefixes.nbytes; i++) {
+       for_each_insn_prefix(insn, i, p) {
                insn_attr_t attr;
 
-               attr = inat_get_opcode_attribute(insn->prefixes.bytes[i]);
+               attr = inat_get_opcode_attribute(p);
                switch (attr) {
                case INAT_MAKE_PREFIX(INAT_PFX_CS):
                        idx = INAT_SEG_REG_CS;