[PATCH 2/3] x86/uprobes: Fix not using prefixes.nbytes for loop over prefixes.bytes

[Masami Hiramatsu]

Since the insn.prefixes.nbytes can be bigger than the size of
insn.prefixes.bytes[] when a same prefix is repeated, we have to
check whether the insn.prefixes.bytes[i] != 0 and i < 4 instead
of insn.prefixes.nbytes.

Fixes: 2b1444983508 ("uprobes, mm, x86: Add the ability to install and remove 
uprobes breakpoints")
Cc: sta...@vger.kernel.org
Reported-by: Kees Cook <keesc...@chromium.org>
Signed-off-by: Masami Hiramatsu <mhira...@kernel.org>
---
 arch/x86/kernel/uprobes.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 3fdaa042823d..bb3ea3705b99 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -257,7 +257,7 @@ static bool is_prefix_bad(struct insn *insn)
 {
        int i;
 
-       for (i = 0; i < insn->prefixes.nbytes; i++) {
+       for (i = 0; insn->prefixes.bytes[i] && i < 4; i++) {
                insn_attr_t attr;
 
                attr = inat_get_opcode_attribute(insn->prefixes.bytes[i]);
@@ -746,7 +746,7 @@ static int branch_setup_xol_ops(struct arch_uprobe 
*auprobe, struct insn *insn)
         * Intel and AMD behavior differ in 64-bit mode: Intel ignores 66 
prefix.
         * No one uses these insns, reject any branch insns with such prefix.
         */
-       for (i = 0; i < insn->prefixes.nbytes; i++) {
+       for (i = 0; insn->prefixes.bytes[i] && i < 4; i++) {
                if (insn->prefixes.bytes[i] == 0x66)
                        return -ENOTSUPP;
        }