[PATCH -tip 1/2] x86/kprobes: Fix to check non boostable prefixes correctly

[Masami Hiramatsu]

There are 2 bugs in the can_boost() function because of using
x86 insn decoder. Since the insn->opcode never has a prefix byte,
it can not find CS override prefix in it. And the insn->attr is
the attribute of the opcode, thus inat_is_address_size_prefix(
insn->attr) always returns false.

Fix those by checking each prefix bytes with for_each_insn_prefix
loop and getting the correct attribute for each prefix byte.
Also, this removes unlikely, because this is a slow path.

Fixes: a8d11cd0714f ("kprobes/x86: Consolidate insn decoder users for copying 
code")
Signed-off-by: Masami Hiramatsu <mhira...@kernel.org>
---
 arch/x86/kernel/kprobes/core.c |   17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 89d9f26785c7..503958f15cf9 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -139,6 +139,8 @@ NOKPROBE_SYMBOL(synthesize_relcall);
 int can_boost(struct insn *insn, void *addr)
 {
        kprobe_opcode_t opcode;
+       insn_byte_t prefix;
+       int i;
 
        if (search_exception_tables((unsigned long)addr))
                return 0;       /* Page fault may occur on this address. */
@@ -151,9 +153,14 @@ int can_boost(struct insn *insn, void *addr)
        if (insn->opcode.nbytes != 1)
                return 0;
 
-       /* Can't boost Address-size override prefix */
-       if (unlikely(inat_is_address_size_prefix(insn->attr)))
-               return 0;
+       for_each_insn_prefix(insn, i, prefix) {
+               insn_attr_t attr;
+
+               attr = inat_get_opcode_attribute(prefix);
+               /* Can't boost Address-size override prefix and CS override 
prefix */
+               if (prefix == 0x2e || inat_is_address_size_prefix(attr))
+                       return 0;
+       }
 
        opcode = insn->opcode.bytes[0];
 
@@ -181,8 +188,8 @@ int can_boost(struct insn *insn, void *addr)
                /* indirect jmp is boostable */
                return X86_MODRM_REG(insn->modrm.bytes[0]) == 4;
        default:
-               /* CS override prefix and call are not boostable */
-               return (opcode != 0x2e && opcode != 0x9a);
+               /* call is not boostable */
+               return opcode != 0x9a;
        }
 }