[tip: x86/core] x86/kprobes: Fix to check non boostable prefixes correctly

[tip-bot2 for Masami Hiramatsu]

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     6dd3b8c9f58816a1354be39559f630cd1bd12159
Gitweb:        https://git.kernel.org/tip/6dd3b8c9f58816a1354be39559f630cd1bd12159
Author:        Masami Hiramatsu <mhiramat@xxxxxxxxxx>
AuthorDate:    Thu, 25 Mar 2021 19:08:31 +09:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Thu, 25 Mar 2021 11:37:21 +01:00

x86/kprobes: Fix to check non boostable prefixes correctly

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 <mhiramat@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Link: https://lore.kernel.org/r/161666691162.1120877.2808435205294352583.stgit@devnote2
---
 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 3a14e8a..81e1432 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;
 	}
 }