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

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Date:   Thu, 25 Mar 2021 19:08:31 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Colin Ian King <colin.king@...onical.com>,
        dan.carpenter@...cle.com,
        Muhammad Usama Anjum <musamaanjum@...il.com>
Subject: [PATCH -tip 1/2] 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@...nel.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;
 	}
 }