[RFC PATCH 2/2] x86/kprobes: Prohibit probing on compiler generated CFI checking code

["Masami Hiramatsu (Google)" <mhiramat@kernel.org>]

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Prohibit probing on the compiler generated CFI typeid checking code
because it is used for decoding typeid when CFI error happens.

The compiler generates the following instruction sequence for indirect
call checks on x86;

   movl    -<id>, %r10d       ; 6 bytes
   addl    -4(%reg), %r10d    ; 4 bytes
   je      .Ltmp1             ; 2 bytes
   ud2                        ; <- regs->ip

And handle_cfi_failure() decodes these instructions (movl and addl)
for the typeid and the target address. Thus if we put a kprobe on
those instructions, the decode will fail and report a wrong typeid
and target address.


Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 arch/x86/kernel/kprobes/core.c |   34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index f7f6042eb7e6..fa8c2b41cbaf 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -54,6 +54,7 @@
 #include <asm/insn.h>
 #include <asm/debugreg.h>
 #include <asm/ibt.h>
+#include <asm/cfi.h>
 
 #include "common.h"
 
@@ -293,7 +294,40 @@ static int can_probe(unsigned long paddr)
 #endif
 		addr += insn.length;
 	}
+	if (IS_ENABLED(CONFIG_CFI_CLANG)) {
+		/*
+		 * The compiler generates the following instruction sequence
+		 * for indirect call checks and cfi.c decodes this;
+		 *
+		 *   movl    -<id>, %r10d       ; 6 bytes
+		 *   addl    -4(%reg), %r10d    ; 4 bytes
+		 *   je      .Ltmp1             ; 2 bytes
+		 *   ud2                        ; <- regs->ip
+		 *   .Ltmp1:
+		 *
+		 * Also, these movl and addl are used for showing expected
+		 * type. So those must not be touched.
+		 */
+		__addr = recover_probed_instruction(buf, addr);
+		if (!__addr)
+			return 0;
+
+		if (insn_decode_kernel(&insn, (void *)__addr) < 0)
+			return 0;
+
+		if (insn.opcode.value == 0xBA)
+			offset = 12;
+		else if (insn.opcode.value == 0x3)
+			offset = 6;
+		else
+			goto out;
+
+		/* This movl/addl is used for decoding CFI. */
+		if (is_cfi_trap(addr + offset))
+			return 0;
+	}
 
+out:
 	return (addr == paddr);
 }