kprobes/x86: Don't try to resolve kprobe faults from userspace

This commit:

commit 6f6343f53d133bae516caf3d254bce37d8774625
Author: Masami Hiramatsu <[e-mail]>
Date: Thu Apr 17 17:17:33 2014 +0900

kprobes/x86: Call exception handlers directly from do_int3/do_debug

appears to have inadvertently dropped a check that the int3 came
from kernel mode. Trying to dereference addr when addr is
user-controlled is completely bogus.

Signed-off-by: Andy Lutomirski <[e-mail]>
Acked-by: Masami Hiramatsu <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kernel/kprobes/core.c

···
 	struct kprobe *p;
 	struct kprobe_ctlblk *kcb;
 
+	if (user_mode_vm(regs))
+		return 0;
+
 	addr = (kprobe_opcode_t *)(regs->ip - sizeof(kprobe_opcode_t));
 	/*
 	 * We don't want to be preempted for the entire