x86/kprobes: Inline kprobe_exceptions_notify() into do_general_protection()

The opaque plumbing of #GP from do_general_protection() through
notify_die() into kprobe_exceptions_notify() makes it hard to understand
what's going on.
Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Kees Cook <keescook@chromium.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: kernel-hardening@lists.openwall.com
Cc: dvyukov@google.com
Cc: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: Borislav Petkov <bp@alien8.de>
Link: https://lkml.kernel.org/r/20180828201421.157735-3-jannh@google.com

arch/x86/kernel/kprobes/core.c

@@ -1028,42 +1028,13 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
 		if (fixup_exception(regs, trapnr))
 			return 1;
 
-		/*
-		 * fixup routine could not handle it,
-		 * Let do_page_fault() fix it.
-		 */
+		/* fixup routine could not handle it. */
 	}
 
 	return 0;
 }
 NOKPROBE_SYMBOL(kprobe_fault_handler);
 
-/*
- * Wrapper routine for handling exceptions.
- */
-int kprobe_exceptions_notify(struct notifier_block *self, unsigned long val,
-			     void *data)
-{
-	struct die_args *args = data;
-	int ret = NOTIFY_DONE;
-
-	if (args->regs && user_mode(args->regs))
-		return ret;
-
-	if (val == DIE_GPF) {
-		/*
-		 * To be potentially processing a kprobe fault and to
-		 * trust the result from kprobe_running(), we have
-		 * be non-preemptible.
-		 */
-		if (!preemptible() && kprobe_running() &&
-		    kprobe_fault_handler(args->regs, args->trapnr))
-			ret = NOTIFY_STOP;
-	}
-	return ret;
-}
-NOKPROBE_SYMBOL(kprobe_exceptions_notify);
-
 bool arch_within_kprobe_blacklist(unsigned long addr)
 {
 	bool is_in_entry_trampoline_section = false;

arch/x86/kernel/traps.c

@@ -556,6 +556,16 @@ do_general_protection(struct pt_regs *regs, long error_code)
 
 		tsk->thread.error_code = error_code;
 		tsk->thread.trap_nr = X86_TRAP_GP;
+
+		/*
+		 * To be potentially processing a kprobe fault and to
+		 * trust the result from kprobe_running(), we have to
+		 * be non-preemptible.
+		 */
+		if (!preemptible() && kprobe_running() &&
+		    kprobe_fault_handler(regs, X86_TRAP_GP))
+			return;
+
 		if (notify_die(DIE_GPF, "general protection fault", regs, error_code,
 			       X86_TRAP_GP, SIGSEGV) != NOTIFY_STOP)
 			die("general protection fault", regs, error_code);