android: drivers: Avoid debugfs race in binder

If a /d/binder/proc/[pid] entry is kept open after linux has
torn down the associated process, binder_proc_show can deference
an invalid binder_proc that has been stashed in the debugfs
inode.  Validate that the binder_proc ptr passed into binder_proc_show
has not been freed by looking for it within the global process list
whilst the global lock is held. If the ptr is not valid, print nothing.

Cc: Colin Cross <ccross@android.com>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Dmitry Shmidt <dimitrysh@google.com>
Cc: Rom Lemarchand <romlem@google.com>
Cc: Serban Constantinescu <serban.constantinescu@arm.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Android Kernel Team <kernel-team@android.com>
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>
[jstultz: Minor commit message tweaks]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder.c

@@ -3600,13 +3600,24 @@ static int binder_transactions_show(struct seq_file *m, void *unused)
 
 static int binder_proc_show(struct seq_file *m, void *unused)
 {
+	struct binder_proc *itr;
 	struct binder_proc *proc = m->private;
 	int do_lock = !binder_debug_no_lock;
+	bool valid_proc = false;
 
 	if (do_lock)
 		binder_lock(__func__);
-	seq_puts(m, "binder proc state:\n");
-	print_binder_proc(m, proc, 1);
+
+	hlist_for_each_entry(itr, &binder_procs, proc_node) {
+		if (itr == proc) {
+			valid_proc = true;
+			break;
+		}
+	}
+	if (valid_proc) {
+		seq_puts(m, "binder proc state:\n");
+		print_binder_proc(m, proc, 1);
+	}
 	if (do_lock)
 		binder_unlock(__func__);
 	return 0;