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>

1 files changed, 13 insertions, 2 deletions

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 1db376184955..f0ce9959d14d 100644
--- a/drivers/android/binder.c
+++ b/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;