inotify: race use after free/double free in inotify inode marks

There is a race in the inotify add/rm watch code.  A task can find and
remove a mark which doesn't have all of it's references.  This can
result in a use after free/double free situation.

Task A					Task B
------------				-----------
inotify_new_watch()
 allocate a mark (refcnt == 1)
 add it to the idr
					inotify_rm_watch()
					 inotify_remove_from_idr()
					  fsnotify_put_mark()
					      refcnt hits 0, free
 take reference because we are on idr
 [at this point it is a use after free]
 [time goes on]
 refcnt may hit 0 again, double free

The fix is to take the reference BEFORE the object can be found in the
idr.

Signed-off-by: Eric Paris <eparis@redhat.com>
Cc: <stable@kernel.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
index 40da732eb73c..e46ca685b9be 100644
--- a/fs/notify/inotify/inotify_user.c
+++ b/fs/notify/inotify/inotify_user.c
@@ -546,21 +546,24 @@ retry:
         if (unlikely(!idr_pre_get(&group->inotify_data.idr, GFP_KERNEL)))
                 goto out_err;
 
+        /* we are putting the mark on the idr, take a reference */
+        fsnotify_get_mark(&tmp_ientry->fsn_entry);
+
         spin_lock(&group->inotify_data.idr_lock);
         ret = idr_get_new_above(&group->inotify_data.idr, &tmp_ientry->fsn_entry,
                                 group->inotify_data.last_wd+1,
                                 &tmp_ientry->wd);
         spin_unlock(&group->inotify_data.idr_lock);
         if (ret) {
+                /* we didn't get on the idr, drop the idr reference */
+                fsnotify_put_mark(&tmp_ientry->fsn_entry);
+
                 /* idr was out of memory allocate and try again */
                 if (ret == -EAGAIN)
                         goto retry;
                 goto out_err;
         }
 
-        /* we put the mark on the idr, take a reference */
-        fsnotify_get_mark(&tmp_ientry->fsn_entry);
-
         /* we are on the idr, now get on the inode */
         ret = fsnotify_add_mark(&tmp_ientry->fsn_entry, group, inode);
         if (ret) {