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>

diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
index 40da732..e46ca68 100644 (file)
--- 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) {