fanotify: fix double free of pending permission events

Commit 85816794240b ("fanotify: Fix use after free for permission
events") introduced a double free issue for permission events which are
pending in group's notification queue while group is being destroyed.
These events are freed from fanotify_handle_event() but they are not
removed from groups notification queue and thus they get freed again
from fsnotify_flush_notify().

Fix the problem by removing permission events from notification queue
before freeing them if we skip processing access response.  Also expand
comments in fanotify_release() to explain group shutdown in detail.

Fixes: 85816794240b9659e66e4d9b0df7c6e814e5f603
Signed-off-by: Jan Kara <jack@suse.cz>
Reported-by: Douglas Leeder <douglas.leeder@sophos.com>
Tested-by: Douglas Leeder <douglas.leeder@sophos.com>
Reported-by: Heinrich Schuchard <xypron.glpk@gmx.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index fbf2210823ab..b13992a41bd9 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -359,6 +359,11 @@ static int fanotify_release(struct inode *ignored, struct file *file)
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
         struct fanotify_perm_event_info *event, *next;
 
+        /*
+         * There may be still new events arriving in the notification queue
+         * but since userspace cannot use fanotify fd anymore, no event can
+         * enter or leave access_list by now.
+         */
         spin_lock(&group->fanotify_data.access_lock);
 
         atomic_inc(&group->fanotify_data.bypass_perm);
@@ -373,6 +378,13 @@ static int fanotify_release(struct inode *ignored, struct file *file)
         }
         spin_unlock(&group->fanotify_data.access_lock);
 
+        /*
+         * Since bypass_perm is set, newly queued events will not wait for
+         * access response. Wake up the already sleeping ones now.
+         * synchronize_srcu() in fsnotify_destroy_group() will wait for all
+         * processes sleeping in fanotify_handle_event() waiting for access
+         * response and thus also for all permission events to be freed.
+         */
         wake_up(&group->fanotify_data.access_waitq);
 #endif