fanotify: fix races when adding/removing marks

fanotify: fix races when adding/removing marks

For both adding an event to an existing mark and destroying a mark we
first have to find it via fsnotify_find_[inode|vfsmount]_mark().  But
getting the mark and adding an event (or destroying it) is not done
atomically.  This opens a race where a thread is about to destroy a mark
while another thread still finds the same mark and adds an event to its
mask although it will be destroyed.

Another race exists concerning the excess of a groups number of marks
limit: When a mark is added the number of group marks is checked against
the max number of marks per group and increased afterwards.  Since check
and increment is also not done atomically, this may result in 2 or more
processes passing the check at the same time and increasing the number of
group marks above the allowed limit.

With this patch both races are avoided by doing the concerning operations
with the groups mark mutex locked.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Cc: Eric Paris <eparis@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>