fsnotify: do not set group for a mark before it is on the i_list
fsnotify_add_mark is supposed to add a mark to the g_list and i_list and to
set the group and inode for the mark. fsnotify_destroy_mark_by_entry uses
the fact that ->group != NULL to know if this group should be destroyed or
if it's already been done.
But fsnotify_add_mark sets the group and inode before it actually adds the
mark to the i_list and g_list. This can result in a race in inotify, it
requires 3 threads.
sys_inotify_add_watch("file") sys_inotify_add_watch("file") sys_inotify_rm_watch([a])
inotify_update_watch()
inotify_new_watch()
inotify_add_to_idr()
^--- returns wd = [a]
inotfiy_update_watch()
inotify_new_watch()
inotify_add_to_idr()
fsnotify_add_mark()
^--- returns wd = [b]
returns to userspace;
inotify_idr_find([a])
^--- gives us the pointer from task 1
fsnotify_add_mark()
^--- this is going to set the mark->group and mark->inode fields, but will
return -EEXIST because of the race with [b].
fsnotify_destroy_mark()
^--- since ->group != NULL we call back
into inotify_freeing_mark() which calls
inotify_remove_from_idr([a])
since fsnotify_add_mark() failed we call:
inotify_remove_from_idr([a]) <------WHOOPS it's not in the idr, this could
have been any entry added later!
The fix is to make sure we don't set mark->group until we are sure the mark is
on the inode and fsnotify_add_mark will return success.
Signed-off-by: Eric Paris <eparis@redhat.com>
1 file changed