On Mon, Aug 12, 2019 at 11:20 AM Aaron Goidel <acgoide@xxxxxxxxxxxxx> wrote: > As of now, setting watches on filesystem objects has, at most, applied a > check for read access to the inode, and in the case of fanotify, requires > CAP_SYS_ADMIN. No specific security hook or permission check has been > provided to control the setting of watches. Using any of inotify, dnotify, > or fanotify, it is possible to observe, not only write-like operations, but > even read access to a file. Modeling the watch as being merely a read from > the file is insufficient for the needs of SELinux. This is due to the fact > that read access should not necessarily imply access to information about > when another process reads from a file. Furthermore, fanotify watches grant > more power to an application in the form of permission events. While > notification events are solely, unidirectional (i.e. they only pass > information to the receiving application), permission events are blocking. > Permission events make a request to the receiving application which will > then reply with a decision as to whether or not that action may be > completed. This causes the issue of the watching application having the > ability to exercise control over the triggering process. Without drawing a > distinction within the permission check, the ability to read would imply > the greater ability to control an application. Additionally, mount and > superblock watches apply to all files within the same mount or superblock. > Read access to one file should not necessarily imply the ability to watch > all files accessed within a given mount or superblock. > > In order to solve these issues, a new LSM hook is implemented and has been > placed within the system calls for marking filesystem objects with inotify, > fanotify, and dnotify watches. These calls to the hook are placed at the > point at which the target path has been resolved and are provided with the > path struct, the mask of requested notification events, and the type of > object on which the mark is being set (inode, superblock, or mount). The > mask and obj_type have already been translated into common FS_* values > shared by the entirety of the fs notification infrastructure. The path > struct is passed rather than just the inode so that the mount is available, > particularly for mount watches. This also allows for use of the hook by > pathname-based security modules. However, since the hook is intended for > use even by inode based security modules, it is not placed under the > CONFIG_SECURITY_PATH conditional. Otherwise, the inode-based security > modules would need to enable all of the path hooks, even though they do not > use any of them. > > This only provides a hook at the point of setting a watch, and presumes > that permission to set a particular watch implies the ability to receive > all notification about that object which match the mask. This is all that > is required for SELinux. If other security modules require additional hooks > or infrastructure to control delivery of notification, these can be added > by them. It does not make sense for us to propose hooks for which we have > no implementation. The understanding that all notifications received by the > requesting application are all strictly of a type for which the application > has been granted permission shows that this implementation is sufficient in > its coverage. > > Security modules wishing to provide complete control over fanotify must > also implement a security_file_open hook that validates that the access > requested by the watching application is authorized. Fanotify has the issue > that it returns a file descriptor with the file mode specified during > fanotify_init() to the watching process on event. This is already covered > by the LSM security_file_open hook if the security module implements > checking of the requested file mode there. Otherwise, a watching process > can obtain escalated access to a file for which it has not been authorized. > > The selinux_path_notify hook implementation works by adding five new file > permissions: watch, watch_mount, watch_sb, watch_reads, and watch_with_perm > (descriptions about which will follow), and one new filesystem permission: > watch (which is applied to superblock checks). The hook then decides which > subset of these permissions must be held by the requesting application > based on the contents of the provided mask and the obj_type. The > selinux_file_open hook already checks the requested file mode and therefore > ensures that a watching process cannot escalate its access through > fanotify. > > The watch, watch_mount, and watch_sb permissions are the baseline > permissions for setting a watch on an object and each are a requirement for > any watch to be set on a file, mount, or superblock respectively. It should > be noted that having either of the other two permissions (watch_reads and > watch_with_perm) does not imply the watch, watch_mount, or watch_sb > permission. Superblock watches further require the filesystem watch > permission to the superblock. As there is no labeled object in view for > mounts, there is no specific check for mount watches beyond watch_mount to > the inode. Such a check could be added in the future, if a suitable labeled > object existed representing the mount. > > The watch_reads permission is required to receive notifications from > read-exclusive events on filesystem objects. These events include accessing > a file for the purpose of reading and closing a file which has been opened > read-only. This distinction has been drawn in order to provide a direct > indication in the policy for this otherwise not obvious capability. Read > access to a file should not necessarily imply the ability to observe read > events on a file. > > Finally, watch_with_perm only applies to fanotify masks since it is the > only way to set a mask which allows for the blocking, permission event. > This permission is needed for any watch which is of this type. Though > fanotify requires CAP_SYS_ADMIN, this is insufficient as it gives implicit > trust to root, which we do not do, and does not support least privilege. > > Signed-off-by: Aaron Goidel <acgoide@xxxxxxxxxxxxx> > Acked-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx> > Acked-by: Jan Kara <jack@xxxxxxx> > --- > v3: > - fixed comment style in security hook > > v2: > - move initialization of obj_type up to remove duplicate work > - convert inotify and fanotify flags to common FS_* flags > --- > fs/notify/dnotify/dnotify.c | 15 +++++++-- > fs/notify/fanotify/fanotify_user.c | 19 ++++++++++-- > fs/notify/inotify/inotify_user.c | 14 +++++++-- > include/linux/lsm_hooks.h | 9 +++++- > include/linux/security.h | 10 ++++-- > security/security.c | 6 ++++ > security/selinux/hooks.c | 47 +++++++++++++++++++++++++++++ > security/selinux/include/classmap.h | 5 +-- > 8 files changed, 113 insertions(+), 12 deletions(-) Merged into selinux/next, thanks everyone! -- paul moore www.paul-moore.com
