Re: [PATCH] fanotify, inotify, dnotify, security: add security hook for fs notifications

[Paul Moore]

On Fri, Aug 9, 2019 at 5:06 AM Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> On Thu, Aug 8, 2019 at 9:33 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
> > On Wed, Jul 31, 2019 at 11:35 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>
> > > ---
> > >  fs/notify/dnotify/dnotify.c         | 15 +++++++--
> > >  fs/notify/fanotify/fanotify_user.c  | 27 +++++++++++++++--
> > >  fs/notify/inotify/inotify_user.c    | 13 ++++++--
> > >  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, 120 insertions(+), 12 deletions(-)
> >
> > Other than Casey's comments, and ACK, I'm not seeing much commentary
> > on this patch so FS and LSM folks consider this your last chance - if
> > I don't hear any objections by the end of this week I'll plan on
> > merging this into selinux/next next week.
>
> Please consider it is summer time so people may be on vacation like I was...

This is one of the reasons why I was speaking to the mailing list and
not a particular individual :)

> First a suggestion, take it or leave it.
> The name of the hook _notify() seems misleading to me.
> naming the hook security_path_watch() seems much more
> appropriate and matching the name of the constants FILE__WATCH
> used by selinux.

I guess I'm not too bothered by either name, Aaron?  FWIW, if I was
writing this hook, I would probably name it
security_fsnotify_path(...).

-- 
paul moore
www.paul-moore.com