[RFC PATCH] security, anon_inodes, kvm: enable security support for anon inodes

[Paul Moore]

On Thu, Feb 13, 2020 at 2:41 PM Stephen Smalley <sds at tycho.nsa.gov> wrote:
>
> Add support for labeling and controlling access to files attached to anon
> inodes. Introduce extended interfaces for creating such files to permit
> passing a related file as an input to decide how to label the anon
> inode. Define a security hook for initializing the anon inode security
> attributes. Security attributes are either inherited from a related file
> or determined based on some combination of the creating task and policy
> (in the case of SELinux, using type_transition rules).  As an
> example user of the inheritance support, convert kvm to use the new
> interface for passing the related file so that the anon inode can inherit
> the security attributes of /dev/kvm and provide consistent access control
> for subsequent ioctl operations.  Other users of anon inodes, including
> userfaultfd, will default to the transition-based mechanism instead.
>
> Compared to the series in
> https://lore.kernel.org/selinux/20200211225547.235083-1-dancol@google.com/,
> this approach differs in that it does not require creation of a separate
> anonymous inode for each file (instead storing the per-instance security
> information in the file security blob), it applies labeling and control
> to all users of anonymous inodes rather than requiring opt-in via a new
> flag, it supports labeling based on a related inode if provided,
> it relies on type transitions to compute the label of the anon inode
> when there is no related inode, and it does not require introducing a new
> security class for each user of anonymous inodes.
>
> On the other hand, the approach in this patch does expose the name passed
> by the creator of the anon inode to the policy (an indirect mapping could
> be provided within SELinux if these names aren't considered to be stable),
> requires the definition of type_transition rules to distinguish userfaultfd
> inodes from proc inodes based on type since they share the same class,
> doesn't support denying the creation of anonymous inodes (making the hook
> added by this patch return something other than void is problematic due to
> it being called after the file is already allocated and error handling in
> the callers can't presently account for this scenario and end up calling
> release methods multiple times), and may be more expensive
> (security_transition_sid overhead on each anon inode allocation).
>
> We are primarily posting this RFC patch now so that the two different
> approaches can be concretely compared.  We anticipate a hybrid of the
> two approaches being the likely outcome in the end.  In particular
> if support for allocating a separate inode for each of these files
> is acceptable, then we would favor storing the security information
> in the inode security blob and using it instead of the file security
> blob.

Bringing this back up in hopes of attracting some attention from the
fs-devel crowd and Al.  As Stephen already mentioned, from a SELinux
perspective we would prefer to attach the security blob to the inode
as opposed to the file struct; does anyone have any objections to
that?

-- 
paul moore
www.paul-moore.com