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

[Daniel Colascione]

On Thu, Feb 20, 2020 at 10:11 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
>
> On 2/17/2020 4:14 PM, Paul Moore wrote:
> > 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?
>
> Sorry for the delay - been sick the past few days.
>
> I agree that the inode is a better place than the file for information
> about the inode. This is especially true for Smack, which uses
> multiple extended attributes in some cases. I don't believe that any
> except the access label will be relevant to anonymous inodes, but
> I can imagine security modules with policies that would.
>
> I am always an advocate of full xattr support. It goes a long
> way in reducing the number and complexity of special case interfaces.

It sounds like we have broad consensus on using the inode to hold
security information, implying that anon_inodes should create new
inodes. Do any of the VFS people want to object?