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

[Casey Schaufler]

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.