[PATCH 2/3] Teach SELinux about anonymous inodes

[Stephen Smalley]

On 2/14/20 12:21 PM, Daniel Colascione wrote:
> On Fri, Feb 14, 2020 at 8:38 AM Stephen Smalley <sds at tycho.nsa.gov> wrote:
>> That's assuming you are ok with having to define these type_transition
>> rules for the userfaultfd case instead of having your own separate
>> security class.  Wondering how many different anon inode names/classes
>> there are in the kernel today and how much they change over time; for a
>> small, relatively stable set, separate classes might be ok; for a large,
>> dynamic set, type transitions should scale better.
> 
> I think we can get away without a class per anonymous-inode-type. I do
> wonder whether we need a class for all anonymous inodes, though: if we
> just give them the file class and use the anon inode type name for the
> type_transition rule, isn't it possible that the type_transition rule
> might also fire for plain files with the same names in the last path
> component and the same originating sid? (Maybe I'm not understanding
> type_transition rules properly.) Using a class to encompass all
> anonymous inodes would address this problem (assuming the problem
> exists in the first place).

It shouldn't fire for non-anon inodes because on a (non-anon) file 
creation, security_transition_sid() is passed the parent directory SID 
as the second argument and we only assign task SIDs to /proc/pid 
directories, which don't support (userspace) file creation anyway.

However, in the absence of a matching type_transition rule, we'll end up 
defaulting to the task SID on the anon inode, and without a separate 
class we won't be able to distinguish it from a /proc/pid inode.  So 
that might justify a separate anoninode or similar class.

This however reminded me that for the context_inode case, we not only 
want to inherit the SID but also the sclass from the context_inode. 
That is so that anon inodes created via device node ioctls inherit the 
same SID/class pair as the device node and a single allowx rule can 
govern all ioctl commands on that device.