On Thu, Feb 20, 2020 at 10:11 AM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote: > > On 2/17/2020 4:14 PM, Paul Moore wrote: > > On Thu, Feb 13, 2020 at 2:41 PM Stephen Smalley <sds@xxxxxxxxxxxxx> 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@xxxxxxxxxx/, > >> 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?
