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.
