On Wed, Feb 12, 2020 at 10:59 AM Stephen Smalley <sds@xxxxxxxxxxxxx> wrote: > > On 2/12/20 1:04 PM, Stephen Smalley wrote: > > On 2/12/20 12:19 PM, Daniel Colascione wrote: > >> Thanks for taking a look. > >> > >> On Wed, Feb 12, 2020 at 9:04 AM Stephen Smalley <sds@xxxxxxxxxxxxx> > >> wrote: > >>> > >>> On 2/11/20 5:55 PM, Daniel Colascione wrote: > >>>> Use the secure anonymous inode LSM hook we just added to let SELinux > >>>> policy place restrictions on userfaultfd use. The create operation > >>>> applies to processes creating new instances of these file objects; > >>>> transfer between processes is covered by restrictions on read, write, > >>>> and ioctl access already checked inside selinux_file_receive. > >>>> > >>>> Signed-off-by: Daniel Colascione <dancol@xxxxxxxxxx> > >>> > >>> (please add linux-fsdevel and viro to the cc for future versions of this > >>> patch since it changes the VFS) > >>> > >>>> --- > >>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > >>>> index 1659b59fb5d7..e178f6f40e93 100644 > >>>> --- a/security/selinux/hooks.c > >>>> +++ b/security/selinux/hooks.c > >>>> @@ -2915,6 +2919,69 @@ static int selinux_inode_init_security(struct > >>>> inode *inode, struct inode *dir, > >>>> + > >>>> + /* > >>>> + * We shouldn't be creating secure anonymous inodes before LSM > >>>> + * initialization completes. > >>>> + */ > >>>> + if (unlikely(!selinux_state.initialized)) > >>>> + return -EBUSY; > >>> > >>> I don't think this is viable; any arbitrary actions are possible before > >>> policy is loaded, and a Linux distro can be brought up fully with > >>> SELinux enabled and no policy loaded. You'll just need to have a > >>> default behavior prior to initialization. > >> > >> We'd have to fail open then, I think, and return an S_PRIVATE inode > >> (the regular anon inode). > > > > Not sure why. You aren't doing anything in the hook that actually > > relies on selinux_state.initialized being set (i.e. nothing requires a > > policy). The avc_has_perm() call will just succeed until a policy is > > loaded. So if these inodes are created prior to policy load, they will > > get assigned the task SID (which would be the kernel SID prior to policy > > load or first exec or write to /proc/self/attr/current afterward) and > > UFFD class (in your current code), be permitted, and then once policy is > > loaded any further access will get checked against the kernel SID. > > > >>>> + /* > >>>> + * We only get here once per ephemeral inode. The inode has > >>>> + * been initialized via inode_alloc_security but is otherwise > >>>> + * untouched, so check that the state is as > >>>> + * inode_alloc_security left it. > >>>> + */ > >>>> + BUG_ON(isec->initialized != LABEL_INVALID); > >>>> + BUG_ON(isec->sclass != SECCLASS_FILE); > >>> > >>> I think the kernel discourages overuse of BUG_ON/BUG/... > >> > >> I'm not sure what counts as overuse. > > > > Me either (not my rule) but I'm pretty sure this counts or you'd see a > > lot more of these kinds of BUG_ON() checks throughout. Try to reserve > > them for really critical cases. > > > >>>> + > >>>> +#ifdef CONFIG_USERFAULTFD > >>>> + if (fops == &userfaultfd_fops) > >>>> + isec->sclass = SECCLASS_UFFD; > >>>> +#endif > >>> > >>> Not sure we want or need to introduce a new security class for each user > >>> of anonymous inodes since the permissions should be the same as for > >>> file. > >> > >> The purpose of this change is to apply special policy to userfaultfd > >> FDs in particular. Isn't having a UFFD security class the best way to > >> go about that? (There's no path.) Am I missing something? > > > > It is probably the simplest approach; it just doesn't generalize to all > > users of anonymous inodes. We can distinguish them in one of two ways: > > use a different class like you did (requires a code change every time we > > add a new one and yet another duplicate of the file class) or use a > > different SID/context/type. The latter could be achieved by calling > > security_transition_sid() with the provided name wrapped in a qstr and > > specifying type_transition rules on the name. Then policy could define > > derived types for each domain, ala > > type_transition init self:file "[userfaultfd]" init_userfaultfd; > > type_transition untrusted_app self:file "[userfaultfd]" > > untrusted_app_userfaultfd; > > ... > > > >>> Also not sure we want to be testing fops for each such case. > >> > >> I was also thinking of just providing some kind of context string > >> (maybe the name), which might be friendlier to modules, but the loose > >> coupling kind of scares me, and for this particular application, since > >> UFFD is always in the core and never in a module, checking the fops > >> seems a bit more robust and doesn't hurt anything. > > > > Yes, not sure how the vfs folks feel about either coupling (the > > name-based one or the fops-based one). Neither seems great. > > > >>> We > >>> were looking at possibly leveraging the name as a key and using > >>> security_transition_sid() to generate a distinct SID/context/type for > >>> the inode via type_transition rules in policy. We have some WIP along > >>> those lines. > >> > >> Where? Any chance it would be ready soon? I'd rather not hold up this > >> work for a more general mechanism. > > > > Hopefully will have a patch available soon. But not saying this > > necessarily has to wait either. > > > >>>> + /* > >>>> + * Always give secure anonymous inodes the sid of the > >>>> + * creating task. > >>>> + */ > >>>> + > >>>> + isec->sid = tsec->sid; > >>> > >>> This doesn't generalize for other users of anonymous inodes, e.g. the > >>> /dev/kvm case where we'd rather inherit the SID and class from the > >>> original /dev/kvm inode itself. > >> > >> I think someone mentioned on the first version of this patch that we > >> could make it more flexible if the need arose. If we do want to do it > >> now, we could have the anon_inode security hook accept a "parent" or > >> "context" inode that modules could inspect for the purposes of forming > >> the new inode's SID. Does that make sense to you? > > > > Yes, that's the approach in our current WIP, except we call it a > > "related" inode since it isn't necessarily connected to the anon inode > > in any vfs sense. > > The other key difference in our WIP approach is that we assumed that we > couldn't mandate allocating a separate anon inode for each of these fds > and we wanted to cover all anonymous inodes (not opt-in), so we are > storing the SID/class pair as additional fields in the > file_security_struct and have modified file_has_perm() and others to > look there for anonymous inodes. A separate inode seems like the simpler approach for now, because it means that we have fewer places to check for security information --- and it's not as if an inode is particularly expensive. We can always switch later.
