Re: [PATCH v2 3/6] Teach SELinux about a new userfaultfd class

[Daniel Colascione]

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.