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

[Stephen Smalley <sds@tycho.nsa.gov>]

On 2/12/20 2:11 PM, Stephen Smalley wrote:
> On 2/12/20 2:04 PM, Daniel Colascione wrote:
>> On Wed, Feb 12, 2020 at 10:59 AM Stephen Smalley <sds@tycho.nsa.gov> 
>> 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@tycho.nsa.gov>
>>>>> 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@google.com>
>>>>>>
>>>>>> (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.
> 
> We'd prefer having a separate inode if possible but didn't think that 
> would fly with the vfs folks, especially if we try to apply this to all 
> anonymous inodes. It might be ok for userfaultfd usage as a specific 
> case but there is a reason why anonymous inodes were introduced and 
> creating a separate inode each time defeats that purpose IIUC.  It will 
> be interesting to see how they respond.

I suppose an optimization of your approach could be to only allocate a 
new anon inode if there isn't already one that has the same security 
info (SID/class pair in the SELinux case).