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

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

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.