[RFC][PATCH 00/13] Mount, FS, Block and Keyrings notifications [ver #4]

[Casey Schaufler]

On 6/10/2019 11:22 AM, Andy Lutomirski wrote:
>> On Jun 10, 2019, at 11:01 AM, Casey Schaufler <casey at schaufler-ca.com> wrote:
>>
>>> On 6/10/2019 9:42 AM, Andy Lutomirski wrote:
>>>> On Mon, Jun 10, 2019 at 9:34 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
>>>>> On 6/10/2019 8:21 AM, Stephen Smalley wrote:
>>>>>> On 6/7/19 10:17 AM, David Howells wrote:
>>>>>> Hi Al,
>>>>>>
>>>>>> Here's a set of patches to add a general variable-length notification queue
>>>>>> concept and to add sources of events for:
>>>>>>
>>>>>>  (1) Mount topology events, such as mounting, unmounting, mount expiry,
>>>>>>      mount reconfiguration.
>>>>>>
>>>>>>  (2) Superblock events, such as R/W<->R/O changes, quota overrun and I/O
>>>>>>      errors (not complete yet).
>>>>>>
>>>>>>  (3) Key/keyring events, such as creating, linking and removal of keys.
>>>>>>
>>>>>>  (4) General device events (single common queue) including:
>>>>>>
>>>>>>      - Block layer events, such as device errors
>>>>>>
>>>>>>      - USB subsystem events, such as device/bus attach/remove, device
>>>>>>        reset, device errors.
>>>>>>
>>>>>> One of the reasons for this is so that we can remove the issue of processes
>>>>>> having to repeatedly and regularly scan /proc/mounts, which has proven to
>>>>>> be a system performance problem.  To further aid this, the fsinfo() syscall
>>>>>> on which this patch series depends, provides a way to access superblock and
>>>>>> mount information in binary form without the need to parse /proc/mounts.
>>>>>>
>>>>>>
>>>>>> LSM support is included, but controversial:
>>>>>>
>>>>>>  (1) The creds of the process that did the fput() that reduced the refcount
>>>>>>      to zero are cached in the file struct.
>>>>>>
>>>>>>  (2) __fput() overrides the current creds with the creds from (1) whilst
>>>>>>      doing the cleanup, thereby making sure that the creds seen by the
>>>>>>      destruction notification generated by mntput() appears to come from
>>>>>>      the last fputter.
>>>>>>
>>>>>>  (3) security_post_notification() is called for each queue that we might
>>>>>>      want to post a notification into, thereby allowing the LSM to prevent
>>>>>>      covert communications.
>>>>>>
>>>>>>  (?) Do I need to add security_set_watch(), say, to rule on whether a watch
>>>>>>      may be set in the first place?  I might need to add a variant per
>>>>>>      watch-type.
>>>>>>
>>>>>>  (?) Do I really need to keep track of the process creds in which an
>>>>>>      implicit object destruction happened?  For example, imagine you create
>>>>>>      an fd with fsopen()/fsmount().  It is marked to dissolve the mount it
>>>>>>      refers to on close unless move_mount() clears that flag.  Now, imagine
>>>>>>      someone looking at that fd through procfs at the same time as you exit
>>>>>>      due to an error.  The LSM sees the destruction notification come from
>>>>>>      the looker if they happen to do their fput() after yours.
>>>>> I remain unconvinced that (1), (2), (3), and the final (?) above are a good idea.
>>>>>
>>>>> For SELinux, I would expect that one would implement a collection of per watch-type WATCH permission checks on the target object (or to some well-defined object label like the kernel SID if there is no object) that allow receipt of all notifications of that watch-type for objects related to the target object, where "related to" is defined per watch-type.
>>>>>
>>>>> I wouldn't expect SELinux to implement security_post_notification() at all.  I can't see how one can construct a meaningful, stable policy for it.  I'd argue that the triggering process is not posting the notification; the kernel is posting the notification and the watcher has been authorized to receive it.
>>>> I cannot agree. There is an explicit action by a subject that results
>>>> in information being delivered to an object. Just like a signal or a
>>>> UDP packet delivery. Smack handles this kind of thing just fine. The
>>>> internal mechanism that results in the access is irrelevant from
>>>> this viewpoint. I can understand how a mechanism like SELinux that
>>>> works on finer granularity might view it differently.
>>> I think you really need to give an example of a coherent policy that
>>> needs this.
>> I keep telling you, and you keep ignoring what I say.
>>
>>>  As it stands, your analogy seems confusing.
>> It's pretty simple. I have given both the abstract
>> and examples.
> You gave the /dev/null example, which is inapplicable to this patchset.

That addressed an explicit objection, and pointed out
an exception to a generality you had asserted, which was
not true. It's also a red herring regarding the current
discussion.

>>>  If someone
>>> changes the system clock, we don't restrict who is allowed to be
>>> notified (via, for example, TFD_TIMER_CANCEL_ON_SET) that the clock
>>> was changed based on who changed the clock.
>> That's right. The system clock is not an object that
>> unprivileged processes can modify. In fact, it is not
>> an object at all. If you care to look, you will see that
>> Smack does nothing with the clock.
> And this is different from the mount tree how?

The mount tree can be modified by unprivileged users.
If nothing that unprivileged users can do to the mount
tree can trigger a notification you are correct, the
mount tree is very like the system clock. Is that the
case? 

>>>  Similarly, if someone
>>> tries to receive a packet on a socket, we check whether they have the
>>> right to receive on that socket (from the endpoint in question) and,
>>> if the sender is local, whether the sender can send to that socket.
>>> We do not check whether the sender can send to the receiver.
>> Bzzzt! Smack sure does.
> This seems dubious. I’m still trying to get you to explain to a non-Smack person why this makes sense.

Process A sends a packet to process B.
If A has access to TopSecret data and B is not
allowed to see TopSecret data, the delivery should
be prevented. Is that nonsensical?

>>> The signal example is inapplicable.
>> From a modeling viewpoint the actions are identical.
> This seems incorrect to me

What would be correct then? Some convoluted combination
of system entities that aren't owned or controlled by
any mechanism? 

>  and, I think, to most everyone else reading this.

That's quite the assertion. You may even be correct.

>  Can you explain?
>
> In SELinux-ese, when you write to a file, the subject is the writer and the object is the file.  When you send a signal to a process, the object is the target process.

YES!!!!!!!!!!!!

And when a process triggers a notification it is the subject
and the watching process is the object!

Subject == active entity
Object  == passive entity

Triggering an event is, like calling kill(), an action!