Re: [PATCH 3/7] vfs: Add a mount-notification facility

From: Casey Schaufler
Date: Wed May 29 2019 - 15:32:03 EST

On 5/29/2019 11:11 AM, Jann Horn wrote:
> On Wed, May 29, 2019 at 7:46 PM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>> On 5/29/2019 10:13 AM, Andy Lutomirski wrote:
>>>> On May 29, 2019, at 8:53 AM, Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>>>>> On 5/29/2019 4:00 AM, David Howells wrote:
>>>>> Jann Horn <jannh@xxxxxxxxxx> wrote:
>>>>>>> +void post_mount_notification(struct mount *changed,
>>>>>>> + struct mount_notification *notify)
>>>>>>> +{
>>>>>>> + const struct cred *cred = current_cred();
>>>>>> This current_cred() looks bogus to me. Can't mount topology changes
>>>>>> come from all sorts of places? For example, umount_mnt() from
>>>>>> umount_tree() from dissolve_on_fput() from __fput(), which could
>>>>>> happen pretty much anywhere depending on where the last reference gets
>>>>>> dropped?
>>>>> IIRC, that's what Casey argued is the right thing to do from a security PoV.
>>>>> Casey?
>>>> You need to identify the credential of the subject that triggered
>>>> the event. If it isn't current_cred(), the cred needs to be passed
>>>> in to post_mount_notification(), or derived by some other means.
>>> Taking a step back, why do we care who triggered the event? It seems to me that we should care whether the event happened and whether the *receiver* is permitted to know that.
>> There are two filesystems, "dot" and "dash". I am not allowed
>> to communicate with Fred on the system, and all precautions have
>> been taken to ensure I cannot. Fred asks for notifications on
>> all mount activity. I perform actions that result in notifications
>> on "dot" and "dash". Fred receives notifications and interprets
>> them using Morse code. This is not OK. If Wilma, who *is* allowed
>> to communicate with Fred, does the same actions, he should be
>> allowed to get the messages via Morse.
> In other words, a classic covert channel. You can't really prevent two
> cooperating processes from communicating through a covert channel on a
> modern computer.

That doesn't give you permission to design them in.
Plus, the LSMs that implement mandatory access controls
are going to want to intervene. No unclassified user
should see notifications caused by Top Secret users.

> You can transmit information through the scheduler,
> through hyperthread resource sharing, through CPU data caches, through
> disk contention, through page cache state, through RAM contention, and
> probably dozens of other ways that I can't think of right now.

Yeah, and there's been a lot of activity to reduce those,
which are hard to exploit, as opposed to this, which would
be trivial and obvious.

> There
> have been plenty of papers that demonstrated things like an SSH
> connection between two virtual machines without network access running
> on the same physical host (<>),
> communication between a VM and a browser running on the host system,
> and so on.

So you're saying we shouldn't have mode bits on files because
spectre/meltdown makes them pointless?