Re: [RFC][PATCH 0/8] Mount, FS, Block and Keyrings notifications [ver #2]

From: Andy Lutomirski
Date: Wed Jun 05 2019 - 13:51:49 EST

> On Jun 5, 2019, at 10:01 AM, Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>> On 6/5/2019 9:04 AM, Andy Lutomirski wrote:
>>> On Wed, Jun 5, 2019 at 7:51 AM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>>>> On 6/5/2019 1:41 AM, David Howells wrote:
>>>> Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>>>>> I will try to explain the problem once again. If process A
>>>>> sends a signal (writes information) to process B the kernel
>>>>> checks that either process A has the same UID as process B
>>>>> or that process A has privilege to override that policy.
>>>>> Process B is passive in this access control decision, while
>>>>> process A is active. In the event delivery case, process A
>>>>> does something (e.g. modifies a keyring) that generates an
>>>>> event, which is then sent to process B's event buffer.
>>>> I think this might be the core sticking point here. It looks like two
>>>> different situations:
>>>> (1) A explicitly sends event to B (eg. signalling, sendmsg, etc.)
>>>> (2) A implicitly and unknowingly sends event to B as a side effect of some
>>>> other action (eg. B has a watch for the event A did).
>>>> The LSM treats them as the same: that is B must have MAC authorisation to send
>>>> a message to A.
>>> YES!
>>> Threat is about what you can do, not what you intend to do.
>>> And it would be really great if you put some thought into what
>>> a rational model would be for UID based controls, too.
>>>> But there are problems with not sending the event:
>>>> (1) B's internal state is then corrupt (or, at least, unknowingly invalid).
>>> Then B is a badly written program.
>> Either I'm misunderstanding you or I strongly disagree.
> A program needs to be aware of the conditions under
> which it gets event, *including the possibility that
> it may not get an event that it's not allowed*. Do you
> regularly write programs that go into corrupt states
> if an open() fails? Or where read() returns less than
> the amount of data you ask for?

I do not regularly write programs that handle read() omitting data in the middle of a TCP stream. I also donât write programs that wait for processes to die and need to handle the case where a child is dead, waitid() can see it, but SIGCHLD wasnât sent because âsecurityâ.

>> If B has
>> authority to detect a certain action, and A has authority to perform
>> that action, then refusing to notify B because B is somehow missing
>> some special authorization to be notified by A is nuts.
> You are hand-waving the notion of authority. You are assuming
> that if A can read X and B can read X that A can write B.

No, read it again please. Iâm assuming that if A can *write* X and B can read X then A can send information to B.