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

From: Casey Schaufler
Date: Wed Jun 05 2019 - 14:16:31 EST

On 6/5/2019 10:47 AM, Andy Lutomirski wrote:
>> 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.

That is *not* a valid assumption:

A can write to /dev/null.
B can read from /dev/null.
Does not imply B can read what A wrote.
Does not imply A can send a signal to B.

A can send a UDP datagram to port 3343
B can is bound to port 3343
Does not imply the packet will be delivered