On Jun 5, 2019, at 10:01 AM, Casey Schaufler [off-list ref] wrote:

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On 6/5/2019 9:04 AM, Andy Lutomirski wrote:

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On Wed, Jun 5, 2019 at 7:51 AM Casey Schaufler [off-list ref] wrote:

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On 6/5/2019 1:41 AM, David Howells wrote:
Casey Schaufler [off-list ref] wrote:

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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.

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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 don’t 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 wasn’t sent because “security”.

quoted

 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. I’m assuming that if A can *write* X and B can read X then A can send information to B.