> 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 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”. > >> 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.
