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 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. 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
