On Wed, Jun 05, 2019 at 02:25:33PM -0400, Stephen Smalley wrote: > On 6/5/19 1:47 PM, 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. > > I guess the questions here are: > > 1) How do we handle recursive notification support, since we can't check > that B can read everything below a given directory easily? Perhaps we can > argue that if I have watch permission to / then that implies visibility to > everything below it but that is rather broad. How do you handle fanotify today which I think can do this? thanks, greg k-h
