On 2018-11-18, Daniel Colascione wrote: > > Here's my point: if we're really going to make a new API to manipulate > > processes by their fd, I think we should have at least a decent idea > > of how that API will get extended in the future. Right now, we have > > an extremely awkward situation where opening an fd in /proc requires > > certain capabilities or uids, and using those fds often also checks > > current's capabilities, and the target process may have changed its > > own security context, including gaining privilege via SUID, SGID, or > > LSM transition rules in the mean time. This has been a huge source of > > security bugs. It would be nice to have a model for future APIs that > > avoids these problems. > > > > And I didn't say in my proposal that a process's identity should > > fundamentally change when it calls execve(). I'm suggesting that > > certain operations that could cause a process to gain privilege or > > otherwise require greater permission to introspect (mainly execve) > > could be handled by invalidating the new process management fds. > > Sure, if init re-execs itself, it's still PID 1, but that doesn't > > necessarily mean that: > > > > fd = process_open_management_fd(1); > > [init reexecs] > > process_do_something(fd); > > > > needs to work. > > PID 1 is a bad example here, because it doesn't get recycled. Other > PIDs do. The snippet you gave *does* need to work, in general, because > if exec invalidates the handle, and you need to reopen by PID to > re-establish your right to do something with the process, that process > may in fact have died between the invalidation and your reopen, and > your reopened FD may refer to some other random process. I imagine the error would be -EPERM rather than -ESRCH in this case, which would be incredibly trivial for userspace to differentiate between. If you wish to re-open the path that is also trivial by re-opening through /proc/self/fd/$fd -- which will re-do any permission checks and will guarantee that you are re-opening the same 'struct file' and thus the same 'struct pid'. > The only way around this problem is to have two separate FDs --- one > to represent process identity, which *must* be continuous across > execve, and the other to represent some specific capability, some > ability to do something to that process. It's reasonable to invalidate > capability after execve, but it's not reasonable to invalidate > identity. In concrete terms, I don't see a big advantage to this > separation, and I think a single identity FD combined with > per-operation capability checks is sufficient. And much simpler. I think that the error separation above would trivially allow user-space to know whether the identity or capability of a process being monitored has changed. Currently, all operations on a '/proc/$pid' which you've previously opened and has died will give you -ESRCH. So the above separation I mentioned is entirely consistent with how users are using '/proc/$pid' to check for PID death today. > > I think you're overstating your case. To a pretty good approximation, > > setresuid() allows the caller to remove elements from the set {ruid, > > suid, euid}, unless the caller has CAP_SETUID. If you could ptrace a > > process before it calls setresuid(), you might as well be able to > > ptrace() it after, since you could have just ptraced it and made it > > call setresuid() while still ptracing it. > > What about a child that execs a setuid binary? Yeah, for this reason I think that using -EPERM on operations that we think are not reasonable to allow possibly-less-privileged processes to do -- probably the most reasonable choice would be ptrace_may_access(). > > Similarly, it seems like > > it's probably safe to be able to open an fd that lets you watch the > > exit status of a process, have the process call setresuid(), and still > > see the exit status. > > Is it? That's an open question. Well, if we consider wait4(2) it seems that this is already the case. If you fork+exec a setuid binary you can definitely see its exit code. > > My POLLERR hack, aside from being ugly, > > avoids this particular issue because it merely lets you wait for > > something you already could have observed using readdir(). > > Yes. I mentioned this same issue-punting as the motivation behind > exithand, initially, just reading EOF on exit. One question I have about EOF-on-exit is that if we wish to extend it to allow providing the exit status (which is something we discussed in the original thread), how will multiple-readers be handled in such a scenario? Would we be storing the exit status or siginfo in the equivalent of a locked memfd? -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH