Re: [PATCH] proc: allow killing processes via file descriptors

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2018-11-18, Daniel Colascione <dancol@google.com> 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
<https://www.cyphar.com/>

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