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From: Aleksa Sarai <cyphar@cyphar.com>
To: Daniel Colascione <dancol@google.com>
Cc: Andy Lutomirski <luto@kernel.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	Christian Brauner <christian@brauner.io>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Jann Horn <jannh@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Oleg Nesterov <oleg@redhat.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Tim Murray <timmurray@google.com>,
	Kees Cook <keescook@chromium.org>,
	Jan Engelhardt <jengelh@inai.de>
Subject: Re: [PATCH] proc: allow killing processes via file descriptors
Date: Mon, 19 Nov 2018 06:05:04 +1100	[thread overview]
Message-ID: <20181118190504.ixglsqbn6mxkcdzu@yavin> (raw)
In-Reply-To: <CAKOZuevRq-igh06zS_nsGG400zXrKFCtajpEG9-xgU2+Rtb2Pw@mail.gmail.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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  reply	other threads:[~2018-11-18 19:05 UTC|newest]

Thread overview: 53+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-18 11:17 Christian Brauner
2018-11-18 13:59 ` Daniel Colascione
2018-11-18 15:38   ` Andy Lutomirski
2018-11-18 15:53     ` Daniel Colascione
2018-11-18 16:17       ` Andy Lutomirski
2018-11-18 16:29         ` Daniel Colascione
2018-11-18 17:13           ` Andy Lutomirski
2018-11-18 17:17             ` Daniel Colascione
2018-11-18 17:43               ` Eric W. Biederman
2018-11-18 17:45                 ` Andy Lutomirski
2018-11-18 17:56                 ` Daniel Colascione
2018-11-18 16:33         ` Randy Dunlap
2018-11-18 16:48           ` Daniel Colascione
2018-11-18 17:09             ` Andy Lutomirski
2018-11-18 17:24               ` Daniel Colascione
2018-11-18 17:42                 ` Andy Lutomirski
2018-11-18 17:51                   ` Daniel Colascione
2018-11-18 18:28                     ` Andy Lutomirski
2018-11-18 18:43                       ` Daniel Colascione
2018-11-18 19:05                         ` Aleksa Sarai [this message]
2018-11-18 19:44                           ` Daniel Colascione
2018-11-18 20:15                             ` Christian Brauner
2018-11-18 20:21                               ` Daniel Colascione
2018-11-18 20:28                             ` Andy Lutomirski
2018-11-18 20:32                               ` Daniel Colascione
2018-11-19  1:43                                 ` Andy Lutomirski
2018-11-18 20:43                               ` Christian Brauner
2018-11-18 20:54                                 ` Daniel Colascione
2018-11-18 21:23                                   ` Christian Brauner
2018-11-18 21:30                                     ` Christian Brauner
2018-11-19  0:31                                       ` Daniel Colascione
2018-11-19  0:40                                         ` Christian Brauner
2018-11-19  0:09                             ` Aleksa Sarai
2018-11-19  0:53                               ` Daniel Colascione
2018-11-19  1:16                                 ` Daniel Colascione
2018-11-19 16:13                       ` Dmitry Safonov
2018-11-19 16:26                         ` [PATCH] proc: allow killing processes via file descriptors (Larger pids) Eric W. Biederman
2018-11-19 16:27                         ` [PATCH] proc: allow killing processes via file descriptors Daniel Colascione
2018-11-19 20:21                           ` Aleksa Sarai
2018-11-19  2:47                   ` Al Viro
2018-11-19  3:01                     ` Andy Lutomirski
2018-11-18 17:41     ` Christian Brauner
2018-11-18 17:44       ` Andy Lutomirski
2018-11-18 18:07       ` Daniel Colascione
2018-11-18 18:15         ` Andy Lutomirski
2018-11-18 18:31           ` Daniel Colascione
2018-11-18 19:24         ` Christian Brauner
2018-11-19  0:08         ` Aleksa Sarai
2018-11-19  1:14           ` Daniel Colascione
2018-11-18 16:03 ` Daniel Colascione
2018-11-19 10:56 ` kbuild test robot
2018-11-19 14:15 ` David Laight
2018-11-19 15:49 ` Dave Martin

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