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From: Daniel Colascione <dancol@google.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: 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>, Aleksa Sarai <cyphar@cyphar.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: Sun, 18 Nov 2018 09:51:48 -0800	[thread overview]
Message-ID: <CAKOZuevVk_aH_2TuiNcmxgMa+gHXMBXz6Uu5a6TDjoxjhaE36g@mail.gmail.com> (raw)
In-Reply-To: <CALCETrUeNZPfrSYa9vH5Ukrk1Y+Kb9GkZOh6LkqG6Z9NpK5P0w@mail.gmail.com>

On Sun, Nov 18, 2018 at 9:42 AM, Andy Lutomirski <luto@kernel.org> wrote:
> On Sun, Nov 18, 2018 at 9:24 AM Daniel Colascione <dancol@google.com> wrote:
>> Assuming we don't broaden exit status readability (which would make a
>> lot of things simpler), the exit notification mechanism must work like
>> this: if you can see a process in /proc, you should be able to wait on
>> it. If you learn that process's exit status through some other means
>> --- e.g., you're the process's parent, you can ptrace the process, you
>> have CAP_WHATEVER_IT_IS_ --- then you should be able to learn the fate
>> of the process. Otherwise you just be able to learn that the process
>> exited.
>
> Sounds reasonable to me.  Except for the obvious turd that, if you
> open /proc/PID/whatever, and the process calls execve(), then the
> resulting semantics are awkward at best.

A process calling execve does not give up its logical identity. Lots
of programs exec themselves, e.g., to reload configuration.

>> >  Windows has an easy time of it because
>>
>> Windows has an easier time of it because it doesn't use an ad-hoc
>> ambient authority permission model. In Windows, if you can open a
>> handle to do something, that handle lets you do the thing. Period.
>> There's none of this "well, I opened this process FD, but since I
>> opened it, the process called setuid, so now I can't get its exit
>> status" nonsense. Privilege elevation is always accomplished via a
>> separate call to CreateProcessWithToken, which creates a *new* process
>> with the elevated privileges. An existing process can't suddenly and
>> magically become this special thing that you can't inspect, but that
>> has the same PID and identity as this other process that you used to
>> be able to inspect. The model is just better, because permission is
>> baked into the HANDLE. Now, that ship has sailed. We're stuck with
>> setreuid and exec. But let's be clear about what's causing the
>> complexity.
>
> I'm not entirely sure that ship has sailed.  In the kernel, we already
> have a bit of a distinction between a pid (and tid, etc -- I'm
> referring to struct pid) and a task.  If we make a new
> process-management API, we could put a distinction like this into the
> API.

It would be a disaster to have different APIs give callers a different
idea of process identity over its lifetime. The precedent is
well-established that execve and setreuid do not change a process's
identity. Invaliding some identifiers but not others in response to
supposedly-internal things a process might do under rare circumstances
is creating a bug machine..

> setresuid() has no effect
> here -- if you have W access to the process and the process calls
> setresuid(), you still have W access.

Now you've created a situation in which an operation that security
policy previously blocked now becomes possible, invaliding previous
designs based on the old security invariant. That's the definition of
introducing a security hole.

  reply	other threads:[~2018-11-18 17:51 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 [this message]
2018-11-18 18:28                     ` Andy Lutomirski
2018-11-18 18:43                       ` Daniel Colascione
2018-11-18 19:05                         ` Aleksa Sarai
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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