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Re: On trusting its parent process
Re: On trusting its parent process
Wed, 13 Jul 2005 10:58:38 +0200
Gnus/5.1007 (Gnus v5.10.7) Emacs/21.4 (gnu/linux)
Marcus Brinkmann <address@hidden> writes:
> I wish I knew Plan 9 and EROS better, so I could answer your question.
> I don't understand what Jonathan Shapiro is saying in the mail you
> refer to, so it's very hard to comment.
I'm not familiar with these systems either but it seems relevant to
question this authenticity problem in the framework of the Hurd.
>> 1. The *only* way that a process in plan-9 can obtain a capability is
>> from its parent.
>> This differs very strongly from the status in EROS/Coyotos, where
>> *authentication* capabilities are not considered to be "holes" for
>> purposes of confinement. An implication of this is that the capability
>> which answers the question "Is this capability a capability to an
>> authentic X object" can be widely distributed, and can come to the user
>> in a way that the parent process cannot interfere with.
> So what would such a way be? The only way I see is by a kernel system
> call which performs right amplification, and grants the process the
> authority to authenticate capabilities. Maybe this is how it is done
> in EROS (I vaguely remember something like this, but the details
> escape me).
Shapiro mentions this issue in IPC-Assurance.ps. The problem in this
paper is that servers must be able to determine whether they are using a
TBO ("that is, [...] a process executing code that can be trusted to
respond promptly" and that "will actually execute that code"). Then,
roughly, there is an "Identify" operation that allows a process to check
whether a given process is a product of a constructor. And constructors
are well-known trusted processes.
The point here is that the "Identify" operation is provided by the
kernel and is guaranteed to be authentic.
> Right. It is unclear to me though if this actually is a problem. If
> it is a problem or not seems to depend on other aspects of the system
> design. For example, in EROS, you have constructors, which seem to be
> a bit like "suid" applications in Unix. If you start a suid
> application, who is the parent task? In the Hurd, it is the
> filesystem providing the suid application, _not_ the user initiating
> the execution. The suid application receives an initial execution
> environment which is a mix of the filesystems and the initiators
> execution environment.
Right. Or IOW: the suid bit in Unix and constructors in EROS are a
means do distinguish the very few applications that rely on
authenticity. /bin/passwd is one such example. For most other
applications (those are not suid root), I guess we can say that
authenticity of the system services being used is not required (for
example, unlike `passwd', `xeyes' doesn't rely on / being a capability
to the real root filesystem).
Does this make sense?
Now, one may argue that suid-root provides way more than mere
> So, beside the other doubts I have expressed above, I also have some
> doubts about how the word "parent" is used here.
Well, I don't think it really matters. I was just questioning the
relevance of this authenticity problem in the Hurd. I had not
considered the fact that the suid bit, among other things, does allow to
make sure that a program will only talk to the authentic services.
Re: On trusting its parent process, Lee Braiden, 2005/07/13