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From: Andy Lutomirski <luto@kernel.org>
To: Kees Cook <keescook@chromium.org>
Cc: ksummit <ksummit-discuss@lists.linuxfoundation.org>,
	Andy Lutomirski <luto@kernel.org>
Subject: Re: [Ksummit-discuss] [TECH TOPIC] seccomp
Date: Thu, 15 Aug 2019 11:26:10 -0700
Message-ID: <CALCETrV+tk9irkoRTQCk+Ve37kce3V+7M1rFWwoDD8YqZS3p7Q@mail.gmail.com> (raw)
In-Reply-To: <201908151034.CC0F7BD84@keescook>

On Thu, Aug 15, 2019 at 10:48 AM Kees Cook <keescook@chromium.org> wrote:
> On Wed, Aug 14, 2019 at 10:54:49AM -0700, Andy Lutomirski wrote:
> > After thinking about this a bit more, I think that deferring the main
> > seccomp filter invocation until arguments have been read is too
> > problematic.  It has the ordering issues you're thinking of, but it
> > also has unpleasant effects if one of the reads faults or if
> > SECCOMP_RET_TRACE or SECCOMP_RET_TRAP is used.  I'm thinking that this
> Right, I was actually thinking of the trace/trap as being the race.
> > type of deeper inspection filter should just be a totally separate
> > layer.  Once the main seccomp logic decides that a filterable syscall
> > will be issued then, assuming that no -EFAULT happens, a totally
> > different program should get run with access to arguments.  And there
> > should be a way for the main program to know that the syscall nr in
> > question is filterable on the running kernel.
> Right -- this is how I designed the original prototype: it was
> effectively an LSM that was triggered by seccomp (since LSMs don't know
> anything about syscalls -- their hooks are more generalized). So seccomp
> would set a flag to make the LSM hook pay attention.
> Existing LSMs are system-owner defined, so really something like Landlock
> is needed for a process-owned LSM to be defined. But I worry that LSM
> hooks are still too "deep" in the kernel to have a process-oriented
> filter author who is not a kernel developer make any sense of the
> hooks. They're certainly oriented in a better position to gain the
> intent of a filter. For example, if a filter says "you can't open(2)
> /etc/foo", but it misses saying "you can't openat(2) /etc/foo", that's a
> dumb exposure. The LSM hooks are positioned to say "you can't manipulate
> /etc/foo through any means".
> So, I'm not entirely sure. It needs a clear design that chooses and
> justifies the appropriate "depth" of filtering. And FWIW, the two most
> frequent examples of argument parsing requests have been path-based
> checking and network address checking. So any prototype needs to handle
> these two cases sanely...

But also clone() flag filtering, and new clone() proposals keep
wanting to add structs.  And filtering bpf().  /me runs.

But yes, doing this LSM-style could also make sense.

  reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-19  9:35 Christian Brauner
2019-07-19 12:32 ` Andy Lutomirski
2019-07-20  3:18   ` Kees Cook
2019-08-14 17:54     ` Andy Lutomirski
2019-08-15 17:48       ` Kees Cook
2019-08-15 18:26         ` Andy Lutomirski [this message]
2019-08-15 18:31           ` Christian Brauner
2019-08-15 19:21             ` Andy Lutomirski
2019-07-20  7:23 ` James Morris
2019-07-20  7:41   ` Christian Brauner
2019-07-25 14:18     ` Serge E. Hallyn

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