Re: forkat(int pidfd), execveat(int pidfd), other awful things?

From: Andy Lutomirski <luto@kernel.org>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>,
	 LKML <linux-kernel@vger.kernel.org>, Jann Horn <jann@thejh.net>,
	 Christian Brauner <christian.brauner@canonical.com>
Subject: Re: forkat(int pidfd), execveat(int pidfd), other awful things?
Date: Mon, 1 Feb 2021 10:29:42 -0800	[thread overview]
Message-ID: <CALCETrVGLx5yeHo7ExAmJZmPjVjcJiV7p1JOa4iUaW5DRoEvLQ@mail.gmail.com> (raw)
In-Reply-To: <CAHmME9oHBtR4fBBUY8E_Oi7av-=OjOGkSNhQuMJMHhafCjazBw@mail.gmail.com>

On Mon, Feb 1, 2021 at 9:47 AM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> Hi Andy & others,
>
> I was reversing some NT stuff recently and marveling over how wild and
> crazy things are over in Windows-land. A few things related to process
> creation caught my interest:
>
> - It's possible to create a new process with an *arbitrary parent
> process*, which means it'll then inherit various things like handles
> and security attributes and tokens from that new parent process.
>
> - It's possible to create a new process with the memory space handle
> of a different process. Consider this on Linux, and you have some
> abomination like `forkat(int pidfd)`.

My general thought is that this is an excellent idea, but maybe not
quite in this form.  I do rather like a lot about the NT design,
although I have to say that their actual taste in the structures
passed into APIs is baroque at best.

If we're going to do this, though, can we stay away from fork and and
exec entirely?  Fork is cute but inefficient, and exec is the source
of neverending complexity and bugs in the kernel.  But I also think
that whole project can be decoupled into two almost-orthogonal pieces:

1. Inserting new processes into unusual places in the process tree.
The only part of setuid that really needs kernel help to replace is
for the daemon to be able to make its newly-spawned child be a child
of the process that called out to the daemon. Christian's pidfd
proposal could help here, and there could be a new API that is only a
minor tweak to existing fork/exec to fork-and-reparent.

2. A sane process creation API.  It would be delightful to be able to
create a fully-specified process without forking.  This might end up
being a fairly complicated project, though -- there are a lot of
inherited process properties to be enumerated.

(Bonus #3): binfmts are a pretty big attack surface.  Having a way to
handle all the binfmt magic in userspace might be a nice extension to
#2.

--Andy