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Date: Wed, 28 Jul 2021 12:37:57 -0400
From: John Cotton Ericson <mail@...nEricson.me>
To: LKML <linux-kernel@...r.kernel.org>
Cc: David Laight <David.Laight@...LAB.COM>, Andy Lutomirski
<luto@...nel.org>, "Jason A. Donenfeld" <Jason@...c4.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Jann Horn <jann@...jh.net>,
Christian Brauner <christian.brauner@...onical.com>
Subject: Leveraging pidfs for process creation without fork
Hi,
I was excited to learn about about pidfds the other day, precisely in
hopes that it would open the door to such a "sane process creation API".
I searched the LKML, found this thread, and now hope to rekindle the
discussion; my apologies if there has been more discussion since that I
missed and I am making redundant noise.
----
On Tue, Feb 2, 2021, at 4:23 AM, David Laight wrote:
> From: Andy Lutomirski
> > Sent: 01 February 2021 18:30
> ...
> > 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.
>
> Since you are going to (eventually) load in a program image
> have to do several system calls to create the process isn't
> likely to be a problem.
> So using separate calls for each property isn't really an issue
> and solves the horrid problem of the API structure.
I definitely concur creating an embryonic process and then setting the
properties sounds separately like the right approach. I'm no expert, but
I gather from afar that between BPF and io_uring, plenty of people are
investigating general methods of batched/pipelined communication with
the kernel, and so there's little reason to go around making more ad-hoc
mammoth syscalls for specific sets of tasks.
----
> So you could create an embryonic process that inherits a lot
> of stuff from the current process, the do actions that
> sort out the fds, argv, namespace etc.
> Finally running the new program.
All that sounds good, but I wonder if it would be possible to have a
flag such that inheritance (where practical) would *not* be the default
for new processes. I'm convinced that better security will always be an
uphill battle until privileges/capabilities/resources are *not* shared
by default. Only when more sharing requires monotonically more
programmer effort will productivity/laziness align with the principle of
least privilege.
With fork/exec, there's no good way to achieve this, I think it's safe
to say. But with the embryonic processes method, where one has the
ability to e.g. set/unset file descriptors on the embryo under
construction, it seems quite natural.
This is one wrinkle of interface evolution --- as new sandboxing
mechanisms / namespaces are created, we would either need to create
yet-new "no really, default no-share" flags, or arguably be causing API
breakage as previously "leaking" privileges are patched up. I am hopeful
that either having versioned flags, or thoroughly documenting up-front
that the exact behavior is subject to change as "leaks are plugged" is
OK, but I recognize that the former might be too much complexity and the
latter to weasel-wordy, and therefore the whole idea of "opt-in sharing
only" will have to wait.
----
The security <-> ergonomics aspect is the main point of interest for me,
but there a few random ideas:
1. I originally thought an fd to an embryonic process should in fact
point to the task_struct rather than pid, since there is no risk of the
data becoming useless asynchronously --- an embryonic process is never
scheduled and cannot do anything like exiting on it's own. But there is
no reason an embryonic process need start with just one thread, so
allowing entire embryonic thread groups might actually be virtuous. I
don't know for sure, but I figure in that case it is simpler to just
stick with the pid indirection.
2. Embryonic processes can be "forked at rest" (i.e. just duplicated),
which would allow a regime where they are used as templates for process
creation, duplicated ("forked at rest"), and sent around for other tasks
to spawn processes themselves. If my idea for "opt-in sharing only"
fails per the above, sending around an "as isolated as possible" embryo
template could be a decent fallback.
That's all I got. I hope continuing this design process is of interest
to others.
Cheers,
John
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