linux-kernel - Re: [PATCH v2 2/3] seccomp: Introduce addfd ioctl to seccomp user notifier

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Date:   Fri, 29 May 2020 22:47:12 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     christian.brauner@...ntu.com,
        containers@...ts.linux-foundation.org, cyphar@...har.com,
        jannh@...gle.com, jeffv@...gle.com, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, palmer@...gle.com, rsesek@...gle.com,
        tycho@...ho.ws, Matt Denton <mpdenton@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v2 2/3] seccomp: Introduce addfd ioctl to seccomp user
 notifier

On Sat, May 30, 2020 at 03:58:18AM +0000, Sargun Dhillon wrote:
> Isn't the "right" way to do this to allocate a bunch of file descriptors,
> and fill up the user buffer with them, and then install the files? This
> seems to like half-install the file descriptors and then error out.
> 
> I know that's the current behaviour, but that seems like a bad idea. Do
> we really want to perpetuate this half-broken state? I guess that some
> userspace programs could be depending on this -- and their recovery
> semantics could rely on this. I mean this is 10+ year old code.

Right -- my instincts on this are to leave the behavior as-is. I've
been burned by so many "nothing could possible rely on THIS" cases. ;)

It might be worth adding a comment (or commit log note) that describes
the alternative you suggest here. But I think building a common helper
that does all of the work (and will get used in three^Wfour places now)
is the correct way to refactor this.

Oh hey! Look at scm_detach_fds_compat(). It needs this too. (And it's
missing the cgroup tracking.) That would fix:

48a87cc26c13 ("net: netprio: fd passed in SCM_RIGHTS datagram not set correctly")
d84295067fc7 ("net: net_cls: fd passed in SCM_RIGHTS datagram not set correctly")

So, yes, let's get this fixed up. I'd say first fix the missing sock
update in the compat path (so it can be CCed stable). Then fix the missing
sock update in pidfd_getfd() (so it can be CCed stable), then write the
helper with a refactoring of scm_detach_fds(), scm_detach_fds_compat(),
and pidfd_getfd(). And then add the addfd seccomp user_notif ioctl cmd.

-- 
Kees Cook