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

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Date:   Fri, 29 May 2020 22:22:40 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Sargun Dhillon <sargun@...gun.me>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        Linux API <linux-api@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Chris Palmer <palmer@...gle.com>,
        Robert Sesek <rsesek@...gle.com>,
        Tycho Andersen <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 05:17:24AM +0200, Jann Horn wrote:
> On Sat, May 30, 2020 at 4:43 AM Kees Cook <keescook@...omium.org> wrote:
> > I mean, yes, that's certainly better, but it just seems a shame that
> > everyone has to do the get_unused/put_unused dance just because of how
> > SCM_RIGHTS does this weird put_user() in the middle.
> >
> > Can anyone clarify the expected failure mode from SCM_RIGHTS? Can we
> > move the put_user() after instead?
> 
> Honestly, I think trying to remove file descriptors and such after
> -EFAULT is a waste of time. If userspace runs into -EFAULT, userspace
> is beyond saving and can't really do much other than exit immediately.
> There are a bunch of places that will change state and then throw
> -EFAULT at the end if userspace supplied an invalid address, because
> trying to hold locks across userspace accesses just in case userspace
> supplied a bogus address is kinda silly (and often borderline
> impossible).

Logically, I agree. I'm more worried about the behavioral change -- if
we don't remove the fd on failure, the fd is installed with no
indication to the process that it exists (it won't know the close it --
if it keeps running -- and it may survive across exec). Before, it never
entered the file table.

> You can actually see that even scm_detach_fds() currently just
> silently swallows errors if writing some header fields fails at the
> end.

Yeah, and it's a corner case. But it should be possible (trivial, even)
to clean up on failure to retain the original results.

-- 
Kees Cook