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

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Date:   Sat, 30 May 2020 15:58:27 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Sargun Dhillon <sargun@...gun.me>,
        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

Agreed, we've never bothered with trying to recover from EFAULT. Just
look at kernel/fork.c:_do_fork():
	if (clone_flags & CLONE_PARENT_SETTID)
		put_user(nr, args->parent_tid);

we don't even bother even though we technically could.

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

There's really no point in trying to save a broken scm message imho.