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

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Message-ID: <202005300908.303040EF9@keescook>
Date:   Sat, 30 May 2020 09:09:29 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Jann Horn <jannh@...gle.com>, 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 03:58:27PM +0200, Christian Brauner wrote:
> 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
> [...]
> 
> There's really no point in trying to save a broken scm message imho.

Right -- my concern is about stuffing a fd into a process without it
knowing (this is likely an overly paranoid concern, given that if the
process is getting EFAULT at the end of a list of fds, all the prior
ones will be installed too..)

-- 
Kees Cook