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

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Date:   Mon, 1 Jun 2020 12:02:10 -0700
From:   Sargun Dhillon <sargun@...gun.me>
To:     Kees Cook <keescook@...omium.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>, Jann Horn <jannh@...gle.com>,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        Linux API <linux-api@...r.kernel.org>,
        LKML <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>
Subject: Re: [PATCH v2 2/3] seccomp: Introduce addfd ioctl to seccomp user notifier

On Sat, May 30, 2020 at 9:07 AM Kees Cook <keescook@...omium.org> wrote:
>
> On Sat, May 30, 2020 at 03:08:37PM +0100, Al Viro wrote:
> > On Fri, May 29, 2020 at 07:43:10PM -0700, Kees Cook wrote:
> >
> > > Can anyone clarify the expected failure mode from SCM_RIGHTS? Can we
> > > move the put_user() after instead? I think cleanup would just be:
> > > replace_fd(fd, NULL, 0)
> >
> > Bollocks.
> >
> > Repeat after me: descriptor tables can be shared.  There is no
> > "cleanup" after you've put something there.
>
> Right -- this is what I was trying to ask about, and why I didn't like
> the idea of just leaving the fd in the table on failure. But yeah, there
> is a race if the process is about to fork or something.
>
> So the choice here is how to handle the put_user() failure:
>
> - add the put_user() address to the new helper, as I suggest in [1].
>   (exactly duplicates current behavior)
> - just leave the fd in place (not current behavior: dumps a fd into
>   the process without "agreed" notification).
> - do a double put_user (once before and once after), also in [1].
>   (sort of a best-effort combo of the above two. and SCM_RIGHTS is
>   hardly fast-pth).
>
> -Kees
>
> [1] https://lore.kernel.org/linux-api/202005282345.573B917@keescook/
>
> --
> Kees Cook

I'm going to suggest we stick to the approach of doing[1]:
1. Allocate FD
2. put_user
3. "Receive" and install file into FD

That is the only way to preserve the current behaviour in which userspace
is notified about *every* FD that is received via SCM_RIGHTS. The
scm_detach_fds code as it reads today does effectively what is above,
in that the fd is not installed until *after* the put user. Therefore
if put_user
gets an EFAULT or ENOMEM, it falls through to the MSG_CTRUNC bit.

The approach suggested[2] has a "change" in behaviour, in that (all in
file_receive):
1. Allocate FD
2. Receive file
3. put_user

Based on what Al Viro said, I don't think we can simply add step #4,
being "just" uninstall the FD.

[1]: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2179418.html
[2]: https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2179453.html