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

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Message-ID: <CAG48ez0+BvbLoSc+zcZwnwfOSCFt2LHnUkzzt-d4LQFJYXZC9w@mail.gmail.com>
Date:   Sat, 30 May 2020 05:17:24 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Kees Cook <keescook@...omium.org>
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 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).

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