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From:   Andy Lutomirski <luto@kernel.org>
Date:   Tue, 7 Jan 2020 20:39:10 -0800
Message-ID: <CALCETrX1JZV-KtU-LwAuTv4Dc2yFWdTOKUZnmN_pgbUdC-bgLw@mail.gmail.com>
Subject: Re: [PATCH RFC 1/1] mount: universally disallow mounting over symlinks
To:     Aleksa Sarai <cyphar@cyphar.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        David Howells <dhowells@redhat.com>,
        Eric Biederman <ebiederm@xmission.com>,
        stable <stable@vger.kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Serge Hallyn <serge@hallyn.com>, dev@opencontainers.org,
        Linux Containers <containers@lists.linux-foundation.org>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Mon, Dec 30, 2019 at 12:29 AM Aleksa Sarai <cyphar@cyphar.com> wrote:
>
> On 2019-12-29, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > On Sun, Dec 29, 2019 at 9:21 PM Aleksa Sarai <cyphar@cyphar.com> wrote:
>
> If allowing bind-mounts over symlinks is allowed (which I don't have a
> problem with really), it just means we'll need a few more kernel pieces
> to get this hardening to work. But these features would be useful
> outside of the problems I'm dealing with (O_EMPTYPATH and some kind of
> pidfd-based interface to grab the equivalent of /proc/self/exe and a few
> other such magic-link targets).

As one data point, I would use this ability in virtme: this would
allow me to more reliably mount over /etc/resolve.conf even when it's
a symlink.

(Perhaps I should use overlayfs instead.  Hmm.)