linux-kernel - Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks

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Message-ID: <20200102035920.dsycgxnb6ba2jhz2@yavin.dot.cyphar.com>
Date:   Thu, 2 Jan 2020 14:59:20 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     David Howells <dhowells@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        stable@...r.kernel.org,
        Christian Brauner <christian.brauner@...ntu.com>,
        Serge Hallyn <serge@...lyn.com>, dev@...ncontainers.org,
        containers@...ts.linux-foundation.org, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 0/1] mount: universally disallow mounting over
 symlinks

On 2020-01-01, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Thu, Jan 02, 2020 at 01:44:07AM +1100, Aleksa Sarai wrote:
> 
> > Thanks, this fixes the issue for me (and also fixes another reproducer I
> > found -- mounting a symlink on top of itself then trying to umount it).
> > 
> > Reported-by: Aleksa Sarai <cyphar@...har.com>
> > Tested-by: Aleksa Sarai <cyphar@...har.com>
> 
> Pushed into #fixes.

Thanks. One other thing I noticed is that umount applies to the
underlying symlink rather than the mountpoint on top. So, for example
(using the same scripts I posted in the thread):

  # ln -s /tmp/foo link
  # ./mount_to_symlink /etc/passwd link
  # umount -l link # will attempt to unmount "/tmp/foo"

Is that intentional?

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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