linux-kernel - Re: BUG: Mount ignores mount options

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Date:   Fri, 10 Aug 2018 16:46:39 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     David Howells <dhowells@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        John Johansen <john.johansen@...onical.com>,
        Tejun Heo <tj@...nel.org>, SELinux-NSA <selinux@...ho.nsa.gov>,
        Paul Moore <paul@...l-moore.com>,
        Li Zefan <lizefan@...wei.com>,
        Linux API <linux-api@...r.kernel.org>,
        apparmor@...ts.ubuntu.com,
        Casey Schaufler <casey@...aufler-ca.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Eric Biggers <ebiggers@...gle.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Johannes Weiner <hannes@...xchg.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        tomoyo-dev-en@...ts.sourceforge.jp,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Miklos Szeredi <miklos@...redi.hu>
Subject: Re: BUG: Mount ignores mount options

On Fri, Aug 10, 2018 at 01:06:54PM -0700, Andy Lutomirski wrote:
> If the same block device is visible, with rw access, in two different
> containers, I don't see any anything good can happen.

It's worse than that.  I've fixed a lot of bugs which cause the kernel
to crash, and a few that might be levered into a privilege escalationh
attack, when you mount a maliciously corrupted file system using ext4.
I'm told told the security researcher filed similar reports with the
XFS community, and he was told, "that's what metadata checksums are
for; go away".  Given how much time it takes to work with these
security researchers, I don't blame them.

But in light of that, I'd make a somewhat stronger statement.  If you
let an untrusted container mount arbitrary block devices where they
have rw acccess to the underlying block device, nothing good can
happen.  Period.  :-)

Which is why I don't think the lack of being able to reject
"conflicting mount options" is really all that important.  It
certainly shouldn't block the fsopen patch series.  #1, it's a problem
we have today, and #2, I'm really not all sure supporting bind mounts
via specifying block device was ever a good idea to begin with.  And
#3, while I've been fixing ext4 against security issues caused by
maliciously corrupted file system images, I'm still sure that allowing
untrusted containers access to mount *any* file system via a block
device for which they have r/w access is a Really Bad Idea.

> It seems to me that the current approach mostly involves crossing our fingers.

Agreed!

						- Ted