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

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Date:   Fri, 10 Aug 2018 17:38:52 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>,
        Andy Lutomirski <luto@...nel.org>,
        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 07:54:47PM -0400, Theodore Y. Ts'o wrote:
> On Fri, Aug 10, 2018 at 03:12:34PM -0700, Darrick J. Wong wrote:
> > Hey now, there was a little more nuance to it than that[1][2].  The
> > complaint in the first instance had much more to do with breaking
> > existing V4 filesystems by adding format requirements that mkfs didn't
> > know about when the filesystem was created.  Yes, you can create V4
> > filesystems that will hang the system if the log was totally unformatted
> > and metadata updates are made, but OTOH it's fairly obvious when that
> > happens, you have to be root to mount a disk filesystem, and we try to
> > avoid breaking existing users.
> 
> I wasn't thinking about syzbot reports; I've largely written them off
> as far as file system testing is concerned, but rather Wen Xu at
> Georgia Tech, who is much more reasonable than Dmitry, and has helpeyd
> me out a lot; and has complained that the XFS folks haven't been
> engaging with him.

Ahh, ok.  Yes, Wen has been easier to work with, and gives out
filesystem images.  Hm, I'll go comb the bugzilla again...

> In either case, both security researchers are fuzzing file system
> images, and then fixing the checksums, and discovering that this can
> lead to kernel crashes, and in a few cases, buffer overruns that can
> lead to potential privilege escalations.  Wen can generate reports
> faster than syzbot, but at least he gives me file system images (as
> opposed to having to dig them out of syzbot repro C files) and he
> actually does some analysis and explains what he thinks is going on.

(FWIW I tried to figure out how to add fs image dumping to syzbot and
whoah that was horrifying.

> I don't think anyone was claiming that format requirements should be
> added to ext4 or xfs file systems.  But rather, that kernel code
> should be made more robust against maliciously corrupted file system
> images that have valid checksums.  I've been more willing to work with
> Wen; Dave has expressed the opinion that these are not realistic bug
> reports, and since only root can mount file systems, it's not high
> priority.

I don't think they're high priority either, but they're at least worth
/some/ attention.

> The reason why I bring this up here is that in container land, there
> are those who believe that "container root" should be able to mount
> file systems, and if the "container root" isn't trusted, the fact that
> the "container root" can crash the host kernel, or worse, corrupt the
> host kernel and break out of the container as a result, that would be
> sad.
> 
> I was pretty sure most file system developers are on the same page
> that allowing untrusted "container roots" the ability to mount
> arbitrary block device file systems is insanity.

Agreed.

> Whether or not we try to fix these sorts of bugs submitted by security
> researchers.  :-)

and agreed. :)

--D

> 	  	       	    	       		  - Ted