Re: BUG: Mount ignores mount options

From: Darrick J. Wong
Date: Fri Aug 10 2018 - 20:39:48 EST

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.


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

and agreed. :)


> - Ted