Re: [PATCH 0/7] Initial support for user namespace owned mounts

J. Bruce Fields
2015-07-21T17:37:26Z

On Fri, Jul 17, 2015 at 12:47:35PM +1000, Dave Chinner wrote:
> On Thu, Jul 16, 2015 at 07:42:03PM -0500, Eric W. Biederman wrote:
> > Dave Chinner <david@fromorbit.com> writes:
> >
> > > On Wed, Jul 15, 2015 at 11:47:08PM -0500, Eric W. Biederman wrote:
> > >> Casey Schaufler <casey@schaufler-ca.com> writes:
> > >> > On 7/15/2015 6:08 PM, Andy Lutomirski wrote:
> > >> >> If I mount an unprivileged filesystem, then either the contents were
> > >> >> put there *by me*, in which case letting me access them are fine, or
> > >> >> (with Seth's patches and then some) I control the backing store, in
> > >> >> which case I can do whatever I want regardless of what LSM thinks.
> > >> >>
> > >> >> So I don't see the problem. Why would Smack or any other LSM care at
> > >> >> all, unless it wants to prevent me from mounting the fs in the first
> > >> >> place?
> > >> >
> > >> > First off, I don't cotton to the notion that you should be able
> > >> > to mount filesystems without privilege. But it seems I'm being
> > >> > outvoted on that. I suspect that there are cases where it might
> > >> > be safe, but I can't think of one off the top of my head.
> > >>
> > >> There are two fundamental issues mounting filesystems without privielge,
> > >> by which I actually mean mounting filesystems as the root user in a user
> > >> namespace.
> > >>
> > >> - Are the semantics safe.
> > >> - Is the extra attack surface a problem.
> > >
> > > I think the attack surface this exposes is the biggest problem
> > > facing this proposal.
> >
> > I completely agree.
> >
> > >> Figuring out how to make semantics safe is what we are talking about.
> > >>
> > >> Once we sort out the semantics we can look at the handful of filesystems
> > >> like fuse where the extra attack surface is not a concern.
> > >>
> > >> With that said desktop environments have for a long time been
> > >> automatically mounting whichever filesystem you place in your computer,
> > >> so in practice what this is really about is trying to align the kernel
> > >> with how people use filesystems.
> > >
> > > The key difference is that desktops only do this when you physically
> > > plug in a device. With unprivileged mounts, a hostile attacker
> > > doesn't need physical access to the machine to exploit lurking
> > > kernel filesystem bugs. i.e. they can just use loopback mounts, and
> > > they can keep mounting corrupted images until they find something
> > > that works.
> >
> > Yep. That magnifies the problem quite a bit.
> >
> > > User namespaces are supposed to provide trust separation. The
> > > kernel filesystems simply aren't hardened against unprivileged
> > > attacks from below - there is a trust relationship between root and
> > > the filesystem in that they are the only things that can write to
> > > the disk. Mounts from within a userns destroys this relationship as
> > > the userns root, by definition, is not a trusted actor.
> >
> > I talked to Ted Tso a while back and ext4 is at least in principle
> > already hardened against that kind of attack. I am not certain I
> > believe it, but if it is true I think it is fantastic.
>
> No, it's not. No filesystem is, because to harden against such
> attacks requires complete verification of all metadata when it is
> read from disk, before it is used, or some method or ensuring the
> block was not tampered with. CRCs are not sufficient, because they
> can be tampered with, too.
>
> The only way a filesystem would be able to trust what it reads from
> disk has not been tampered with in a system with untrusted mounts is
> if it has some kind of cryptographically secure signature in the
> metadata and the attacker is unable to access the key for that
> signature.

Preventing tampering is a little different from protecting the kernel
from attack, isn't it? I thought the latter was what people were asking
about.

So, for example, a screwed up on-disk directory structure shouldn't
result in creating a cycle in the dcache and then deadlocking.

--b.

> No filesystem we have has that capability and AFAIA there
> are no plans for any filesystem to implement such tamper detection.
> And no, ext4 encryption does not provide this because it only stores
> the values and data in encrypted format and does not protect
> metadata from tampering when it is not mounted.
>
> If we don't have crypto signatures in metadata, then XFS is probably
> the most robust against tampering as it does a lot more checking of
> the on-disk metadata before it is used than any other filesystem
> (i.e. see the verifier infrastructure that does corruption checks
> after read (in io completion) and before write (in io submission)
> to catch bad metadata before it is used by the kernel, or before it
> is written to disk by the kernel.
>
> However, these checks are far from comprehensive. we can only check
> internal consistency of the metadata objects in the block, and even
> then we really only can check for values within range rather than
> absolute correctness. e.g. we can check a dirent has a valid name,
> length, ftype and inode number, but we can't validate that the inode
> is actually allocated or not because that requires a lookup in the
> allocated inode btree. We *trust* that inode number to be
> allocated and valid because it is in metadata the filesystem wrote.
>
> For inode numbers that come from untrusted sources (NFS,
> open-by-handle, etc) we have a flag that does inode number
> validation on lookup (XFS_IGET_UNTRUSTED) to check against trusted
> metadata (i.e. the allocated inode btrees), but that is expensive
> and so not done on inodes that we pull directly from metadata that
> has come from disk. Indeed, we still trust on-disk metadata to be
> correct to validate that other metadata canbe trusted, so if one
> structure can be tampered with, so can others.
>
> IOWs, if we cannot trust one part of the filesystem metadata to be
> correct, then we cannot trust that filesystem *at all*, *for
> anything*. And even running fsck doesn't restore trust - all it does
> is tell us that any modification that was made is not a detectable
> inconsistency that needs fixing.
>
> > At this point any setting of the FS_USER_MOUNT flag I figure needs to go
> > through the filesystem maintainers tree and they need to be aware of and
> > agree to deal with the attack from below issue.
> >
> > The one filesystem I truly expect we can make work is fuse. fuse has
> > been designed to deal with some variation of the attack from below issue
> > since day one. We looked at what the patches to fuse would look like
> > with the current state of the vfs and it was not pretty.
> >
> > We very much need to sort through as much as possible at the vfs layer,
> > and in generic code. Allow everyone to see what is going on and how
> > it works before preceeding forward with enabling any filesystems.
>
> The VFS protects us from attacks from above the filesystem, not
> below. The VFS plays no part in validating the on-disk structure of
> a filesystem which is what attacks from below will be attempting to
> exploit.
>
> > I truly hope we can find a small set of block device filesystems that we
> > can harden from attack below. That would allow linux to have serious
> > defenses against evil usb stick attacks. I think that is going to take
> > a lot of careful coding, testing and validation and advancing the state
> > of the art to get there.
>
> Somehow, I just can't see that happening.
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com
> --
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