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Re: [libvirt] [PATCH 03/18] security: Include security_util



On Wed, Nov 28, 2018 at 09:35:49AM +0100, Michal Privoznik wrote:
> On 11/27/18 4:20 PM, Daniel P. Berrangé wrote:
> > On Fri, Nov 23, 2018 at 09:43:21AM +0100, Michal Privoznik wrote:
> > 
> >> +/* There are four namespaces available (xattr(7)):
> > 
> > s/available/available on Linux/
> > 
> > FreeBSD only supports 'user' and 'system' namespaces
> 
> D'oh!
> 
> > 
> >> + *
> >> + *  user - can be modified by anybody,
> >> + *  system - used by ACLs
> >> + *  security - used by SELinux
> >> + *  trusted - accessibly by CAP_SYS_ADMIN processes only
> >> + *
> >> + * Looks like the last one is way to go.
> > 
> > That prevents the QEMU driver using this functionality on any
> > non-Linux host.
> > 
> > The key problem we obviously face is that of the QEMU process
> > being able to modify the xattrs maliciously. 'trusted' namespace
> > solves this for Linux but unsolved for BSD/macOS.
> > 
> > I can only think of two alternative ways to deal with this
> > 
> >  - Use a sidecar file. eg $FILEPATH.libvirt.json
> >    Works ok for plain files. Troublesome for device nodes.
> >    Would have to use a file in /var/run/libvirt/devs/$DEVNODE
> >    perhaps ?
> 
> I rather not create files. If there is a bug and libvirt doesn't remove
> the file on the last restore (or it gets refcounting wrong or something)
> then it requires user intervention.

Isn't that true of this series too - ie if we update the xattr for
refcount wrong, or fail to remove the xattr on last restore ?

Feels like we have the same possible source of bugs, just against
an unlink() call rather than a setxattr() call.

The existance of libvirt's sidecar files on disk would be more
obviously to an admin.

That said tellin people to "rm" the file is kinda dangerous if
they typo and rm the real disk file that's alongside.  Using a
completely 100% separate directory would solve that.

> Not only that, but the file would need to be owned by root:root (in
> order to avoid malicious users mangling its contents) which might not be
> possible at all times (e.g. root squashed NFS). On the other hand, NFS
> itself doesn't support XATTRs currently so these patches do not with
> that filesystem.

Yeah, root sqaush  NFS is horrible :-(

> 
> > 
> >  - Use 'user' label but add a cryptographic signature
> >    as a further attribute. Doesn't prevent tampering
> >    but lets us throw away the data when tempering is
> >    detected.
> 
> I'm not sure how this would work with multiple daemons. They would have
> to have the key shared among them. I mean, if one daemon sets the
> attribute and signs it with its own key and then comes another daemon
> and updates the attribute it needs to sign it with the very same key
> otherwise the signature would be invalidated from the first daemon POV.

Yeah, it would need shared keys in some manner which is a very big
pain point.

> 
> > 
> > Did you consider either of these, or any other possible
> > options ?  I'm still loathe to bake in a solution that will
> > only work on Linux, despite 99% of our userbase being Linux.
> 
> Not really. I focused on Linux and haven't realized BSD doesn't support
> trusted.
> 
> Now that I am playing with this it looks like 'system' while being
> available on both Linux and BSD can't really be set on Linux. But it can
> be set on BSD (for privileged user only of course). I'm wondering if
> using 'trusted' on Linux and 'system' on BSD is the way to go. This
> could work except for cases where BSD and Linux meet, which can be only
> on NFS (right?) which doesn't support XATTRs anyway. Therefore I think
> we are safe, aren't we?

I expect eventually NFSv4 may get xattr support - there's a 1 year old
RFC for it:

  https://tools.ietf.org/html/rfc8276

Any app using xattr is going to face this same problem unless they only
use the common "user" namespace

Maybe justusing trusted (Linux) and system (BSD) is good enough for
us to get started, and we can allow for a different system (side car
files, or attribute signatures) in future if it becomes needed.

Regards,
Daniel
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