Re: [Qemu-devel] [PATCH v2 4/4] 9pfs: local: metadata file for the VirtFS root

[Greg Kurz <groug@kaod.org>]

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On Wed, 24 May 2017 01:08:55 +0200
Leo Gaspard <leo@gaspard.io> wrote:

> On 05/23/2017 07:13 PM, Eric Blake wrote:> We have to block
> VIRTFS_META_DIR at any depth in the hierarchy, but
> > can/should we change the blocking of VIRTFS_META_ROOT_FILE to only
> > happen at the root directory, rather than at all directories?  On the
> > other hand, if you can simultaneously map /path/to/a for one mount
> > point, and /path/to/a/b for another, then the root file for B is visible
> > at a lower depth than the root file for A, and blocking the metafile
> > name everywhere means that the mount A can't influence the behavior on
> > the mount for B.  
> 
> If you take this kind of vulnerabilities into account, then you also
> have to consider a mix of mapped-file and mapped-attr mounts, or even
> worse a proxy with a mapped-file mount (which I think is currently
> vulnerable to this threat if the "proxy" path points above the
> "local,security_model=mapped-file" path, as the check is done in
> "local_" functions, which are I guess not used for proxy-type virtfs)
> 
> I'm clearly not saying it's an invalid attack (there is no explicit
> documentation stating it's insecure to "nest" virtual mounts"), just
> saying it's a much larger attack surface than one internal to virtfs
> mapped-file only. Then the question of what is reasonably possible to
> forbid to the user arises, and that's not one I could answer.
> 

The attack surface is infinite. Untrusted code that could help a guest to
forge custom metadata may reside anywhere actually, not necessarily in
another QEMU-based guest... 

My motivation to hide VIRTFS_META_ROOT_FILE everywhere was more for
consistency (ie. open(VIRTFS_META_ROOT_FILE) always fails, no matter
the cwd) and for simplicity.

Cheers,

--
Greg

> Cheers & HTH,
> Leo
> 


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