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

[Leo Gaspard]

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.

Cheers & HTH,
Leo

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