Re: [Qemu-devel] Filtering files passing through MTP devices

[Omer Katz]

What would be a simpler way to do this so that the guest machine would
still be able to recognize the USB drive?
Right now we're triggering a script whenever udev recognizes that a USB
drive is plugged in.
The script copies the allowed files to a certain folder. The guest has a
cron job that periodically triggers scp from that folder to the guest.
That's a very complicated flow and I just described only the flow which
files are imported. I even omitted some of the moving parts since they are
irrelevant.
Long story short our architecture is very complicated because of this
issue.

On Wed, Apr 25, 2018, 7:17 PM Bandan Das <address@hidden> wrote:

> Omer Katz <address@hidden> writes:
>
> > We're connecting USB drives that we want the guests to copy files from.
> > The user should only be allowed to copy certain files into the system.
> > The same thing goes for copying files to the USB drive. We only allow
> > certain files to be exported from the guest.
>
> If I understand your problem correctly, this should be doable by plugging
> in
> your logic into usb_mtp_write_data for the write side and
> usb_mtp_handle_data
> for the read side. The write probably doesn't need a lot, you trigger an
> error
> response the moment your data has something you don't want and discard the
> new file.
> For the read, though, you probably have to read the whole file first,
> which is not what the current code is doing (I think).
>
> Apart from that dev-mtp.c is implementing a MTP server based on the MTP
> spec and adding
> something like this would be confusing, I also feel that this is too
> specific a usecase
> and as Daniel said, there are perhaps simpler ways of doing it.
>
> Bandan
>
> > On Wed, Apr 25, 2018, 12:57 PM Daniel P. Berrangé <address@hidden>
> > wrote:
> >
> >> On Mon, Apr 23, 2018 at 03:10:32PM +0000, Omer Katz wrote:
> >> > Hi everyone,
> >> >
> >> > We have a use case that requires us to only allow certain files to
> pass
> >> > through to the guest machine from USB storage devices.
> >> >
> >> > I was told on IRC that such a feature does not exist but the easiest
> way
> >> to
> >> > achieve our goal is to contribute a patch the the MTP device driver
> since
> >> > other drivers operate on a filesystem level instead of a file level
> which
> >> > is what we need.
> >>
> >> IMHO the easiest way to stop the guest accessing files is to simply not
> >> put them in the directory that you are exporting the guest in the first
> >> place. If you have a directory that has some files you don't want
> accessed
> >> and can't remove them, then perhaps create a second directory and use
> >> symlinks or hardlinks to pull in files from the original directory.
> >>
> >> > The plan is to pass the contents of each file to a program through
> stdin
> >> > and decide based on the exit code if the file should be allowed to
> pass
> >> > through to the guest or not.
> >>
> >> I can't say I like this idea. It is a really very inefficient and heavy
> >> solution.
> >>
> >> > Since this is the first time I'm contributing to QEMU I'd like some
> >> > guidance to where the filtering code should be.
> >> > https://github.com/qemu/qemu/blob/master/hw/usb/dev-mtp.c doesn't
> look
> >> that
> >> > complicated but I still need to understand it better to continue.
> >> > Furthermore, I need to know where to add such a command line option to
> >> > point QEMU to the filtering program.
> >> >
> >> > Would such a patch be accepted if all the requirements above are met?
> >>
> >> Can you explain the usage scenario you have in more details, rather than
> >> just the high level abstract.
> >>
> >>
> >> Regards,
> >> Daniel
> >> --
> >> |: https://berrange.com      -o-
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> >> |: https://libvirt.org         -o-
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> >>
>