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From: Gerd Hoffmann <kraxel@redhat.com>
Date: Fri, 23 Oct 2015 08:56:26 +0200
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Subject: Re: [Qemu-devel] fw_cfg DMA security
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To: Jordan Justen <jordan.l.justen@intel.com>
Cc: markmb@redhat.com, qemu-devel@nongnu.org

  Hi,

> One complication I thought of was that it might be tricky to deal with
> the implications of allowing this DMA to specify any old address to
> fill with fw_cfg data.
> 
> So, for example, since Red Hat is working on SMM. Would a DMA to SMRAM
> be protected?
> 
> I haven't watched the fw_cfg DMA discussion too closely, but has this
> been thought about?

Yes.  That problem isn't new and it isn't specific to fw_cfg.  You also
don't want grant dma access to smram/tseg to your ide/sata/scsi
controller or NIC.

> One idea I had was that near the end of the firmware boot, the
> firmware could trigger fw_cfg in QEMU to stop supporting DMA until a
> reset.

Should not be needed.  We have address spaces in qemu, and the
smram/tseg regions are explicitly excluded (when enabled) from dma-able
memory.

mark: when writing a fw_cfg_dma tests it is a good idea to add a
testcase for this, so make sure this works as intended and to avoid
security-sensitive regressions.

cheers,
  Gerd