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Re: [Xen-devel] [PATCH for-4.13] docs/xl: Document pci-assignable state


  • To: Ian Jackson <ian.jackson@xxxxxxxxxx>
  • From: George Dunlap <george.dunlap@xxxxxxxxxx>
  • Date: Tue, 26 Nov 2019 14:26:20 +0000
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Paul Durrant <paul.durrant@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Wei Liu <wl@xxxxxxx>
  • Delivery-date: Tue, 26 Nov 2019 14:26:27 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 11/26/19 2:14 PM, Ian Jackson wrote:
> George Dunlap writes ("[PATCH for-4.13] docs/xl: Document pci-assignable 
> state"):
>>  =item B<pci-assignable-remove> [I<-r>] I<BDF>
> ...
>> +Make the device at PCI Bus/Device/Function BDF not assignable to
>> +guests.  This will at least unbind the device from pciback, and
>> +re-assign it from the "quarantine domain" back to domain 0.  If the -r
>> +option is specified, it will also attempt to re-bind the device to its
>> +original driver, making it usable by Domain 0 again.  If the device is
>> +not bound to pciback, it will return success.
>> +
>> +Note that this functionality will work even for devices which were not
>> +made assignable by B<pci-assignable-add>.  This can be used to allow
>> +dom0 to access devices which were automatically quarantined by Xen
>> +after domain destruction as a result of Xen's B<iommu=quarantine>
>> +command-line default.
> 
> What are the security implications of doing this if the device might
> still be doing DMA or something ?

Then the device might scribble over any memory dom0 has access to.
Function-level reset will theoretically stop this, but fundamentally we
have to consider it unreliable in the general case.  Same thing for
assigning to a different guest.

I kind of feel like the discussion of the security risks inherent in pci
passthrough belong in a separate document, but perhaps a brief mention
here would be helpful.  Perhaps the following?

"As always, this should only be done if you trust the guest, or are
confident that the particular device you're re-assigning to dom0 will
cancel all in-flight DMA on FLR."

 -George


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