ISSUE DESCRIPTION
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Qemu allows guests to not only read, but also write all parts of the PCI config space (but not extended config space) of passed through PCI devices not explicitly dealt with for (partial) emulation purposes.
IMPACT
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Since the effect depends on the specific purpose of the the config space field, it's not possbile to give a general statement about the exact impact on the host or other guests. Privilege escalation, host crash (Denial of Service), and leaked information all cannot be excluded.
VULNERABLE SYSTEMS
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Xen versions 3.3 and onwards are vulnerable due to supporting PCI pass-through.
Only x86 systems are vulnerable. ARM systems are not vulnerable.
Only HVM guests with their device model run in Dom0 can take advantage of this vulnerability.
Only HVM guests which have been granted access to physical PCI devices (`PCI passthrough') can take advantage of this vulnerability.
MITIGATION
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This issue can be avoided by not assigning PCI devices to untrusted HVM guests.
This issue can also be avoided by only using PV guests.
It can also be avoided by configuring HVM guests with their device model run in a separate (stub) domain. (When using xl, this can be requested with "device_model_stubdomain_override=1" in the domain configuration file.)
Acknowledgements:
Red Hat would like to thank the Xen project for reporting this issue.
Statement:
This issue does affect then Xen packages as shipped with Red Hat Enterprise Linux 5.
Red Hat Enterprise Linux 5 is now in Production 3 Phase of the support and maintenance life cycle. This has been rated as having Low security impact and is not currently planned to be addressed in future updates. For additional information, refer to the Red Hat Enterprise Linux Life Cycle: https://access.redhat.com/support/policy/updates/errata/.