Information

Advisory

XSA-302

Public release

2019-10-31 12:00

Updated

2019-10-31 12:30

Version

5

CVE(s)

CVE-2019-18424

Title

passed through PCI devices may corrupt host memory after deassignment

Filesadvisory-302.txt (signed advisory file)
xsa302.meta
xsa302-4.9/0001-IOMMU-add-missing-HVM-check.patch
xsa302-4.9/0002-passthrough-quarantine-PCI-devices.patch
xsa302-4.10/0001-IOMMU-add-missing-HVM-check.patch
xsa302-4.10/0002-passthrough-quarantine-PCI-devices.patch
xsa302-4.11/0001-IOMMU-add-missing-HVM-check.patch
xsa302-4.11/0002-passthrough-quarantine-PCI-devices.patch
xsa302-4.12/0001-IOMMU-add-missing-HVM-check.patch
xsa302-4.12/0002-passthrough-quarantine-PCI-devices.patch
xsa302/0001-passthrough-quarantine-PCI-devices.patchAdvisory

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256

        Xen Security Advisory CVE-2019-18424 / XSA-302
                           version 5

passed through PCI devices may corrupt host memory after deassignment

UPDATES IN VERSION 5

Public release.

The patches are broken on ARM (which is not affected by the issue). Don’t apply the patches on ARM. See Resolution.

ISSUE DESCRIPTION

When a PCI device is assigned to an untrusted domain, it is possible for that domain to program the device to DMA to an arbitrary address. The IOMMU is used to protect the host from malicious DMA by making sure that the device addresses can only target memory assigned to the guest. However, when the guest domain is torn down, or the device is deassigned, the device is assigned back to dom0, thus allowing any in-flight DMA to potentially target critical host data.

IMPACT

An untrusted domain with access to a physical device can DMA into host memory, leading to privilege escalation.

VULNERABLE SYSTEMS

Only systems where guests are given direct access to physical devices capable of DMA (PCI pass-through) are vulnerable. Systems which do not use PCI pass-through are not vulnerable.

MITIGATION

In some configurations, use of passthrough can be replaced with a higher-level protocol such as Xen PV block or network devices.

CREDITS

This issue was discovered by Paul Durrant of Citrix.

RESOLUTION

Applying the appropriate attached patchset should resolve this issue. For Xen 4.9 and earlier at least the first patch of XSA-299 (whitespace cleanup) is also needed for XSA-302 to apply.

Unfortunately, at the time of writing, these patches have not been tested to our satisfaction.

The patches are known to break on ARM. ARM is not affected by the issue, so do not apply these patches on ARM systems. (On x86, there is a latent bug but the patches are good to use.)

xsa302/*.patch xen-unstable xsa302-4.12/*.patch Xen 4.12.x xsa302-4.11/*.patch Xen 4.11.x xsa302-4.10/*.patch Xen 4.10.x xsa302-4.9/*.patch Xen 4.9.x, Xen 4.8.x

$ sha256sum xsa302* xsa302*/* d722d1bed2440a5d35f0fd041e4a77966b7d26980a0f874d38d48710db0b9ebd xsa302.meta 703faced133ca21142f484acd8cf16578258e12ae0cf1413a5d9252f1e099465 xsa302-4.9/0001-IOMMU-add-missing-HVM-check.patch edb4753b91fa66e2f4b51d0075d106fc28d8451241ba482a33c2db4be53f21d1 xsa302-4.9/0002-passthrough-quarantine-PCI-devices.patch 3c79107d8fd94807543443192fb31f3d188912c208f4dbda61f1f2ff92701afc xsa302-4.10/0001-IOMMU-add-missing-HVM-check.patch 2a76add5a907baf0217e57e2a4dca91a6a8ce84c67b9ff87be1bcbb1f29efdc6 xsa302-4.10/0002-passthrough-quarantine-PCI-devices.patch a75723160c52c2c65d563905d0904b587beda1cfb6ca3ee18fb70e79818d3faa xsa302-4.11/0001-IOMMU-add-missing-HVM-check.patch 48b9dae7adbe2438dcaa00f969532d835061cb4a06ab2bf47ada2afb644de4c5 xsa302-4.11/0002-passthrough-quarantine-PCI-devices.patch a21efa6cae14e87318ca3927f0ac310aee2dd1323f2dbf040c0fe80789d78712 xsa302-4.12/0001-IOMMU-add-missing-HVM-check.patch 0a95f750ad1d5eb1838b6488e4ac188acdc2e568eb21b26306d5af2980bffb58 xsa302-4.12/0002-passthrough-quarantine-PCI-devices.patch 11d7015960eab265b1f9ce372dd14597b6c4cc7907d77ed3eed14d161dd50e5c xsa302/0001-passthrough-quarantine-PCI-devices.patch $

DEPLOYMENT DURING EMBARGO

Deployment of the *patches* described above (or others which are substantially similar) is permitted during the embargo, even on public-facing systems with untrusted guest users and administrators.

But: Distribution of updated software is prohibited (except to other members of the predisclosure list).

Also: deployment of the reconfiguration *mitigation* is NOT permitted (except where all the affected systems and VMs are administered and used only by organisations which are members of the Xen Project Security Issues Predisclosure List). Specifically, deployment on public cloud systems is NOT permitted.

This is because this reconfiguration reveals that a PCI passthrough vulnerability is involved.

Deployment of that migitation is permitted only AFTER the embargo ends.

Predisclosure list members who wish to deploy significantly different patches and/or mitigations, please contact the Xen Project Security Team.

(Note: this during-embargo deployment notice is retained in post-embargo publicly released Xen Project advisories, even though it is then no longer applicable. This is to enable the community to have oversight of the Xen Project Security Team’s decisionmaking.)

For more information about permissible uses of embargoed information, consult the Xen Project community’s agreed Security Policy: http://www.xenproject.org/security-policy.html -----BEGIN PGP SIGNATURE-----

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Xenproject.org Security Team