Information

Advisory

XSA-366

Public release

2021-02-18 11:46

Updated

2021-02-23 16:36

Version

2

CVE(s)

CVE-2021-27379

Title

missed flush in XSA-321 backport

Filesadvisory-366.txt (signed advisory file)
xsa366.meta
xsa366-4.11.patchAdvisory

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

        Xen Security Advisory CVE-2021-27379 / XSA-366
                          version 2

               missed flush in XSA-321 backport

UPDATES IN VERSION 2

CVE assigned.

Fixed erroneous reference to XSA-320; should have read XSA-321.

ISSUE DESCRIPTION

An oversight was made when backporting XSA-321, leading entries in the IOMMU not being properly updated under certain circumstances.

IMPACT

A malicious guest may be able to retain read/write DMA access to frames returned to Xen’s free pool, and later reused for another purpose. Host crashes (leading to a Denial of Service) and privilege escalation cannot be ruled out.

VULNERABLE SYSTEMS

Xen versions up to 4.11, from at least 3.2 onwards, are affected. Xen versions 4.12 and newer are not affected.

Only x86 Intel systems are affected. x86 AMD as well as Arm systems are not affected.

Only x86 HVM guests using hardware assisted paging (HAP), having a passed through PCI device assigned, and having page table sharing enabled can leverage the vulnerability. Note that page table sharing will be enabled (by default) only if Xen considers IOMMU and CPU large page size support compatible.

MITIGATION

Suppressing the use of page table sharing will avoid the vulnerability (command line option “iommu=no-sharept”).

Suppressing the use of large HAP pages will avoid the vulnerability (command line options “hap_2mb=no hap_1gb=no”).

Not passing through PCI devices to HVM guests will avoid the vulnerability.

CREDITS

This issue was reported as a bug by M. Vefa Bicakci, and recognized as a security issue by Roger Pau Monne of Citrix.

RESOLUTION

Applying the appropriate attached patch resolves this issue.

Note that patches for released versions are generally prepared to apply to the stable branches, and may not apply cleanly to the most recent release tarball. Downstreams are encouraged to update to the tip of the stable branch before applying these patches.

xsa366-4.11.patch Xen 4.11.x

$ sha256sum xsa366* 3131c9487b9446655e2e21df4ccf1e003bec471881396d7b2b1a0939f5cbae96 xsa366.meta 8c8c18ca8425e6167535c3cf774ffeb9dcb4572e81c8d2ff4a73fefede2d4d94 xsa366-4.11.patch $

NOTE REGARDING LACK OF EMBARGO

This was reported and debugged publicly, before the security implications were apparent. -----BEGIN PGP SIGNATURE-----

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Xenproject.org Security Team