Date: Tue, 15 Aug 2017 12:05:57 +0000 From: Xen.org security team <security@....org> To: xen-announce@...ts.xen.org, xen-devel@...ts.xen.org, xen-users@...ts.xen.org, oss-security@...ts.openwall.com CC: Xen.org security team <security-team-members@....org> Subject: Xen Security Advisory 229 (CVE-2017-12134) - linux: Fix Xen block IO merge-ability calculation -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory CVE-2017-12134 / XSA-229 version 3 linux: Fix Xen block IO merge-ability calculation UPDATES IN VERSION 3 ==================== Public release. ISSUE DESCRIPTION ================= The block layer in Linux may choose to merge adjacent block IO requests. When Linux is running as a Xen guest, the default merging algorithm is replaced with a Xen-specific one. When Linux is running as an x86 PV guest, some BIO's are erroneously merged, corrupting the data stream to/from the block device. This can result in incorrect access to an uncontrolled adjacent frame. IMPACT ====== A buggy or malicious guest can cause Linux to read or write incorrect memory when processing a block stream. This could leak information from other guests in the system or from Xen itself, or be used to DoS or escalate privilege within the system. VULNERABLE SYSTEMS ================== All x86 Xen systems using pvops Linux in a backend role (either as dom0, or as a disk device driver domain) are affected. This includes upstream Linux versions 2.6.37 and later. Systems using the older classic-linux fork are not affected. All PV x86 domains doing block IO on behalf of a guest, including dom0 and any PV driver domains, are vulnerable. (Any HVM driver domains running are not vulnerable.) This includes Xen vbd backends such as blkback, but also direct IO performed for the guest via eg qemu. ARM systems are not affected. The vulnerability is only exposed if the underlying block device has request merging enabled. See Mitigation. The vulnerability is only exposed to configurations which use grant mapping as a transport mechanism for the block data. Configurations which use exclusively grant copy are not vulnerable. MITIGATION ========== Disable bio merges on all relevant underlying backend block devices. For example, echo 2 > /sys/block/nvme0n1/queue/nomerges CREDITS ======= This issue was discovered by Jan H. Schönherr of Amazon. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. xsa229.patch Linux $ sha256sum xsa229* 5f96c72c8c5a971d52f5540475a3fc6f4fef2071ec772ef21392fdc238eda858 xsa229.patch $ DEPLOYMENT DURING EMBARGO ========================= Deployment of the patches and/or mitigations 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). 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----- Version: GnuPG v1 iQEcBAEBCAAGBQJZkuNWAAoJEIP+FMlX6CvZBt4H/3tpKPBmzTaI5yKPdBf6wU7L hjmKG6QROeWV+EX3wmmmRi+iG0M90hDYFCTmhdNY4sjCdDEFDMB1KM8XA/LwHlz2 3gX6TVKQ/cXQRJFhlWSZQUDDd5jPqZzDK7KnhS2DC+MjnKvnnuS6N2ibIfaHJmUG HL6VdS7GZ8Z434mgOZskWPFn5xeaWd1vXGV+GI9Ih2RRn/axe6l0RSzgDpfeGB3T hVRQdy9wW4aXrnnUXEuuz5JNlTU1fuGXGz7W5BDP8mu9l/dzmDye6NOgVqo5wAkz +l/fRbFrjdO9JnKDpASDjGuoOCZgkBBxmG2wUz8COi6JTA5X0IRysG5OMOYZ/KU= =lyzV -----END PGP SIGNATURE----- Download attachment "xsa229.patch" of type "application/octet-stream" (2451 bytes)
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