Date: Wed, 11 Dec 2019 12:09:18 +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 309 v3 (CVE-2019-19578) - Linear pagetable use / entry miscounts -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory CVE-2019-19578 / XSA-309 version 3 Linear pagetable use / entry miscounts UPDATES IN VERSION 3 ==================== Public release. Updated metadata to add 4.13, update StableRef's ISSUE DESCRIPTION ================= "Linear pagetables" is a technique which involves either pointing a pagetable at itself, or to another pagetable of the same or higher level. Xen has limited support for linear pagetables: A page may either point to itself, or point to another pagetable of the same level (i.e., L2 to L2, L3 to L3, and so on). XSA-240 introduced an additional restriction that limited the "depth" of such chains by allowing pages to either *point to* other pages of the same level, or *be pointed to* by other pages of the same level, but not both. To implement this, we keep track of the number of outstanding times a page points to or is pointed to another page table, to prevent both from happening at the same time. Unfortunately, the original commit introducing this reset this count when resuming validation of a partially-validated pagetable, incorrectly dropping some "linear_pt_entry" counts. If an attacker could engineer such a situation to occur, they might be able to make loops or other arbitrary chains of linear pagetables, as described in XSA-240. IMPACT ====== A malicious or buggy PV guest may cause the hypervisor to crash, resulting in Denial of Service (DoS) affecting the entire host. Privilege escalation and information leaks cannot be excluded. VULNERABLE SYSTEMS ================== All versions of Xen are vulnerable. Only x86 systems are affected. Arm systems are not affected. Only x86 PV guests can leverage the vulnerability. x86 HVM and PVH guests cannot leverage the vulnerability. Only systems which have enabled linear pagetables are vulnerable. Systems which have disabled linear pagetables, either by selecting CONFIG_PV_LINEAR_PT=n when building the hypervisor, or adding pv-linear-pt=false on the command-line, are not vulnerable. MITIGATION ========== If you don't have any guests which need linear pagetables, you can disable the feature by adding pv-linear-pt=false to your Xen command-line. NetBSD is known to use linear pagetables; Linux and MiniOS are known not to use linear pagetables. CREDITS ======= This issue was discovered by Manuel Bouyer and diagnosed as a security issue by Jan Beulich of SUSE. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. xsa309.patch xen-unstable, Xen 4.13 - Xen 4.8 $ sha256sum xsa309* ddd00dfbc85bada4e4cee8a51b989e3138cc47c58992657054246bc95c8ae34d xsa309.meta 0e4b75f4416624de698f3ed619c28418917ab0a5c9663c1641804e1d0a0dec1b xsa309.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). Deployment of the `pv-linear-pt=false` 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 someone may notice the feature going away, and armed with the knowledge of where the issue is, re-discover it. Deployment of the mitigation 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----- iQFABAEBCAAqFiEEI+MiLBRfRHX6gGCng/4UyVfoK9kFAl3w3FwMHHBncEB4ZW4u b3JnAAoJEIP+FMlX6CvZt+QIAL4wU2XUXRQZFk4uS9m4EYV3tlzOidJVcAOvr4pC x9O0rCRrUTnXvaqDj/X7fqPC4e/uHy4yPgg2gnRqb4y/jXJexPBkY/fsZJ64JdWJ Fo+0a9CK8IrlzhXFcxVff49kUC3Vv/X2FMa5mY07wfg3ww2qyh9rUiKSFEX4B8vV 6lfMbFZNyOiO2vm1RnQzUCRnUeHnLXmR22BIvwLX6496qoI/ubHDBOK8NX0RU81e N1wdKlOlfmX1SuXfYzKPcdulmKLHnxiVgxG5FAsaQ5At3luA0+WEn5scoBXG99uB e6EkbmDpLabceQufMPR7Bvad3uVSzg3qLe/NvW4bd4Fvzb0= =Td+m -----END PGP SIGNATURE----- Download attachment "xsa309.meta" of type "application/octet-stream" (2043 bytes) Download attachment "xsa309.patch" of type "application/octet-stream" (2193 bytes)
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