Date: Tue, 04 May 2021 12:02:06 +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 370 v2 (CVE-2021-28689) - x86: Speculative vulnerabilities with bare (non-shim) 32-bit PV guests -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory CVE-2021-28689 / XSA-370 version 2 x86: Speculative vulnerabilities with bare (non-shim) 32-bit PV guests UPDATES IN VERSION 2 ==================== Note that the patch is docs-only and the affected version ranges, in the files summary of the Resolution section. Public release. ISSUE DESCRIPTION ================= 32-bit x86 PV guest kernels run in ring 1. At the time when Xen was developed, this area of the i386 architecture was rarely used, which is why Xen was able to use it to implement paravirtualisation, Xen's novel approach to virtualization. In AMD64, Xen had to use a different implementation approach, so Xen does not use ring 1 to support 64-bit guests. With the focus now being on 64-bit systems, and the availability of explicit hardware support for virtualization, fixing speculation issues in ring 1 is not a priority for processor companies. Indirect Branch Restricted Speculation (IBRS) is an architectural x86 extension put together to combat speculative execution sidechannel attacks, including Spectre v2. It was retrofitted in microcode to existing CPUs. For more details on Spectre v2, see: http://xenbits.xen.org/xsa/advisory-254.html However, IBRS does not architecturally protect ring 0 from predictions learnt in ring 1. For more details, see: https://software.intel.com/security-software-guidance/deep-dives/deep-dive-indirect-branch-restricted-speculation Similar situations may exist with other mitigations for other kinds of speculative execution attacks. The situation is quite likely to be similar for speculative execution attacks which have yet to be discovered, disclosed, or mitigated. IMPACT ====== A malicious 32-bit guest kernel may be able to mount a Spectre v2 attack against Xen, despite the presence hardware protections being active. It therefore might be able to infer the contents of arbitrary host memory, including memory assigned to other guests. VULNERABLE SYSTEMS ================== Systems running all versions of Xen are affected. Only x86 systems are vulnerable, and only CPUs which are potentially vulnerable to Spectre v2. Consult your hardware manufacturer. The vulnerability can only be exploited by 32-bit PV guests which are not run in PV-Shim. MITIGATION ========== Running 32-bit PV guests under PV-Shim avoids the vulnerability when Spectre v2 protections are otherwise enabled on the system. PV shim is available and fully security-supported in all security-supported versions of Xen. Using shim is the recommended configuration. Not running 32-bit PV guests avoids the vulnerability. CREDITS ======= This issue was discovered by Jann Horn of Google Project Zero. RESOLUTION ========== There is no resolution available, and none is ever expected. The patches provided only update the security support statement. The first patch is an unavoidable consequence of the discussions above; the support status described is in effect immediately. The security team does not consider the support status listed in patch 1 to be particularly useful; however, we do not feel we have the authority to completely de-support non-shim 32-bit PV guests without community consultation. The second patch is the long-term support status the security team proposes to the community. It will not become effective until three weeks after the XSA-370 embargo lifts, and only if there are no objections raised before that point. If you need security support for un-shimmed 32-bit PV guests, please make your voice heard on xen-devel@...ts.xenproject.org (or to security@...project.org) as soon as possible after the embargo lifts. xsa370/*.patch Xen unstable (docs only) <no fix available> Xen (all versions) $ sha256sum xsa370* xsa370*/* ffb6e1be6a849b8e6930386d70817f53970f3d71a0a89980565c87070e85a7e2 xsa370.meta 45c11df550f1900663a388106d6625e84fa280881e613825c830b1984f87b3a9 xsa370/0001-SUPPORT.md-Document-speculative-attacks-status-of-no.patch 48dfe434bcdf4f08b623b639079fd1c9f9b1939b279200550dbae7736340cb53 xsa370/0002-SUPPORT.md-Un-shimmed-32-bit-PV-guests-are-no-longer.patch $ BARE 32 BIT PV SECURITY SUPPORT STATUS ====================================== This advisory discloses only a (very serious) information disclosure vulnerability exploitable by bare 32 bit PV guests, using speculative execution. We are considering further entirely withdrawing security support for configurations with non-shim 32 bit PV guests. Any such decision, including the precise scope of the (de)support, will be made following public community discussion. The result of that public process will be a patch to the security support statement, backported (as applicable) to the relevant trees. NOTE REGARDING EMBARGO ====================== In principle, the fact that the new CPU facilities are not capable of protecting ring 0 Xen from a ring 1 PV guest, might be gleaned from the hardware vendor documentation. Howver, in practice this docuemntation is so difficult to find and interpret that the implications discussed in this advisory are not recognised widely, if at all. 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----- iQFABAEBCAAqFiEEI+MiLBRfRHX6gGCng/4UyVfoK9kFAmCRH6YMHHBncEB4ZW4u b3JnAAoJEIP+FMlX6CvZNZIIAJiJsQvTRMfiBJ5+Yg4gyT7/T4vVkLZ4+j8FBlXL 1+SnIcOu5wgU0tmOADl58us9nZVZfo6X5xV4A+oJwrYvunI/1oGn27ylr3c0FYUH PLSa8bGIw3BeeAGEpADL3rPIQtTeiokpGlkRSNaAz1N8kKypcY+4Ds4Pjtgz3Gd4 gk2y7U2wReV7OItk7Sp1lstyBdda1qClXedKJa+dENSzsf/6/o9Nad8sgCosMj+k dx65CNgUWC2JRsMq+4fMTwhE2CtIh9IL4ylv7RyqI/ICW8UTMS2XOnALyjVIu1bI 96HCYrSCNclebmHI1385PV3CXUk6Goue0EDk3FxRTaBv7SM= =YLXZ -----END PGP SIGNATURE----- Download attachment "xsa370.meta" of type "application/octet-stream" (339 bytes) Download attachment "xsa370/0001-SUPPORT.md-Document-speculative-attacks-status-of-no.patch" of type "application/octet-stream" (1798 bytes) Download attachment "xsa370/0002-SUPPORT.md-Un-shimmed-32-bit-PV-guests-are-no-longer.patch" of type "application/octet-stream" (2390 bytes)
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