Date: Tue, 08 Jun 2021 17:04:30 +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 375 v2 (CVE-2021-0089) - Speculative Code Store Bypass -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory CVE-2021-0089 / XSA-375 version 2 Speculative Code Store Bypass UPDATES IN VERSION 2 ==================== New 4.12 backport (also targeting 4.11), addressing a build issue. Discuss the need for SPECULATIVE_HARDEN_BRANCH in Resolution. Provide Arm information links. Public release. ISSUE DESCRIPTION ================= Modern superscalar processors may employ sophisticated decoding and caching of the instruction stream to improve performance. However, a consequence is that self-modifying code updates may not take effect instantly. Whatever the architectural guarantees, some CPUs have microarchitectural behaviour whereby the stale instruction stream may be speculatively decoded and executed. Speculation of this form can suffer from type confusion in registers, and potentially leak data. For more details, see: https://www.vusec.net/projects/fpvi-scsb https://www.amd.com/en/corporate-product-security-bulletin-amd-sb-1003 https://software.intel.com/content/www/us/en/develop/articles/software-security-guidance/advisory-guidance/speculative-code-store-bypass.html https://software.intel.com/content/www/us/en/develop/articles/software-security-guidance/advisory-guidance/floating-point-value-injection.html https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability/frequently-asked-questions#scsb https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability/frequently-asked-questions#fvpi IMPACT ====== In attacker 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. Whether a CPU is potentially vulnerable depends on its microarchitecture. Consult your hardware vendor. Xen running on ARM does not have runtime self-modying code, so is believed to be not vulnerable, irrespective of any hardware susceptibility. Xen running on x86 does have runtime self-modying code as part of emulation, and is believed to be potentially vulnerable. Xen is not vulnerable if retpoline or lfence mitigations for Spectre v2 protection are active. Protections depend on compiler support (as indicated by INDIRECT_THUNK), and a runtime setting (BTI-Thunk): # xl dmesg | grep -e INDIRECT_THUNK -e BTI-Thunk (XEN) Compiled-in support: INDIRECT_THUNK SHADOW_PAGING (XEN) Xen settings: BTI-Thunk RETPOLINE, SPEC_CTRL: IBRS+ SSBD-, Other: SRB_LOCK+ IBPB L1D_FLUSH VERW BRANCH_HARDEN BTI-Thunk as either RETPOLINE or LFENCE prevents the vulnerability. MITIGATION ========== If Spectre v2 support is compiled in, but JMP is used by default, RETPOLINE or LFENCE can be selected with `spec-ctrl=bti-thunk=retpoline` or `spec-ctrl=bti-thunk=lfence`. CREDITS ======= This issue was discovered by Enrico Barberis, Hany Ragab, Herbert Bos, and Cristiano Giuffrida from the VUSec group at VU Amsterdam. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. Note that in 4.13 and newer the patch will only take effect when the SPECULATIVE_HARDEN_BRANCH hypervisor config option is enabled. 4.12 and older do not have such an option, and the change will take effect unconditionally. 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. xsa375.patch xen-unstable - 4.14.x xsa375-4.13.patch Xen 4.13.x xsa375-4.12.patch Xen 4.12.x - 4.11.x $ sha256sum xsa375* 367d5bb97c942b9f744a57645df87148772c0879de6f351f36f88147f3958e83 xsa375.meta 301ef80da837bc2af36a0958f35f42f4d267b20ec6e91ae5faf2616167ef49f8 xsa375.patch dc024daf17242b6477a16a349754a94b2b25cbbfd8c14475741b778710a44c93 xsa375-4.12.patch f70511d843c6617b932da11ffe857e2e3aa3834ccff07d4d0beba90d63a3dae2 xsa375-4.13.patch $ NOTE CONCERNING CVE-2021-0086 ============================= Floating Point Value Injection (FPVI) was discovered and disclosed in the same research as SCSB. Xen on x86 does in some cases emulate floating point operations with guest provided inputs, but does not have subsequent control flow dependent on results, transient or otherwise, of the operation. Therefore, we believe Xen is not vulnerable to FPVI, irrespective of any hardware susceptibility. 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/4UyVfoK9kFAmC/oxIMHHBncEB4ZW4u b3JnAAoJEIP+FMlX6CvZ0+QH/190a0VhQlorqC7eY2kt+l09S5chHL4AqfAxhBWT pxbgNcNiuUXhGRQEfxEV/CRBGnUDy5TNwtyHlJqSYm89hqVv3Dh5IbVcRK0DGV7R x9YLlESaKx97e/SaSDHZ3XtwSXa/es+O6Vmn4X67UZI7jpv8EU89fxa3Fv1fuNhv Ud8BGW2WXJ1SEW3XIT7/gz/xza1fFtv/rIew+jpnlsu6qSrlE/3pZHLOqI5Wa2n9 LklxwoGmB9JyIV8Me0tOCqiLKEOTGnS1JZiug07N2TmlxjiHj76KrVysTDqRdkFD R/C8wfmwlOSCddUPnj6uB81fH7C7I02yVTefpYwIBmI7ldc= =dP+p -----END PGP SIGNATURE----- Download attachment "xsa375.meta" of type "application/octet-stream" (1821 bytes) Download attachment "xsa375.patch" of type "application/octet-stream" (2467 bytes) Download attachment "xsa375-4.12.patch" of type "application/octet-stream" (2492 bytes) Download attachment "xsa375-4.13.patch" of type "application/octet-stream" (2473 bytes)
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