Date: Tue, 14 Nov 2023 14:01:02 +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 446 v2 (CVE-2023-46836) - x86: BTC/SRSO fixes not fully effective -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory CVE-2023-46836 / XSA-446 version 2 x86: BTC/SRSO fixes not fully effective UPDATES IN VERSION 2 ==================== Grammar fixes. Public release. ISSUE DESCRIPTION ================= The fixes for XSA-422 (Branch Type Confusion) and XSA-434 (Speculative Return Stack Overflow) are not IRQ-safe. It was believed that the mitigations always operated in contexts with IRQs disabled. However, the original XSA-254 fix for Meltdown (XPTI) deliberately left interrupts enabled on two entry paths; one unconditionally, and one conditionally on whether XPTI was active. As BTC/SRSO and Meltdown affect different CPU vendors, the mitigations are not active together by default. Therefore, there is a race condition whereby a malicious PV guest can bypass BTC/SRSO protections and launch a BTC/SRSO attack against Xen. IMPACT ====== An attacker in a PV guest might be able to infer the contents of memory belonging to other guests. VULNERABLE SYSTEMS ================== All versions of Xen are vulnerable. Xen is only vulnerable in default configurations on AMD and Hygon CPUs. Xen is not believed to be vulnerable in default configurations on CPUs from other hardware vendors. Only PV guests can leverage the vulnerability. MITIGATION ========== Running only HVM or PVH VMs will avoid the vulnerability. CREDITS ======= This issue was discovered by Andrew Cooper of XenServer. 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. xsa446.patch xen-unstable - Xen 4.15.x $ sha256sum xsa446* ed27ad5f36af31233e25c80daefb8b0078eeb18cacbc1923fdd6f10f0b394201 xsa446.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----- iQFABAEBCAAqFiEEI+MiLBRfRHX6gGCng/4UyVfoK9kFAmVTfRgMHHBncEB4ZW4u b3JnAAoJEIP+FMlX6CvZfLoH/iZJzkNK4d6vUrx8F5Srm8mAIDMGL4fPvJz00IsO 7h7+/wz0+FdnaWgT/12kHjIJv7p38rNkyJ3UC3p55NFFGUXKQxaKjJ6YU70IdHmY zbQDdYd2eB9dGbAq2NEkZibtg5mhhThBsQw9Sf+YZuSzOV5xRWiEhnBGz7l4+Dym bM7vuusZo3/iUc0WgE+p+j85QmzgTFdt7VEUYY2mSTFud+hDYtvx62Ej3AkwCRdu I0JbGYcRaDR9RPDae2d9yvz0+E473rFgOSX6DqZLjnQ+UQivZ7eo8soJD87qY4Jh OrEDMQWysSNiT90NYWZ+HxsRRZVjPVPoxX6EWEkwC7+CffI= =2Xtx -----END PGP SIGNATURE----- Download attachment "xsa446.patch" of type "application/octet-stream" (4424 bytes)
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