-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory CVE-2020-15852 / XSA-329 version 4 Linux ioperm bitmap context switching issues UPDATES IN VERSION 4 ==================== Normalize version tags. ISSUE DESCRIPTION ================= Linux 5.5 overhauled the internal state handling for the iopl() and ioperm() system calls. Unfortunately, one aspect on context switch wasn't wired up correctly for the Xen PVOps case. IMPACT ====== IO port permissions don't get rescinded when context switching to an unprivileged task. Therefore, all userspace can use the IO ports granted to the most recently scheduled task with IO port permissions. VULNERABLE SYSTEMS ================== Only x86 guests are vulnerable. All versions of Linux from 5.5 are potentially vulnerable. Linux is only vulnerable when running as x86 PV guest. Linux is not vulnerable when running as an x86 HVM/PVH guests. The vulnerability can only be exploited in domains which have been granted access to IO ports by Xen. This is typically only the hardware domain, and guests configured with PCI Passthrough. MITIGATION ========== Running only HVM/PVH guests avoids the vulnerability. CREDITS ======= This issue was discovered by Andy Lutomirski. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. xsa329.patch Linux master - 5.5 $ sha256sum xsa329* cdb5ac9bfd21192b5965e8ec0a1c4fcf12d0a94a962a8158cd27810e6aa362f0 xsa329.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deployment notice is retained in post-embargo publicly released Xen Project advisories, even though it is then no longer applicable.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/4UyVfoK9kFAmV8b/oMHHBncEB4ZW4u b3JnAAoJEIP+FMlX6CvZOpwH/jPq9mA6aBN9xZ1zwoECY+AJWBSBlhLv+ktIV7oH Z3qxqGRlfqF4us+y8U5AXkW5UyEwxNjLERcAvOQ7zOQtClg37SxkjKjFT5AQc3ya obEyqutOJasz+Hl5IkzmtbuShms/gyD1k/XHIFoUUwl9V4P6HuQxSsz3+UORuA7y iw1mLwTnEZ6C4M7Q5vsZzNkFbJAlxaxTomFhE2cL1BfKjDplM3IOKK0LYIe1MVEZ 14gETPAXm3HX/vy7h4rCTeyhJunN4ZA6Cf5TGGXgbwHNm8sY8H0A3bda9Bu/qXK0 Xua0+uVD7hTrsqk0nbmSEjTUF4d9TG8Re2iatiF7utBB00Y= =5YmF -----END PGP SIGNATURE-----