Date: Fri, 19 Jul 2019 13:53:34 +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 300 v2 - Linux: No grant table and foreign mapping limits -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory XSA-300 version 2 Linux: No grant table and foreign mapping limits UPDATES IN VERSION 2 ==================== Drop inapplicable "Deployment during embargo" section. Rewrite for clarity, and to remove most references to dom0. The issue is equally applicable to domU's providing backend services. Add information about the arbitrary limit for userspace backends. ISSUE DESCRIPTION ================= Virtual device backends and device models running in domain 0, or other backend driver domains, need to be able to map guest memory (either via grant mappings, or via the foreign mapping interface). Inside Xen, mapped grants are tracked by the maptrack structure. The size of this structure is chosen during domain creation, and has a fixed upper bound for the lifetime of the domain. For Linux to keep track of these mappings, it needs to have a page structure for each one. In practice the number of page structures is usually limited. In PV guests, a range of pfns are typically set aside at boot ("pre-ballooned") for this purpose. For HVM/PVH and Arm guests, no memory is set aside to begin with. In either case, when more of this "foreign / grant map pfn space" is needed, Linux will balloon out extra pages to use for this purpose. Unfortunately, in Linux, there are no limits, either on the total amount of memory which the domain will attempt to balloon out, nor on the amount of "foreign / grant map" memory which any individual guest can consume. For Linux userspace backends (e.g. QEMU) which use /dev/xen/gnttab or /proc/xen/gnttab, there is an arbitrary mapping limit which, if hit, will prevent further mappings from being established. As a result, a malicious guest may be able to, with crafted requests, cause a backend Linux domain to either: 1) Fill the maptrack table in Xen and/or hit the userspace limit. This will starve I/O from other guests served by the same backend. 2) Balloon out sufficient RAM to cause it to swap excessively, or run completely out of memory. This may starve all operations from the domain, including I/O from other guests, or may cause a crash of the domain. IMPACT ====== Guest may be able to crash backend Linux domains, or starve operations inside the domain, including the processing of guest I/O requests (Guest Denial-of-Service). If the backend is domain 0, which is the most common configuration, then host-wide operations may be starved, or the host may crash (Host Denial-of-Service). VULNERABLE SYSTEMS ================== All versions of Linux are vulnerable. Only Linux guests acting as backend domains for other guests may be exploited. All Arm domains are vulnerable, as are x86 PVH/HVM guests. The vulnerability of x86 PV guests depends on how they were configured at boot. MITIGATION ========== PV guests can be constructed with "pre-ballooned" memory, by building it with maxmem > memory. See `man 5 xl.cfg` for full details of these two parameters. For PV dom0, these are controlled by Xen's "dom0_mem=$X,max:$Y" command line parameter. The larger the difference between memory and maxmem, the more space Linux has to fill with grant/foreign mappings before it will start ballooning out real memory to satisfy further mapping requests. This makes the attack more difficult to accomplish. CREDITS ======= This issue was discovered by Julien Grall of ARM. RESOLUTION ========== Applying the appropriate attached patch resolves the backend memory exhaustion issue. NOTE: This does NOT fix the guest starvation issue. Fixing fixing this issue is more complex, and it was determined that it was better to work on a robust fix for the issue in public. This advisory will be updated when fixes are available. xsa300-linux-5.2.patch Linux 4.4 ... 5.2 $ sha256sum xsa300* 9c8a9aec52b147f8e8ef41444e1dd11803bacf3bd4d0f6efa863b16f7a9621ac xsa300-linux-5.2.patch $ NOTE ON LACK OF EMBARGO ======================= The lack of predisclosure is due to a short schedule set by the discoverer, and efforts to resolve the advisory wording. -----BEGIN PGP SIGNATURE----- iQFABAEBCAAqFiEEI+MiLBRfRHX6gGCng/4UyVfoK9kFAl0xyy0MHHBncEB4ZW4u b3JnAAoJEIP+FMlX6CvZyzUH/3hhOLPLuiTnKQd3idx0iIrpRkQfcdl9pxWWARWx xiVKyyMIajokrq5besT01Ztizz6B80DN+m4W14yi+j8nDyR3W4v/JriZQY48Tj1i nd+jvBGfvQcjNc5WaVjBtU/x9j0HDCUrBP+uJMGdt9jl6fppvMwnBcv/OeEvl/eE TjwEMs/RQ69LcjpwGGPSAh8AR2i1+oL3LiHtwO31hdkw0Ritqa32Uw4c+ENuo/OE PApIX8O8TMgRX0/LriGy6dtlb/L4SljTPa592EHH1cPfDelHmzpWEeIx77nbq8v/ /Ex6Gjd/19ArWvofxQkQk1+aNfvBPnPCaboc7JrlCuFEDP4= =OcOD -----END PGP SIGNATURE----- Download attachment "xsa300-linux-5.2.patch" of type "application/octet-stream" (2278 bytes)
Powered by blists - more mailing lists
Please check out the Open Source Software Security Wiki, which is counterpart to this mailing list.
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.