164 lines
6.3 KiB
ReStructuredText
164 lines
6.3 KiB
ReStructuredText
iTLB multihit
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=============
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iTLB multihit is an erratum where some processors may incur a machine check
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error, possibly resulting in an unrecoverable CPU lockup, when an
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instruction fetch hits multiple entries in the instruction TLB. This can
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occur when the page size is changed along with either the physical address
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or cache type. A malicious guest running on a virtualized system can
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exploit this erratum to perform a denial of service attack.
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Affected processors
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-------------------
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Variations of this erratum are present on most Intel Core and Xeon processor
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models. The erratum is not present on:
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- non-Intel processors
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- Some Atoms (Airmont, Bonnell, Goldmont, GoldmontPlus, Saltwell, Silvermont)
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- Intel processors that have the PSCHANGE_MC_NO bit set in the
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IA32_ARCH_CAPABILITIES MSR.
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Related CVEs
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------------
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The following CVE entry is related to this issue:
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============== =================================================
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CVE-2018-12207 Machine Check Error Avoidance on Page Size Change
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============== =================================================
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Problem
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-------
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Privileged software, including OS and virtual machine managers (VMM), are in
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charge of memory management. A key component in memory management is the control
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of the page tables. Modern processors use virtual memory, a technique that creates
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the illusion of a very large memory for processors. This virtual space is split
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into pages of a given size. Page tables translate virtual addresses to physical
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addresses.
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To reduce latency when performing a virtual to physical address translation,
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processors include a structure, called TLB, that caches recent translations.
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There are separate TLBs for instruction (iTLB) and data (dTLB).
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Under this errata, instructions are fetched from a linear address translated
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using a 4 KB translation cached in the iTLB. Privileged software modifies the
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paging structure so that the same linear address using large page size (2 MB, 4
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MB, 1 GB) with a different physical address or memory type. After the page
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structure modification but before the software invalidates any iTLB entries for
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the linear address, a code fetch that happens on the same linear address may
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cause a machine-check error which can result in a system hang or shutdown.
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Attack scenarios
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----------------
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Attacks against the iTLB multihit erratum can be mounted from malicious
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guests in a virtualized system.
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iTLB multihit system information
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--------------------------------
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The Linux kernel provides a sysfs interface to enumerate the current iTLB
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multihit status of the system:whether the system is vulnerable and which
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mitigations are active. The relevant sysfs file is:
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/sys/devices/system/cpu/vulnerabilities/itlb_multihit
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The possible values in this file are:
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.. list-table::
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* - Not affected
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- The processor is not vulnerable.
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* - KVM: Mitigation: Split huge pages
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- Software changes mitigate this issue.
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* - KVM: Vulnerable
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- The processor is vulnerable, but no mitigation enabled
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Enumeration of the erratum
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--------------------------------
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A new bit has been allocated in the IA32_ARCH_CAPABILITIES (PSCHANGE_MC_NO) msr
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and will be set on CPU's which are mitigated against this issue.
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======================================= =========== ===============================
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IA32_ARCH_CAPABILITIES MSR Not present Possibly vulnerable,check model
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IA32_ARCH_CAPABILITIES[PSCHANGE_MC_NO] '0' Likely vulnerable,check model
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IA32_ARCH_CAPABILITIES[PSCHANGE_MC_NO] '1' Not vulnerable
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======================================= =========== ===============================
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Mitigation mechanism
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-------------------------
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This erratum can be mitigated by restricting the use of large page sizes to
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non-executable pages. This forces all iTLB entries to be 4K, and removes
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the possibility of multiple hits.
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In order to mitigate the vulnerability, KVM initially marks all huge pages
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as non-executable. If the guest attempts to execute in one of those pages,
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the page is broken down into 4K pages, which are then marked executable.
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If EPT is disabled or not available on the host, KVM is in control of TLB
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flushes and the problematic situation cannot happen. However, the shadow
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EPT paging mechanism used by nested virtualization is vulnerable, because
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the nested guest can trigger multiple iTLB hits by modifying its own
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(non-nested) page tables. For simplicity, KVM will make large pages
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non-executable in all shadow paging modes.
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Mitigation control on the kernel command line and KVM - module parameter
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------------------------------------------------------------------------
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The KVM hypervisor mitigation mechanism for marking huge pages as
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non-executable can be controlled with a module parameter "nx_huge_pages=".
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The kernel command line allows to control the iTLB multihit mitigations at
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boot time with the option "kvm.nx_huge_pages=".
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The valid arguments for these options are:
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========== ================================================================
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force Mitigation is enabled. In this case, the mitigation implements
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non-executable huge pages in Linux kernel KVM module. All huge
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pages in the EPT are marked as non-executable.
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If a guest attempts to execute in one of those pages, the page is
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broken down into 4K pages, which are then marked executable.
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off Mitigation is disabled.
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auto Enable mitigation only if the platform is affected and the kernel
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was not booted with the "mitigations=off" command line parameter.
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This is the default option.
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========== ================================================================
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Mitigation selection guide
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--------------------------
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1. No virtualization in use
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The system is protected by the kernel unconditionally and no further
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action is required.
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2. Virtualization with trusted guests
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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If the guest comes from a trusted source, you may assume that the guest will
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not attempt to maliciously exploit these errata and no further action is
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required.
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3. Virtualization with untrusted guests
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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If the guest comes from an untrusted source, the guest host kernel will need
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to apply iTLB multihit mitigation via the kernel command line or kvm
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module parameter.
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