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