| L1TF - L1 Terminal Fault |
| ======================== |
| |
| L1 Terminal Fault is a hardware vulnerability which allows unprivileged |
| speculative access to data which is available in the Level 1 Data Cache |
| when the page table entry controlling the virtual address, which is used |
| for the access, has the Present bit cleared or other reserved bits set. |
| |
| Affected processors |
| ------------------- |
| |
| This vulnerability affects a wide range of Intel processors. The |
| vulnerability is not present on: |
| |
| - Processors from AMD, Centaur and other non Intel vendors |
| |
| - Older processor models, where the CPU family is < 6 |
| |
| - A range of Intel ATOM processors (Cedarview, Cloverview, Lincroft, |
| Penwell, Pineview, Silvermont, Airmont, Merrifield) |
| |
| - The Intel XEON PHI family |
| |
| - Intel processors which have the ARCH_CAP_RDCL_NO bit set in the |
| IA32_ARCH_CAPABILITIES MSR. If the bit is set the CPU is not affected |
| by the Meltdown vulnerability either. These CPUs should become |
| available by end of 2018. |
| |
| Whether a processor is affected or not can be read out from the L1TF |
| vulnerability file in sysfs. See :ref:`l1tf_sys_info`. |
| |
| Related CVEs |
| ------------ |
| |
| The following CVE entries are related to the L1TF vulnerability: |
| |
| ============= ================= ============================== |
| CVE-2018-3615 L1 Terminal Fault SGX related aspects |
| CVE-2018-3620 L1 Terminal Fault OS, SMM related aspects |
| CVE-2018-3646 L1 Terminal Fault Virtualization related aspects |
| ============= ================= ============================== |
| |
| Problem |
| ------- |
| |
| If an instruction accesses a virtual address for which the relevant page |
| table entry (PTE) has the Present bit cleared or other reserved bits set, |
| then speculative execution ignores the invalid PTE and loads the referenced |
| data if it is present in the Level 1 Data Cache, as if the page referenced |
| by the address bits in the PTE was still present and accessible. |
| |
| While this is a purely speculative mechanism and the instruction will raise |
| a page fault when it is retired eventually, the pure act of loading the |
| data and making it available to other speculative instructions opens up the |
| opportunity for side channel attacks to unprivileged malicious code, |
| similar to the Meltdown attack. |
| |
| While Meltdown breaks the user space to kernel space protection, L1TF |
| allows to attack any physical memory address in the system and the attack |
| works across all protection domains. It allows an attack of SGX and also |
| works from inside virtual machines because the speculation bypasses the |
| extended page table (EPT) protection mechanism. |
| |
| |
| Attack scenarios |
| ---------------- |
| |
| 1. Malicious user space |
| ^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| Operating Systems store arbitrary information in the address bits of a |
| PTE which is marked non present. This allows a malicious user space |
| application to attack the physical memory to which these PTEs resolve. |
| In some cases user-space can maliciously influence the information |
| encoded in the address bits of the PTE, thus making attacks more |
| deterministic and more practical. |
| |
| The Linux kernel contains a mitigation for this attack vector, PTE |
| inversion, which is permanently enabled and has no performance |
| impact. The kernel ensures that the address bits of PTEs, which are not |
| marked present, never point to cacheable physical memory space. |
| |
| A system with an up to date kernel is protected against attacks from |
| malicious user space applications. |
| |
| 2. Malicious guest in a virtual machine |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| The fact that L1TF breaks all domain protections allows malicious guest |
| OSes, which can control the PTEs directly, and malicious guest user |
| space applications, which run on an unprotected guest kernel lacking the |
| PTE inversion mitigation for L1TF, to attack physical host memory. |
| |
| A special aspect of L1TF in the context of virtualization is symmetric |
| multi threading (SMT). The Intel implementation of SMT is called |
| HyperThreading. The fact that Hyperthreads on the affected processors |
| share the L1 Data Cache (L1D) is important for this. As the flaw allows |
| only to attack data which is present in L1D, a malicious guest running |
| on one Hyperthread can attack the data which is brought into the L1D by |
| the context which runs on the sibling Hyperthread of the same physical |
| core. This context can be host OS, host user space or a different guest. |
| |
| If the processor does not support Extended Page Tables, the attack is |
| only possible, when the hypervisor does not sanitize the content of the |
| effective (shadow) page tables. |
| |
| While solutions exist to mitigate these attack vectors fully, these |
| mitigations are not enabled by default in the Linux kernel because they |
| can affect performance significantly. The kernel provides several |
| mechanisms which can be utilized to address the problem depending on the |
| deployment scenario. The mitigations, their protection scope and impact |
| are described in the next sections. |
| |
| The default mitigations and the rationale for choosing them are explained |
| at the end of this document. See :ref:`default_mitigations`. |
| |
| .. _l1tf_sys_info: |
| |
| L1TF system information |
| ----------------------- |
| |
| The Linux kernel provides a sysfs interface to enumerate the current L1TF |
| status of the system: whether the system is vulnerable, and which |
| mitigations are active. The relevant sysfs file is: |
| |
| /sys/devices/system/cpu/vulnerabilities/l1tf |
| |
| The possible values in this file are: |
| |
| =========================== =============================== |
| 'Not affected' The processor is not vulnerable |
| 'Mitigation: PTE Inversion' The host protection is active |
| =========================== =============================== |
| |
| If KVM/VMX is enabled and the processor is vulnerable then the following |
| information is appended to the 'Mitigation: PTE Inversion' part: |
| |
| - SMT status: |
| |
| ===================== ================ |
| 'VMX: SMT vulnerable' SMT is enabled |
| 'VMX: SMT disabled' SMT is disabled |
| ===================== ================ |
| |
| - L1D Flush mode: |
| |
| ================================ ==================================== |
| 'L1D vulnerable' L1D flushing is disabled |
| |
| 'L1D conditional cache flushes' L1D flush is conditionally enabled |
| |
| 'L1D cache flushes' L1D flush is unconditionally enabled |
| ================================ ==================================== |
| |
| The resulting grade of protection is discussed in the following sections. |
| |
| |
| Host mitigation mechanism |
| ------------------------- |
| |
| The kernel is unconditionally protected against L1TF attacks from malicious |
| user space running on the host. |
| |
| |
| Guest mitigation mechanisms |
| --------------------------- |
| |
| .. _l1d_flush: |
| |
| 1. L1D flush on VMENTER |
| ^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| To make sure that a guest cannot attack data which is present in the L1D |
| the hypervisor flushes the L1D before entering the guest. |
| |
| Flushing the L1D evicts not only the data which should not be accessed |
| by a potentially malicious guest, it also flushes the guest |
| data. Flushing the L1D has a performance impact as the processor has to |
| bring the flushed guest data back into the L1D. Depending on the |
| frequency of VMEXIT/VMENTER and the type of computations in the guest |
| performance degradation in the range of 1% to 50% has been observed. For |
| scenarios where guest VMEXIT/VMENTER are rare the performance impact is |
| minimal. Virtio and mechanisms like posted interrupts are designed to |
| confine the VMEXITs to a bare minimum, but specific configurations and |
| application scenarios might still suffer from a high VMEXIT rate. |
| |
| The kernel provides two L1D flush modes: |
| - conditional ('cond') |
| - unconditional ('always') |
| |
| The conditional mode avoids L1D flushing after VMEXITs which execute |
| only audited code paths before the corresponding VMENTER. These code |
| paths have been verified that they cannot expose secrets or other |
| interesting data to an attacker, but they can leak information about the |
| address space layout of the hypervisor. |
| |
| Unconditional mode flushes L1D on all VMENTER invocations and provides |
| maximum protection. It has a higher overhead than the conditional |
| mode. The overhead cannot be quantified correctly as it depends on the |
| workload scenario and the resulting number of VMEXITs. |
| |
| The general recommendation is to enable L1D flush on VMENTER. The kernel |
| defaults to conditional mode on affected processors. |
| |
| **Note**, that L1D flush does not prevent the SMT problem because the |
| sibling thread will also bring back its data into the L1D which makes it |
| attackable again. |
| |
| L1D flush can be controlled by the administrator via the kernel command |
| line and sysfs control files. See :ref:`mitigation_control_command_line` |
| and :ref:`mitigation_control_kvm`. |
| |
| .. _guest_confinement: |
| |
| 2. Guest VCPU confinement to dedicated physical cores |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| To address the SMT problem, it is possible to make a guest or a group of |
| guests affine to one or more physical cores. The proper mechanism for |
| that is to utilize exclusive cpusets to ensure that no other guest or |
| host tasks can run on these cores. |
| |
| If only a single guest or related guests run on sibling SMT threads on |
| the same physical core then they can only attack their own memory and |
| restricted parts of the host memory. |
| |
| Host memory is attackable, when one of the sibling SMT threads runs in |
| host OS (hypervisor) context and the other in guest context. The amount |
| of valuable information from the host OS context depends on the context |
| which the host OS executes, i.e. interrupts, soft interrupts and kernel |
| threads. The amount of valuable data from these contexts cannot be |
| declared as non-interesting for an attacker without deep inspection of |
| the code. |
| |
| **Note**, that assigning guests to a fixed set of physical cores affects |
| the ability of the scheduler to do load balancing and might have |
| negative effects on CPU utilization depending on the hosting |
| scenario. Disabling SMT might be a viable alternative for particular |
| scenarios. |
| |
| For further information about confining guests to a single or to a group |
| of cores consult the cpusets documentation: |
| |
| https://www.kernel.org/doc/Documentation/cgroup-v1/cpusets.txt |
| |
| .. _interrupt_isolation: |
| |
| 3. Interrupt affinity |
| ^^^^^^^^^^^^^^^^^^^^^ |
| |
| Interrupts can be made affine to logical CPUs. This is not universally |
| true because there are types of interrupts which are truly per CPU |
| interrupts, e.g. the local timer interrupt. Aside of that multi queue |
| devices affine their interrupts to single CPUs or groups of CPUs per |
| queue without allowing the administrator to control the affinities. |
| |
| Moving the interrupts, which can be affinity controlled, away from CPUs |
| which run untrusted guests, reduces the attack vector space. |
| |
| Whether the interrupts with are affine to CPUs, which run untrusted |
| guests, provide interesting data for an attacker depends on the system |
| configuration and the scenarios which run on the system. While for some |
| of the interrupts it can be assumed that they won't expose interesting |
| information beyond exposing hints about the host OS memory layout, there |
| is no way to make general assumptions. |
| |
| Interrupt affinity can be controlled by the administrator via the |
| /proc/irq/$NR/smp_affinity[_list] files. Limited documentation is |
| available at: |
| |
| https://www.kernel.org/doc/Documentation/IRQ-affinity.txt |
| |
| .. _smt_control: |
| |
| 4. SMT control |
| ^^^^^^^^^^^^^^ |
| |
| To prevent the SMT issues of L1TF it might be necessary to disable SMT |
| completely. Disabling SMT can have a significant performance impact, but |
| the impact depends on the hosting scenario and the type of workloads. |
| The impact of disabling SMT needs also to be weighted against the impact |
| of other mitigation solutions like confining guests to dedicated cores. |
| |
| The kernel provides a sysfs interface to retrieve the status of SMT and |
| to control it. It also provides a kernel command line interface to |
| control SMT. |
| |
| The kernel command line interface consists of the following options: |
| |
| =========== ========================================================== |
| nosmt Affects the bring up of the secondary CPUs during boot. The |
| kernel tries to bring all present CPUs online during the |
| boot process. "nosmt" makes sure that from each physical |
| core only one - the so called primary (hyper) thread is |
| activated. Due to a design flaw of Intel processors related |
| to Machine Check Exceptions the non primary siblings have |
| to be brought up at least partially and are then shut down |
| again. "nosmt" can be undone via the sysfs interface. |
| |
| nosmt=force Has the same effect as "nosmt" but it does not allow to |
| undo the SMT disable via the sysfs interface. |
| =========== ========================================================== |
| |
| The sysfs interface provides two files: |
| |
| - /sys/devices/system/cpu/smt/control |
| - /sys/devices/system/cpu/smt/active |
| |
| /sys/devices/system/cpu/smt/control: |
| |
| This file allows to read out the SMT control state and provides the |
| ability to disable or (re)enable SMT. The possible states are: |
| |
| ============== =================================================== |
| on SMT is supported by the CPU and enabled. All |
| logical CPUs can be onlined and offlined without |
| restrictions. |
| |
| off SMT is supported by the CPU and disabled. Only |
| the so called primary SMT threads can be onlined |
| and offlined without restrictions. An attempt to |
| online a non-primary sibling is rejected |
| |
| forceoff Same as 'off' but the state cannot be controlled. |
| Attempts to write to the control file are rejected. |
| |
| notsupported The processor does not support SMT. It's therefore |
| not affected by the SMT implications of L1TF. |
| Attempts to write to the control file are rejected. |
| ============== =================================================== |
| |
| The possible states which can be written into this file to control SMT |
| state are: |
| |
| - on |
| - off |
| - forceoff |
| |
| /sys/devices/system/cpu/smt/active: |
| |
| This file reports whether SMT is enabled and active, i.e. if on any |
| physical core two or more sibling threads are online. |
| |
| SMT control is also possible at boot time via the l1tf kernel command |
| line parameter in combination with L1D flush control. See |
| :ref:`mitigation_control_command_line`. |
| |
| 5. Disabling EPT |
| ^^^^^^^^^^^^^^^^ |
| |
| Disabling EPT for virtual machines provides full mitigation for L1TF even |
| with SMT enabled, because the effective page tables for guests are |
| managed and sanitized by the hypervisor. Though disabling EPT has a |
| significant performance impact especially when the Meltdown mitigation |
| KPTI is enabled. |
| |
| EPT can be disabled in the hypervisor via the 'kvm-intel.ept' parameter. |
| |
| There is ongoing research and development for new mitigation mechanisms to |
| address the performance impact of disabling SMT or EPT. |
| |
| .. _mitigation_control_command_line: |
| |
| Mitigation control on the kernel command line |
| --------------------------------------------- |
| |
| The kernel command line allows to control the L1TF mitigations at boot |
| time with the option "l1tf=". The valid arguments for this option are: |
| |
| ============ ============================================================= |
| full Provides all available mitigations for the L1TF |
| vulnerability. Disables SMT and enables all mitigations in |
| the hypervisors, i.e. unconditional L1D flushing |
| |
| SMT control and L1D flush control via the sysfs interface |
| is still possible after boot. Hypervisors will issue a |
| warning when the first VM is started in a potentially |
| insecure configuration, i.e. SMT enabled or L1D flush |
| disabled. |
| |
| full,force Same as 'full', but disables SMT and L1D flush runtime |
| control. Implies the 'nosmt=force' command line option. |
| (i.e. sysfs control of SMT is disabled.) |
| |
| flush Leaves SMT enabled and enables the default hypervisor |
| mitigation, i.e. conditional L1D flushing |
| |
| SMT control and L1D flush control via the sysfs interface |
| is still possible after boot. Hypervisors will issue a |
| warning when the first VM is started in a potentially |
| insecure configuration, i.e. SMT enabled or L1D flush |
| disabled. |
| |
| flush,nosmt Disables SMT and enables the default hypervisor mitigation, |
| i.e. conditional L1D flushing. |
| |
| SMT control and L1D flush control via the sysfs interface |
| is still possible after boot. Hypervisors will issue a |
| warning when the first VM is started in a potentially |
| insecure configuration, i.e. SMT enabled or L1D flush |
| disabled. |
| |
| flush,nowarn Same as 'flush', but hypervisors will not warn when a VM is |
| started in a potentially insecure configuration. |
| |
| off Disables hypervisor mitigations and doesn't emit any |
| warnings. |
| It also drops the swap size and available RAM limit restrictions |
| on both hypervisor and bare metal. |
| |
| ============ ============================================================= |
| |
| The default is 'flush'. For details about L1D flushing see :ref:`l1d_flush`. |
| |
| |
| .. _mitigation_control_kvm: |
| |
| Mitigation control for KVM - module parameter |
| ------------------------------------------------------------- |
| |
| The KVM hypervisor mitigation mechanism, flushing the L1D cache when |
| entering a guest, can be controlled with a module parameter. |
| |
| The option/parameter is "kvm-intel.vmentry_l1d_flush=". It takes the |
| following arguments: |
| |
| ============ ============================================================== |
| always L1D cache flush on every VMENTER. |
| |
| cond Flush L1D on VMENTER only when the code between VMEXIT and |
| VMENTER can leak host memory which is considered |
| interesting for an attacker. This still can leak host memory |
| which allows e.g. to determine the hosts address space layout. |
| |
| never Disables the mitigation |
| ============ ============================================================== |
| |
| The parameter can be provided on the kernel command line, as a module |
| parameter when loading the modules and at runtime modified via the sysfs |
| file: |
| |
| /sys/module/kvm_intel/parameters/vmentry_l1d_flush |
| |
| The default is 'cond'. If 'l1tf=full,force' is given on the kernel command |
| line, then 'always' is enforced and the kvm-intel.vmentry_l1d_flush |
| module parameter is ignored and writes to the sysfs file are rejected. |
| |
| .. _mitigation_selection: |
| |
| 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 and the guest OS kernel is |
| guaranteed to have the L1TF mitigations in place the system is fully |
| protected against L1TF and no further action is required. |
| |
| To avoid the overhead of the default L1D flushing on VMENTER the |
| administrator can disable the flushing via the kernel command line and |
| sysfs control files. See :ref:`mitigation_control_command_line` and |
| :ref:`mitigation_control_kvm`. |
| |
| |
| 3. Virtualization with untrusted guests |
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| |
| 3.1. SMT not supported or disabled |
| """""""""""""""""""""""""""""""""" |
| |
| If SMT is not supported by the processor or disabled in the BIOS or by |
| the kernel, it's only required to enforce L1D flushing on VMENTER. |
| |
| Conditional L1D flushing is the default behaviour and can be tuned. See |
| :ref:`mitigation_control_command_line` and :ref:`mitigation_control_kvm`. |
| |
| 3.2. EPT not supported or disabled |
| """""""""""""""""""""""""""""""""" |
| |
| If EPT is not supported by the processor or disabled in the hypervisor, |
| the system is fully protected. SMT can stay enabled and L1D flushing on |
| VMENTER is not required. |
| |
| EPT can be disabled in the hypervisor via the 'kvm-intel.ept' parameter. |
| |
| 3.3. SMT and EPT supported and active |
| """"""""""""""""""""""""""""""""""""" |
| |
| If SMT and EPT are supported and active then various degrees of |
| mitigations can be employed: |
| |
| - L1D flushing on VMENTER: |
| |
| L1D flushing on VMENTER is the minimal protection requirement, but it |
| is only potent in combination with other mitigation methods. |
| |
| Conditional L1D flushing is the default behaviour and can be tuned. See |
| :ref:`mitigation_control_command_line` and :ref:`mitigation_control_kvm`. |
| |
| - Guest confinement: |
| |
| Confinement of guests to a single or a group of physical cores which |
| are not running any other processes, can reduce the attack surface |
| significantly, but interrupts, soft interrupts and kernel threads can |
| still expose valuable data to a potential attacker. See |
| :ref:`guest_confinement`. |
| |
| - Interrupt isolation: |
| |
| Isolating the guest CPUs from interrupts can reduce the attack surface |
| further, but still allows a malicious guest to explore a limited amount |
| of host physical memory. This can at least be used to gain knowledge |
| about the host address space layout. The interrupts which have a fixed |
| affinity to the CPUs which run the untrusted guests can depending on |
| the scenario still trigger soft interrupts and schedule kernel threads |
| which might expose valuable information. See |
| :ref:`interrupt_isolation`. |
| |
| The above three mitigation methods combined can provide protection to a |
| certain degree, but the risk of the remaining attack surface has to be |
| carefully analyzed. For full protection the following methods are |
| available: |
| |
| - Disabling SMT: |
| |
| Disabling SMT and enforcing the L1D flushing provides the maximum |
| amount of protection. This mitigation is not depending on any of the |
| above mitigation methods. |
| |
| SMT control and L1D flushing can be tuned by the command line |
| parameters 'nosmt', 'l1tf', 'kvm-intel.vmentry_l1d_flush' and at run |
| time with the matching sysfs control files. See :ref:`smt_control`, |
| :ref:`mitigation_control_command_line` and |
| :ref:`mitigation_control_kvm`. |
| |
| - Disabling EPT: |
| |
| Disabling EPT provides the maximum amount of protection as well. It is |
| not depending on any of the above mitigation methods. SMT can stay |
| enabled and L1D flushing is not required, but the performance impact is |
| significant. |
| |
| EPT can be disabled in the hypervisor via the 'kvm-intel.ept' |
| parameter. |
| |
| 3.4. Nested virtual machines |
| """""""""""""""""""""""""""" |
| |
| When nested virtualization is in use, three operating systems are involved: |
| the bare metal hypervisor, the nested hypervisor and the nested virtual |
| machine. VMENTER operations from the nested hypervisor into the nested |
| guest will always be processed by the bare metal hypervisor. If KVM is the |
| bare metal hypervisor it will: |
| |
| - Flush the L1D cache on every switch from the nested hypervisor to the |
| nested virtual machine, so that the nested hypervisor's secrets are not |
| exposed to the nested virtual machine; |
| |
| - Flush the L1D cache on every switch from the nested virtual machine to |
| the nested hypervisor; this is a complex operation, and flushing the L1D |
| cache avoids that the bare metal hypervisor's secrets are exposed to the |
| nested virtual machine; |
| |
| - Instruct the nested hypervisor to not perform any L1D cache flush. This |
| is an optimization to avoid double L1D flushing. |
| |
| |
| .. _default_mitigations: |
| |
| Default mitigations |
| ------------------- |
| |
| The kernel default mitigations for vulnerable processors are: |
| |
| - PTE inversion to protect against malicious user space. This is done |
| unconditionally and cannot be controlled. The swap storage is limited |
| to ~16TB. |
| |
| - L1D conditional flushing on VMENTER when EPT is enabled for |
| a guest. |
| |
| The kernel does not by default enforce the disabling of SMT, which leaves |
| SMT systems vulnerable when running untrusted guests with EPT enabled. |
| |
| The rationale for this choice is: |
| |
| - Force disabling SMT can break existing setups, especially with |
| unattended updates. |
| |
| - If regular users run untrusted guests on their machine, then L1TF is |
| just an add on to other malware which might be embedded in an untrusted |
| guest, e.g. spam-bots or attacks on the local network. |
| |
| There is no technical way to prevent a user from running untrusted code |
| on their machines blindly. |
| |
| - It's technically extremely unlikely and from today's knowledge even |
| impossible that L1TF can be exploited via the most popular attack |
| mechanisms like JavaScript because these mechanisms have no way to |
| control PTEs. If this would be possible and not other mitigation would |
| be possible, then the default might be different. |
| |
| - The administrators of cloud and hosting setups have to carefully |
| analyze the risk for their scenarios and make the appropriate |
| mitigation choices, which might even vary across their deployed |
| machines and also result in other changes of their overall setup. |
| There is no way for the kernel to provide a sensible default for this |
| kind of scenarios. |