Messages in this thread Patch in this message | | | From | Julien Desfossez <> | Subject | [RFC PATCH v7 22/23] Documentation: Add documentation on core scheduling | Date | Fri, 28 Aug 2020 15:51:23 -0400 |
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From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Co-developed-by: Vineeth Pillai <viremana@linux.microsoft.com> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org> Signed-off-by: Vineeth Pillai <viremana@linux.microsoft.com> --- .../admin-guide/hw-vuln/core-scheduling.rst | 253 ++++++++++++++++++ Documentation/admin-guide/hw-vuln/index.rst | 1 + 2 files changed, 254 insertions(+) create mode 100644 Documentation/admin-guide/hw-vuln/core-scheduling.rst
diff --git a/Documentation/admin-guide/hw-vuln/core-scheduling.rst b/Documentation/admin-guide/hw-vuln/core-scheduling.rst new file mode 100644 index 000000000000..7ebece93f1d3 --- /dev/null +++ b/Documentation/admin-guide/hw-vuln/core-scheduling.rst @@ -0,0 +1,253 @@ +Core Scheduling +================ +MDS and L1TF mitigations do not protect from cross-HT attacks (attacker running +on one HT with victim running on another). For proper mitigation of this, +core scheduling support is available via the ``CONFIG_SCHED_CORE`` config option. +Using this feature, userspace defines groups of tasks that trust each other. +The core scheduler uses this information to make sure that tasks that do not +trust each other will never run simultaneously on a core while still meeting +the scheduler's requirements. + +Usage +----- +The current interface implementation is just for testing and uses CPU +controller CGroups which will change soon. A ``cpu.tag`` file has been added to +the CPU controller CGroup. If the content of this file is 1, then all the +CGroup's tasks trust each other and are allowed to run concurrently on a core's +hyperthreads (also called siblings). + +As mention, the interface is for testing purposes and has drawbacks. Trusted +tasks have to be grouped into CPU CGroup which is not always possible +depending on the system's existing CGroup configuration, where trusted tasks +could already be in different CPU CGroups. Also, this feature will have a hard +dependency on CGroups and systems with CGroups disabled would not be able to +use core scheduling so another API is needed in conjunction with +CGroups. See `Future work`_ for other API proposals. + +Design/Implementation +--------------------- +Each task that is tagged is assigned a cookie internally in the kernel. As +mentioned in `Usage`_, tasks with the same cookie value are assumed to trust +each other and share a core. + +The basic idea is that, every schedule event tries to select tasks for all the +siblings of a core such that all the selected tasks running on a core are +trusted (same cookie) at any point in time. Kernel threads are assumed trusted. +The idle task is considered special, in that it trusts every thing. + +During a schedule() event on any sibling of a core, the highest priority task for +that core is picked and assigned to the sibling calling schedule() if it has it +enqueued. For rest of the siblings in the core, highest priority task with the +same cookie is selected if there is one runnable in their individual run +queues. If a task with same cookie is not available, the idle task is selected. +Idle task is globally trusted. + +Once a task has been selected for all the siblings in the core, an IPI is sent to +siblings for whom a new task was selected. Siblings on receiving the IPI, will +switch to the new task immediately. If an idle task is selected for a sibling, +then the sibling is considered to be in a "forced idle" state. i.e., it may +have tasks on its on runqueue to run, however it will still have to run idle. +More on this in the next section. + +Forced-idling of tasks +--------------------- +The scheduler tries its best to find tasks that trust each other such that all +tasks selected to be scheduled are of the highest priority in a core. However, +it is possible that some runqueues had tasks that were incompatibile with the +highest priority ones in the core. Favoring security over fairness, one or more +siblings could be forced to select a lower priority task if the highest +priority task is not trusted with respect to the core wide highest priority +task. If a sibling does not have a trusted task to run, it will be forced idle +by the scheduler(idle thread is scheduled to run). + +When the highest priorty task is selected to run, a reschedule-IPI is sent to +the sibling to force it into idle. This results in 4 cases which need to be +considered depending on whether a VM or a regular usermode process was running +on either HT: + +:: + HT1 (attack) HT2 (victim) + + A idle -> user space user space -> idle + + B idle -> user space guest -> idle + + C idle -> guest user space -> idle + + D idle -> guest guest -> idle + +Note that for better performance, we do not wait for the destination CPU +(victim) to enter idle mode. This is because the sending of the IPI would bring +the destination CPU immediately into kernel mode from user space, or VMEXIT +in the case of guests. At best, this would only leak some scheduler metadata +which may not be worth protecting. It is also possible that the IPI is received +too late on some architectures, but this has not been observed in the case of +x86. + +Kernel protection from untrusted tasks +-------------------------------------- +The scheduler on its own cannot protect the kernel executing concurrently with +an untrusted task in a core. This is because the scheduler is unaware of +interrupts/syscalls at scheduling time. To mitigate this, we send an IPI to +siblings on kernel entry. This forces the sibling to enter kernel mode and it +waits before returning to user until all siblings of the core has left kernel +mode. For good performance, we send an IPI only if it is detected that the +core is running tasks that have been marked for core scheduling. If a sibling +is running kernel threads or is idle, no IPI is sent. + +For easier testing, a temporary (not intended for mainline) patch is included +in this series to make kernel protection configurable via a +``CONFIG_SCHED_CORE_KERNEL_PROTECTION`` config option or a +``sched_core_kernel_protection`` boot parameter. + +Other ideas for kernel protection which are + +1. Changing interrupt affinities to a trusted core which does not execute untrusted tasks +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +By changing the interrupt affinities to a designated safe-CPU which runs +only trusted tasks, IRQ data can be protected. One issue is this involves +giving up a full CPU core of the system to run safe tasks. Another is that, +per-cpu interrupts such as the local timer interrupt cannot have their +affinity changed. also, sensitive timer callbacks such as the random entropy timer +can run in softirq on return from these interrupts and expose sensitive +data. In the future, that could be mitigated by forcing softirqs into threaded +mode by utilizing a mechanism similar to ``PREEMPT_RT``. + +Yet another issue with this is, for multiqueue devices with managed +interrupts, the IRQ affinities cannot be changed however it could be +possible to force a reduced number of queues which would in turn allow to +shield one or two CPUs from such interrupts and queue handling for the price +of indirection. + +2. Running IRQs as threaded-IRQs +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +This would result in forcing IRQs into the scheduler which would then provide +the process-context mitigation. However, not all interrupts can be threaded. +Also this does nothing about syscall entries. + +3. Kernel Address Space Isolation +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +System calls could run in a much restricted address space which is +guarenteed not to leak any sensitive data. There are practical limitation in +implementing this - the main concern being how to decide on an address space +that is guarenteed to not have any sensitive data. + +4. Limited cookie-based protection +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +On a system call, change the cookie to the system trusted cookie and initiate a +schedule event. This would be better than pausing all the siblings during the +entire duration for the system call, but still would be a huge hit to the +performance. + +Trust model +----------- +Core scheduling understands trust relationships by assignment of a cookie to +related tasks using the above mentioned interface. When a system with core +scheduling boots, all tasks are considered to trust each other. This is because +the scheduler does not have information about trust relationships. That is, all +tasks have a default cookie value of 0. This cookie value is also considered +the system-wide cookie value and the IRQ-pausing mitigation is avoided if +siblings are running these cookie-0 tasks. + +By default, all system processes on boot are considered trusted and userspace +has to explicitly use the interfaces mentioned above to group sets of tasks. +Tasks within the group trust each other, but not those outside. Tasks outside +the group don't trust the task inside. + +Limitations +----------- +Core scheduling tries to guarentee that only trusted tasks run concurrently on a +core. But there could be small window of time during which untrusted tasks run +concurrently or kernel could be running concurrently with a task not trusted by +kernel. + +1. IPI processing delays +^^^^^^^^^^^^^^^^^^^^^^^^ +Core scheduling selects only trusted tasks to run together. IPI is used to notify +the siblings to switch to the new task. But there could be hardware delays in +receiving of the IPI on some arch (on x86, this has not been observed). This may +cause an attacker task to start running on a cpu before its siblings receive the +IPI. Even though cache is flushed on entry to user mode, victim tasks on siblings +may populate data in the cache and micro acrhitectural buffers after the attacker +starts to run and this is a possibility for data leak. + +Open cross-HT issues that core scheduling does not solve +-------------------------------------------------------- +1. For MDS +^^^^^^^^^^ +Core scheduling cannot protect against MDS attacks between an HT running in +user mode and another running in kernel mode. Even though both HTs run tasks +which trust each other, kernel memory is still considered untrusted. Such +attacks are possible for any combination of sibling CPU modes (host or guest mode). + +2. For L1TF +^^^^^^^^^^^ +Core scheduling cannot protect against a L1TF guest attackers exploiting a +guest or host victim. This is because the guest attacker can craft invalid +PTEs which are not inverted due to a vulnerable guest kernel. The only +solution is to disable EPT. + +For both MDS and L1TF, if the guest vCPU is configured to not trust each +other (by tagging separately), then the guest to guest attacks would go away. +Or it could be a system admin policy which considers guest to guest attacks as +a guest problem. + +Another approach to resolve these would be to make every untrusted task on the +system to not trust every other untrusted task. While this could reduce +parallelism of the untrusted tasks, it would still solve the above issues while +allowing system processes (trusted tasks) to share a core. + +Use cases +--------- +The main use case for Core scheduling is mitigating the cross-HT vulnerabilities +with SMT enabled. There are other use cases where this feature could be used: + +- Isolating tasks that needs a whole core: Examples include realtime tasks, tasks + that uses SIMD instructions etc. +- Gang scheduling: Requirements for a group of tasks that needs to be scheduled + together could also be realized using core scheduling. One example is vcpus of + a VM. + +Future work +----------- +1. API Proposals +^^^^^^^^^^^^^^^^ + +As mentioned in `Usage`_ section, various API proposals are listed here: + +- ``prctl`` : We can pass in a tag and all tasks with same tag set by prctl forms + a trusted group. + +- ``sched_setattr`` : Similar to prctl, but has the advantage that tasks could be + tagged by other tasks with appropriate permissions. + +- ``Auto Tagging`` : Related tasks are tagged automatically. Relation could be, + threads of the same process, tasks by a user, group or session etc. + +- Dedicated CGroup or procfs/sysfs interface for grouping trusted tasks. This could + be combined with prctl/sched_setattr as well. + +2. Auto-tagging of KVM vCPU threads +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +To make configuration easier, it would be great if KVM auto-tags vCPU threads +such that a given VM only trusts other vCPUs of the same VM. Or something more +aggressive like assiging a vCPU thread a unique tag. + +3. Auto-tagging of processes by default +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Currently core scheduling does not prevent 'unconfigured' tasks from being +co-scheduled on the same core. In other words, everything trusts everything +else by default. If a user wants everything default untrusted, a CONFIG option +could be added to assign every task with a unique tag by default. + +4. Auto-tagging on fork +^^^^^^^^^^^^^^^^^^^^^^^ +Currently, on fork a thread is added to the same trust-domain as the parent. For +systems which want all tasks to have a unique tag, it could be desirable to assign +a unique tag to a task so that the parent does not trust the child by default. + +5. Skipping per-HT mitigations if task is trusted +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +If core scheduling is enabled, by default all tasks trust each other as +mentioned above. In such scenario, it may be desirable to skip the same-HT +mitigations on return to the trusted user-mode to improve performance. diff --git a/Documentation/admin-guide/hw-vuln/index.rst b/Documentation/admin-guide/hw-vuln/index.rst index ca4dbdd9016d..f12cda55538b 100644 --- a/Documentation/admin-guide/hw-vuln/index.rst +++ b/Documentation/admin-guide/hw-vuln/index.rst @@ -15,3 +15,4 @@ are configurable at compile, boot or run time. tsx_async_abort multihit.rst special-register-buffer-data-sampling.rst + core-scheduling.rst -- 2.17.1
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