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Subject: [PATCH v10 4/5] Documentation: Add core scheduling documentation
Date:   Fri, 22 Jan 2021 20:17:03 -0500
Message-Id: <20210123011704.1901835-5-joel@joelfernandes.org>
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Document the usecases, design and interfaces for core scheduling.

Co-developed-by: Chris Hyser <chris.hyser@oracle.com>
Co-developed-by: Vineeth Pillai <viremana@linux.microsoft.com>
Co-developed-by: Josh Don <joshdon@google.com>
Signed-off-by: Josh Don <joshdon@google.com>
Signed-off-by: Vineeth Pillai <viremana@linux.microsoft.com>
Signed-off-by: Chris Hyser <chris.hyser@oracle.com>
Tested-by: Julien Desfossez <jdesfossez@digitalocean.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 .../admin-guide/hw-vuln/core-scheduling.rst   | 263 ++++++++++++++++++
 Documentation/admin-guide/hw-vuln/index.rst   |   1 +
 2 files changed, 264 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..a795747c706a
--- /dev/null
+++ b/Documentation/admin-guide/hw-vuln/core-scheduling.rst
@@ -0,0 +1,263 @@
+Core Scheduling
+***************
+Core scheduling support allows userspace to define groups of tasks that can
+share a core. These groups can be specified either for security usecases (one
+group of tasks don't trust another), or for performance usecases (some
+workloads may benefit from running on the same core as they don't need the same
+hardware resources of the shared core).
+
+Security usecase
+----------------
+A cross-HT attack involves the attacker and victim running on different
+Hyper Threads of the same core. MDS and L1TF are examples of such attacks.
+Without core scheduling, the only full mitigation of cross-HT attacks is to
+disable Hyper Threading (HT). Core scheduling allows HT to be turned on safely
+by ensuring that trusted tasks can share a core. This increase in core sharing
+can improvement performance, however it is not guaranteed that performance will
+always improve, though that is seen to be the case with a number of real world
+workloads. In theory, core scheduling aims to perform at least as good as when
+Hyper Threading is disabled. In practice, this is mostly the case though not
+always: as synchronizing scheduling decisions across 2 or more CPUs in a core
+involves additional overhead - especially when the system is lightly loaded
+(``total_threads <= N/2``, where N is the total number of CPUs).
+
+Usage
+-----
+Core scheduling support is enabled via the ``CONFIG_SCHED_CORE`` config option.
+Using this feature, userspace defines groups of tasks that can be co-scheduled
+on the same core.
+The core scheduler uses this information to make sure that tasks that are not
+in the same group never run simultaneously on a core, while doing its best to
+satisfy the system's scheduling requirements.
+
+There are 2 ways to use core-scheduling:
+
+CGroup
+######
+Core scheduling adds additional files to the CPU controller CGroup:
+
+* ``cpu.core_tag``
+Writing ``1`` into this file results in all tasks in the group getting tagged.
+This results in all the CGroup's tasks allowed to run concurrently on a core's
+hyperthreads (also called siblings).
+
+The file being a value of ``0`` means the tag state of the CGroup is inherited
+from its parent hierarchy. If any ancestor of the CGroup is tagged, then the
+group is tagged.
+
+.. note:: Once a CGroup is tagged via cpu.core_tag, it is not possible to set this
+          for any descendant of the tagged group.
+
+.. note:: When a CGroup is not tagged, all the tasks within the group can share
+          a core with kernel threads and untagged system threads. For this reason,
+          if a group has ``cpu.core_tag`` of 0, it is considered to be trusted.
+
+prctl interface
+###############
+A ``prtcl(2)`` command ``PR_SCHED_CORE_SHARE`` provides an interface for the
+creation of and admission and removal of tasks from core scheduling groups.
+
+::
+
+    #include <sys/prctl.h>
+
+    int prctl(int option, unsigned long arg2, unsigned long arg3,
+            unsigned long arg4, unsigned long arg5);
+
+option:
+    ``PR_SCHED_CORE_SHARE``
+
+arg2:
+    - ``PR_SCHED_CORE_CLEAR            0  -- clear core_sched cookie of pid``
+    - ``PR_SCHED_CORE_SHARE_FROM       1  -- get core_sched cookie from pid``
+    - ``PR_SCHED_CORE_SHARE_TO         2  -- push core_sched cookie to pid``
+
+arg3:
+    ``tid`` of the task for which the operation applies
+
+arg4 and arg5:
+    MUST be equal to 0.
+
+Creation
+~~~~~~~~
+Creation is accomplished by sharing a ''cookie'' from a process not currently in
+a core scheduling group.
+
+::
+
+    if (prctl(PR_SCHED_CORE_SHARE, PR_SCHED_CORE_SHARE_FROM, src_tid, 0, 0) < 0)
+            handle_error("src_tid sched_core failed");
+
+Removal
+~~~~~~~
+Removing a task from a core scheduling group is done by:
+
+::
+
+    if (prctl(PR_SCHED_CORE_SHARE, PR_SCHED_CORE_SHARE_CLEAR, clr_tid, 0, 0) < 0)
+             handle_error("clr_tid sched_core failed");
+
+Cookie Transferal
+~~~~~~~~~~~~~~~~~
+Transferring a cookie between the current and other tasks is possible using
+PR_SCHED_CORE_SHARE_FROM and PR_SCHED_CORE_SHARE_TO to inherit a cookie from a
+specified task or a share a cookie with a task. In combination this allows a
+simple helper program to pull a cookie from a task in an existing core
+scheduling group and share it with already running tasks.
+
+::
+
+    if (prctl(PR_SCHED_CORE_SHARE, PR_SCHED_CORE_SHARE_FROM, from_tid, 0, 0) < 0)
+            handle_error("from_tid sched_core failed");
+
+    if (prctl(PR_SCHED_CORE_SHARE, PR_SCHED_CORE_SHARE_TO, to_tid, 0, 0) < 0)
+            handle_error("to_tid sched_core failed");
+
+
+.. note:: The core-sharing granted with ``prctl(2)`` will be subject to
+          core-sharing restrictions specified by the CGroup interface. For example,
+          if tasks T1 and T2 are a part of 2 different tagged CGroups, then they will
+          not share a core even if ``prctl(2)`` is called. This is analogous to how
+          affinities are set using the cpuset interface.
+
+It is important to note that, on a ``clone(2)`` syscall with ``CLONE_THREAD`` set,
+the child will be assigned the same ''cookie'' as its parent and thus in the
+same core scheduling group.  In the security usecase, a ``CLONE_THREAD`` child
+can access its parent's address space anyway (``CLONE_THREAD`` requires
+``CLONE_SIGHAND`` which requires ``CLONE_VM``), so there's no point in not
+allowing them to share a core. If a different behavior is desired, the child
+thread can call ``prctl(2)`` as needed.  This behavior is specific to the
+``prctl(2)`` interface. For the CGroup interface, the child of a fork always
+shares a core with its parent.  On the other hand, if a parent was previously
+tagged via ``prctl(2)`` and does a regular ``fork(2)`` syscall, the child will
+receive a unique tag.
+
+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, as it trusts everything and everything
+trusts it.
+
+During a schedule() event on any sibling of a core, the highest priority task on
+the sibling's core is picked and assigned to the sibling calling schedule(), if
+the sibling has the task 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 incompatible 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 priority 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.
+
+Trust model
+-----------
+Core scheduling maintains trust relationships amongst groups of tasks by
+assigning the tag of them with the same cookie value.
+When a system with core scheduling boots, all tasks are considered to trust
+each other. This is because the core scheduler does not have information about
+trust relationships until userspace uses the above mentioned interfaces, to
+communicate them. In other words, all tasks have a default cookie value of 0.
+and are considered system-wide trusted. The stunning of siblings running
+cookie-0 tasks is also avoided.
+
+Once userspace uses the above mentioned interfaces to group sets of tasks, tasks
+within such groups are considered to trust each other, but do not trust those
+outside. Tasks outside the group also don't trust tasks within.
+
+Limitations in core-scheduling
+------------------------------
+Core scheduling tries to guarantee 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 architectural 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 an L1TF guest attacker 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 (Extended Page Tables).
+
+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.
+
+3. Protecting the kernel (IRQ, syscall, VMEXIT)
+###############################################
+This section is a work in progress. The main point here is entry into the
+kernel is not protected from attackers on a sibling.
+
+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.
diff --git a/Documentation/admin-guide/hw-vuln/index.rst b/Documentation/admin-guide/hw-vuln/index.rst
index 21710f8609fe..361ccbbd9e54 100644
--- a/Documentation/admin-guide/hw-vuln/index.rst
+++ b/Documentation/admin-guide/hw-vuln/index.rst
@@ -16,3 +16,4 @@ are configurable at compile, boot or run time.
    multihit.rst
    special-register-buffer-data-sampling.rst
    l1d_flush.rst
+   core-scheduling.rst
-- 
2.30.0.280.ga3ce27912f-goog