Re: [PATCH v8 -tip 25/26] Documentation: Add core scheduling documentation

[Randy Dunlap]

Hi Joel,

On 10/19/20 6:43 PM, Joel Fernandes (Google) wrote:
> Document the usecases, design and interfaces for core scheduling.
>
> Co-developed-by: Vineeth Pillai <viremana@xxxxxxxxxxxxxxxxxxx>
> Tested-by: Julien Desfossez <jdesfossez@xxxxxxxxxxxxxxxx>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
>   .../admin-guide/hw-vuln/core-scheduling.rst   | 312 ++++++++++++++++++
>   Documentation/admin-guide/hw-vuln/index.rst   |   1 +
>   2 files changed, 313 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..eacafbb8fa3f
> --- /dev/null
> +++ b/Documentation/admin-guide/hw-vuln/core-scheduling.rst
> @@ -0,0 +1,312 @@
> +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 practise, 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``).

N is 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 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 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.tag``
> +Writing ``1`` into this file results in all tasks in the group get tagged. This

                                                                  getting
or                                                                being

> +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 inheritted

                                                                        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.tag, it is not possible to set this
> +          for any descendant of the tagged group. For finer grained control, the
> +          ``cpu.tag_color`` file described next may be used.
> +
> +.. 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.tag`` of 0, it is considered to be trusted.
> +
> +* ``cpu.tag_color``
> +For finer grained control over core sharing, a color can also be set in
> +addition to the tag. This allows to further control core sharing between child
> +CGroups within an already tagged CGroup. The color and the tag are both used to
> +generate a `cookie` which is used by the scheduler to identify the group.
> +
> +Upto 256 different colors can be set (0-255) by writing into this file.

  Up to

> +
> +A sample real-world usage of this file follows:
> +
> +Google uses DAC controls to make ``cpu.tag`` writeable only by root and the

$search tells me "writable".

> +``cpu.tag_color`` can be changed by anyone.
> +
> +The hierarchy looks like this:
> +::
> +  Root group
> +     / \
> +    A   B    (These are created by the root daemon - borglet).
> +   / \   \
> +  C   D   E  (These are created by AppEngine within the container).
> +
> +A and B are containers for 2 different jobs or apps that are created by a root
> +daemon called borglet. borglet then tags each of these group with the ``cpu.tag``
> +file. The job itself can create additional child CGroups which are colored by
> +the container's AppEngine with the ``cpu.tag_color`` file.
> +
> +The reason why Google uses this 2-level tagging system is that AppEngine wants to
> +allow a subset of child CGroups within a tagged parent CGroup to be co-scheduled on a
> +core while not being co-scheduled with other child CGroups. Think of these
> +child CGroups as belonging to the same customer or project.  Because these
> +child CGroups are created by AppEngine, they are not tracked by borglet (the
> +root daemon), therefore borglet won't have a chance to set a color for them.
> +That's where cpu.tag_color file comes in. A color could be set by AppEngine,
> +and once set, the normal tasks within the subcgroup would not be able to
> +overwrite it. This is enforced by promoting the permission of the
> +``cpu.tag_color`` file in cgroupfs.
> +
> +The color is an 8-bit value allowing for upto 256 unique colors.

                                            up to

> +
> +.. note:: Once a CGroup is colored, none of its descendants can be re-colored. Also
> +          coloring of a CGroup is possible only if either the group or one of its
> +          ancestors were tagged via the ``cpu.tag`` file.

                       was

> +
> +prctl interface
> +###############
> +A ``prtcl(2)`` command ``PR_SCHED_CORE_SHARE`` is available to a process to request
> +sharing a core with another process.  For example, consider 2 processes ``P1``
> +and ``P2`` with PIDs 100 and 200. If process ``P1`` calls
> +``prctl(PR_SCHED_CORE_SHARE, 200)``, the kernel makes ``P1`` share a core with ``P2``.
> +The kernel performs ptrace access mode checks before granting the request.
> +
> +.. note:: This operation is not commutative. P1 calling
> +          ``prctl(PR_SCHED_CORE_SHARE, pidof(P2)`` is not the same as P2 calling the
> +          same for P1. The former case is P1 joining P2's group of processes
> +          (which P2 would have joined with ``prctl(2)`` prior to P1's ``prctl(2)``).
> +
> +.. note:: The core-sharing granted with prctl(2) will be subject to
> +          core-sharing restrictions specified by the CGroup interface. For example
> +          if P1 and P2 are a part of 2 different tagged CGroups, then they will
> +          not share a core even if a prctl(2) call is made. This is analogous
> +          to how affinities are set using the cpuset interface.
> +
> +It is important to note that, on a ``CLONE_THREAD`` ``clone(2)`` syscall, the child
> +will be assigned the same tag as its parent and thus be allowed to share a core
> +with them. is design choice is because, for the security usecase, a

              ^^^ missing subject ...

> +``CLONE_THREAD`` child can access its parent's address space anyway, 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 share's a core with its parent's.  On the other hand, if a parent

               shares         with its parent's what?  "parent's" is possessive.


> +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, in that it trusts every thing.

                                        or                everything.

> +
> +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

                                                           too many          it     it
They are not the same "it,", are they?

> +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

                                                                    no comma^

> +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

                                                                  I.e.,

> +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

                                                          incompatible

> +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).

          scheduler (idle

> +
> +When the highest priorty task is selected to run, a reschedule-IPI is sent to

                    priority

> +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

  drop one space ^^

> +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, an IPI is sent to
> +siblings on kernel entry (syscall and IRQ). This IPI forces the sibling to enter
> +kernel mode and wait before returning to user until all siblings of the
> +core have left kernel mode. This process is also known as stunning.  For good
> +performance, an IPI is sent only to a sibling only if it is running a tagged
> +task. If a sibling is running a kernel thread or is idle, no IPI is sent.
> +
> +The kernel protection feature can be turned off on the kernel command line by
> +passing ``sched_core_protect_kernel=0``.
> +
> +Other alternative ideas discussed for kernel protection are listed below just
> +for completeness. They all have limitations:
> +
> +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

                     Also,

> +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 ``CONFIG_PREEMPT_RT``.
> +
> +Yet another issue with this is, for multiqueue devices with managed
> +interrupts, the IRQ affinities cannot be changed however it could be

                                            changed; however,

> +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

   guaranteed                                                     limitations in


> +implementing this - the main concern being how to decide on an address space
> +that is guarenteed to not have any sensitive data.

           guaranteed

> +
> +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 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
> +-----------
> +Core scheduling tries to guarentee that only trusted tasks run concurrently on a

                            guarantee

> +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

                                                CPU

> +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

                                            architectural

> +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

                                          an           attacker


> +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.

huh?  what is EPT?  where is it documented/discussed?

> +
> +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

                                                                           vCPUs

> +  a VM.
> +
> +Future work
> +-----------
> +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 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

Might be an indentation problem there?  Can't tell for sure.


thanks.