Adding upstream version 5.10.209.upstream/5.10.209upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+Microarchitectural Data Sampling (MDS) mitigation
+=================================================
+
+.. _mds:
+
+Overview
+--------
+
+Microarchitectural Data Sampling (MDS) is a family of side channel attacks
+on internal buffers in Intel CPUs. The variants are:
+
+ - Microarchitectural Store Buffer Data Sampling (MSBDS) (CVE-2018-12126)
+ - Microarchitectural Fill Buffer Data Sampling (MFBDS) (CVE-2018-12130)
+ - Microarchitectural Load Port Data Sampling (MLPDS) (CVE-2018-12127)
+ - Microarchitectural Data Sampling Uncacheable Memory (MDSUM) (CVE-2019-11091)
+
+MSBDS leaks Store Buffer Entries which can be speculatively forwarded to a
+dependent load (store-to-load forwarding) as an optimization. The forward
+can also happen to a faulting or assisting load operation for a different
+memory address, which can be exploited under certain conditions. Store
+buffers are partitioned between Hyper-Threads so cross thread forwarding is
+not possible. But if a thread enters or exits a sleep state the store
+buffer is repartitioned which can expose data from one thread to the other.
+
+MFBDS leaks Fill Buffer Entries. Fill buffers are used internally to manage
+L1 miss situations and to hold data which is returned or sent in response
+to a memory or I/O operation. Fill buffers can forward data to a load
+operation and also write data to the cache. When the fill buffer is
+deallocated it can retain the stale data of the preceding operations which
+can then be forwarded to a faulting or assisting load operation, which can
+be exploited under certain conditions. Fill buffers are shared between
+Hyper-Threads so cross thread leakage is possible.
+
+MLPDS leaks Load Port Data. Load ports are used to perform load operations
+from memory or I/O. The received data is then forwarded to the register
+file or a subsequent operation. In some implementations the Load Port can
+contain stale data from a previous operation which can be forwarded to
+faulting or assisting loads under certain conditions, which again can be
+exploited eventually. Load ports are shared between Hyper-Threads so cross
+thread leakage is possible.
+
+MDSUM is a special case of MSBDS, MFBDS and MLPDS. An uncacheable load from
+memory that takes a fault or assist can leave data in a microarchitectural
+structure that may later be observed using one of the same methods used by
+MSBDS, MFBDS or MLPDS.
+
+Exposure assumptions
+--------------------
+
+It is assumed that attack code resides in user space or in a guest with one
+exception. The rationale behind this assumption is that the code construct
+needed for exploiting MDS requires:
+
+ - to control the load to trigger a fault or assist
+
+ - to have a disclosure gadget which exposes the speculatively accessed
+   data for consumption through a side channel.
+
+ - to control the pointer through which the disclosure gadget exposes the
+   data
+
+The existence of such a construct in the kernel cannot be excluded with
+100% certainty, but the complexity involved makes it extremly unlikely.
+
+There is one exception, which is untrusted BPF. The functionality of
+untrusted BPF is limited, but it needs to be thoroughly investigated
+whether it can be used to create such a construct.
+
+
+Mitigation strategy
+-------------------
+
+All variants have the same mitigation strategy at least for the single CPU
+thread case (SMT off): Force the CPU to clear the affected buffers.
+
+This is achieved by using the otherwise unused and obsolete VERW
+instruction in combination with a microcode update. The microcode clears
+the affected CPU buffers when the VERW instruction is executed.
+
+For virtualization there are two ways to achieve CPU buffer
+clearing. Either the modified VERW instruction or via the L1D Flush
+command. The latter is issued when L1TF mitigation is enabled so the extra
+VERW can be avoided. If the CPU is not affected by L1TF then VERW needs to
+be issued.
+
+If the VERW instruction with the supplied segment selector argument is
+executed on a CPU without the microcode update there is no side effect
+other than a small number of pointlessly wasted CPU cycles.
+
+This does not protect against cross Hyper-Thread attacks except for MSBDS
+which is only exploitable cross Hyper-thread when one of the Hyper-Threads
+enters a C-state.
+
+The kernel provides a function to invoke the buffer clearing:
+
+    mds_clear_cpu_buffers()
+
+The mitigation is invoked on kernel/userspace, hypervisor/guest and C-state
+(idle) transitions.
+
+As a special quirk to address virtualization scenarios where the host has
+the microcode updated, but the hypervisor does not (yet) expose the
+MD_CLEAR CPUID bit to guests, the kernel issues the VERW instruction in the
+hope that it might actually clear the buffers. The state is reflected
+accordingly.
+
+According to current knowledge additional mitigations inside the kernel
+itself are not required because the necessary gadgets to expose the leaked
+data cannot be controlled in a way which allows exploitation from malicious
+user space or VM guests.
+
+Kernel internal mitigation modes
+--------------------------------
+
+ ======= ============================================================
+ off      Mitigation is disabled. Either the CPU is not affected or
+          mds=off is supplied on the kernel command line
+
+ full     Mitigation is enabled. CPU is affected and MD_CLEAR is
+          advertised in CPUID.
+
+ vmwerv	  Mitigation is enabled. CPU is affected and MD_CLEAR is not
+	  advertised in CPUID. That is mainly for virtualization
+	  scenarios where the host has the updated microcode but the
+	  hypervisor does not expose MD_CLEAR in CPUID. It's a best
+	  effort approach without guarantee.
+ ======= ============================================================
+
+If the CPU is affected and mds=off is not supplied on the kernel command
+line then the kernel selects the appropriate mitigation mode depending on
+the availability of the MD_CLEAR CPUID bit.
+
+Mitigation points
+-----------------
+
+1. Return to user space
+^^^^^^^^^^^^^^^^^^^^^^^
+
+   When transitioning from kernel to user space the CPU buffers are flushed
+   on affected CPUs when the mitigation is not disabled on the kernel
+   command line. The migitation is enabled through the static key
+   mds_user_clear.
+
+   The mitigation is invoked in prepare_exit_to_usermode() which covers
+   all but one of the kernel to user space transitions.  The exception
+   is when we return from a Non Maskable Interrupt (NMI), which is
+   handled directly in do_nmi().
+
+   (The reason that NMI is special is that prepare_exit_to_usermode() can
+    enable IRQs.  In NMI context, NMIs are blocked, and we don't want to
+    enable IRQs with NMIs blocked.)
+
+
+2. C-State transition
+^^^^^^^^^^^^^^^^^^^^^
+
+   When a CPU goes idle and enters a C-State the CPU buffers need to be
+   cleared on affected CPUs when SMT is active. This addresses the
+   repartitioning of the store buffer when one of the Hyper-Threads enters
+   a C-State.
+
+   When SMT is inactive, i.e. either the CPU does not support it or all
+   sibling threads are offline CPU buffer clearing is not required.
+
+   The idle clearing is enabled on CPUs which are only affected by MSBDS
+   and not by any other MDS variant. The other MDS variants cannot be
+   protected against cross Hyper-Thread attacks because the Fill Buffer and
+   the Load Ports are shared. So on CPUs affected by other variants, the
+   idle clearing would be a window dressing exercise and is therefore not
+   activated.
+
+   The invocation is controlled by the static key mds_idle_clear which is
+   switched depending on the chosen mitigation mode and the SMT state of
+   the system.
+
+   The buffer clear is only invoked before entering the C-State to prevent
+   that stale data from the idling CPU from spilling to the Hyper-Thread
+   sibling after the store buffer got repartitioned and all entries are
+   available to the non idle sibling.
+
+   When coming out of idle the store buffer is partitioned again so each
+   sibling has half of it available. The back from idle CPU could be then
+   speculatively exposed to contents of the sibling. The buffers are
+   flushed either on exit to user space or on VMENTER so malicious code
+   in user space or the guest cannot speculatively access them.
+
+   The mitigation is hooked into all variants of halt()/mwait(), but does
+   not cover the legacy ACPI IO-Port mechanism because the ACPI idle driver
+   has been superseded by the intel_idle driver around 2010 and is
+   preferred on all affected CPUs which are expected to gain the MD_CLEAR
+   functionality in microcode. Aside of that the IO-Port mechanism is a
+   legacy interface which is only used on older systems which are either
+   not affected or do not receive microcode updates anymore.