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diff --git a/Documentation/x86/mds.rst b/Documentation/x86/mds.rst new file mode 100644 index 000000000..5d4330be2 --- /dev/null +++ b/Documentation/x86/mds.rst @@ -0,0 +1,193 @@ +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. |