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diff --git a/Documentation/admin-guide/hw-vuln/cross-thread-rsb.rst b/Documentation/admin-guide/hw-vuln/cross-thread-rsb.rst new file mode 100644 index 000000000000..875616d675fe --- /dev/null +++ b/Documentation/admin-guide/hw-vuln/cross-thread-rsb.rst @@ -0,0 +1,91 @@ + +.. SPDX-License-Identifier: GPL-2.0 + +Cross-Thread Return Address Predictions +======================================= + +Certain AMD and Hygon processors are subject to a cross-thread return address +predictions vulnerability. When running in SMT mode and one sibling thread +transitions out of C0 state, the other sibling thread could use return target +predictions from the sibling thread that transitioned out of C0. + +The Spectre v2 mitigations protect the Linux kernel, as it fills the return +address prediction entries with safe targets when context switching to the idle +thread. However, KVM does allow a VMM to prevent exiting guest mode when +transitioning out of C0. This could result in a guest-controlled return target +being consumed by the sibling thread. + +Affected processors +------------------- + +The following CPUs are vulnerable: + + - AMD Family 17h processors + - Hygon Family 18h processors + +Related CVEs +------------ + +The following CVE entry is related to this issue: + + ============== ======================================= + CVE-2022-27672 Cross-Thread Return Address Predictions + ============== ======================================= + +Problem +------- + +Affected SMT-capable processors support 1T and 2T modes of execution when SMT +is enabled. In 2T mode, both threads in a core are executing code. For the +processor core to enter 1T mode, it is required that one of the threads +requests to transition out of the C0 state. This can be communicated with the +HLT instruction or with an MWAIT instruction that requests non-C0. +When the thread re-enters the C0 state, the processor transitions back +to 2T mode, assuming the other thread is also still in C0 state. + +In affected processors, the return address predictor (RAP) is partitioned +depending on the SMT mode. For instance, in 2T mode each thread uses a private +16-entry RAP, but in 1T mode, the active thread uses a 32-entry RAP. Upon +transition between 1T/2T mode, the RAP contents are not modified but the RAP +pointers (which control the next return target to use for predictions) may +change. This behavior may result in return targets from one SMT thread being +used by RET predictions in the sibling thread following a 1T/2T switch. In +particular, a RET instruction executed immediately after a transition to 1T may +use a return target from the thread that just became idle. In theory, this +could lead to information disclosure if the return targets used do not come +from trustworthy code. + +Attack scenarios +---------------- + +An attack can be mounted on affected processors by performing a series of CALL +instructions with targeted return locations and then transitioning out of C0 +state. + +Mitigation mechanism +-------------------- + +Before entering idle state, the kernel context switches to the idle thread. The +context switch fills the RAP entries (referred to as the RSB in Linux) with safe +targets by performing a sequence of CALL instructions. + +Prevent a guest VM from directly putting the processor into an idle state by +intercepting HLT and MWAIT instructions. + +Both mitigations are required to fully address this issue. + +Mitigation control on the kernel command line +--------------------------------------------- + +Use existing Spectre v2 mitigations that will fill the RSB on context switch. + +Mitigation control for KVM - module parameter +--------------------------------------------- + +By default, the KVM hypervisor mitigates this issue by intercepting guest +attempts to transition out of C0. A VMM can use the KVM_CAP_X86_DISABLE_EXITS +capability to override those interceptions, but since this is not common, the +mitigation that covers this path is not enabled by default. + +The mitigation for the KVM_CAP_X86_DISABLE_EXITS capability can be turned on +using the boolean module parameter mitigate_smt_rsb, e.g. ``kvm.mitigate_smt_rsb=1``. |