Indirect Branch Predictor Barrier

Published: 01/03/2018

The indirect branch predictor barrier (IBPB) is an indirect branch control mechanism that establishes a barrier, preventing software that executed before the barrier from controlling the predicted targets of indirect branches executed after the barrier on the same logical processor. A processor supports IBPB if it enumerates CPUID.(EAX=7H,ECX=0):EDX[26] as 1. IBPB can be used to help mitigate Branch Target Injection. 

IBPB does not define a new mode of processor operation that controls the branch predictors, unlike Indirect Branch Restricted Speculation (IBRS) and Single Thread Indirect Branch Predictors (STIBP). As a result, it is not enabled by setting a bit in the IA32_SPEC_CTRL MSR. Instead, IBPB is a “command” that software executes when necessary.

Software executes an IBPB command by writing the IA32_PRED_CMD MSR to set bit 0 (IBPB). This can be done either using the WRMSRM instruction or as part of a VMX transition that loads the MSR from an MSR-load area. Software that executed before the IBPB command cannot control the predicted targets of indirect branches executed after the command on the same logical processor. The IA32_PRED_CMD MSR is write-only, and it is not necessary to clear the IBPB bit before writing it with a value of 1.

IBPB can be used in conjunction with IBRS to account for cases that IBRS does not cover:

  • As noted in the “Indirect Branch Restricted Speculation (IBRS)” description, IBRS does not prevent software from controlling the predicted target of an indirect branch of unrelated software (for example, a different user application or a different virtual machine) executed at the same predictor mode. Software can prevent such control by executing an IBPB command when changing the identity of software operating at a particular predictor mode (for example, when changing user applications or virtual machines).
  • Software may choose to clear IA32_SPEC_CTRL.IBRS in certain situations (like for execution with CPL = 3 in VMX root operation). In such cases, software can use an IBPB command on certain transitions (for example, after running an untrusted virtual machine) to prevent software that executed earlier from controlling the predicted targets of indirect branches executed subsequently with IBRS disabled.

Note References in this document to indirect branches are only to near call indirect, near jump indirect, and near return instructions. Refer to Speculative Execution Side Channel Mitigations for further details. 

 

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