The indirect branch predictor barrier is less of a barrier than hoped
Six years after the Spectre transient execution processor design flaws were disclosed, efforts to patch the problem continue to fall short.and leaks the hash of the root password from the Set User ID process on recent Intel processors. The researchers claim they successfully conducted such an attack.refers to a set of attacks made possible because of the way processors conduct speculative execution - a performance optimization technique that involves making calculations in advance.
Branch prediction is a form of speculative execution, and modern processors use it to make guesses about the path a program will take. It's related to branch target prediction, which attempts to predict the target address of the next instruction to be executed in a given branch. Spectre attacks try to make the branch predictor forward an incorrect prediction – such that when the processor executes the associated instructions, it accesses out-of-bounds memory that contains secrets like passwords or encryption keys. Subsequent operations on the memory area storing secrets may allow the attacker to infer those secrets by observing side-channels – such as CPU cache accesses and power fluctuations.
The indirect branch predictor barrier was intended as a defense against Spectre v2 attacks on x86 Intel and AMD chips. IBPB is designed to prevent forwarding of previously learned indirect branch target predictions for speculative execution.
In their technical summary, Wikner and Razavi observe:"This microcode update was, however, not available in Ubuntu repositories at the time of writing this paper.""Because AMD’s issue was previously known and tracked under AMD-SB-1040, AMD considers the issue a software bug," the researchers explain."We are currently working with the Linux kernel maintainers to merge our proposed software patch.
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