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Add a new function, WithDataIndependentTiming, which takes a function as an argument, and encloses it with calls to set/unset the DIT PSTATE bit on Arm64. Since DIT is OS thread-local, for the duration of the execution of WithDataIndependentTiming, we lock the goroutine to the OS thread, using LockOSThread. For long running operations, this is likely to not be performant, but we expect this to be tightly scoped around cryptographic operations that have bounded execution times. If locking to the OS thread turns out to be too slow, another option is to add a bit to the g state indicating if a goroutine has DIT enabled, and then have the scheduler enable/disable DIT when scheduling a g. Additionally, we add a new GODEBUG, dataindependenttiming, which allows setting DIT for an entire program. Running a program with dataindependenttiming=1 enables DIT for the program during initialization. In an ideal world PSTATE.DIT would be inherited from the parent thread, so we'd only need to set it in the main thread and then all subsequent threads would inherit the value. While this does happen in the Linux kernel [0], it is not the case for darwin [1]. Rather than add complex logic to only set it on darwin for each new thread, we just unconditionally set it in mstart1 and cgocallbackg1 regardless of the OS. DIT will already impose some overhead, and the cost of setting the bit is only ~two instructions (CALL, MSR), so it should be cheap enough. Fixes #66450 Updates #49702 [0] |
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