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Comment by blattimwind | Hacker News Reader

> The lack of speculation in the ARM1176, Cortex-A7, and Cortex-A53 cores used in Raspberry Pi render us immune to attacks of the sort.

I didn't check, but these will almost certainly have branch prediction. What they probably lack is a predictor advanced enough to speculate on indirect branches, which AIUI is the primary vector of Spectre.

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Branch prediction alone is insufficient. Speculative execution alone is insufficient. You need speculative memory loads for any of these attacks to work.

The Cortex-A53 branch predictor [1] does prefetching to keep the core fed. This ensures that the instructions are ready for decoding, but has no architectural effects beyond the L1 instruction cache, which is already a well-studied timing sidechannel.

[1]: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc....

What about the fact that these instructions might get partially executed in the pipeline before the branch gets resolved and the pipeline flushed? If a mis-fetched instruction can reach the LSU stage before the pipeline gets flushed, it might serve as a speculative memory load...

They're not partially executed. The branch predictor only fetches instructions. They might be decoded, but it's not an out-of-order processor-- pipeline stages only proceed if the previous phase is correct.

Here's the Cortex-A53 pipeline: https://www.anandtech.com/show/11441/dynamiq-and-arms-new-cp...

It's an in-order CPU, so that "issue" phase (pipeline step 5) stalls until the instruction pointer is resolved. Instructions must be issued to the "AGU Load" functional unit, which is what actually performs the read and pulls data into the cache hierarchy.

Note also that a single speculative memory load is insufficient for Spectre. You need two speculative memory loads.

ARM Ltd has a list of vulnerable cores [1], the above are not listed.

[1] https://developer.arm.com/support/security-update

I didn't imply they are vulnerable.

Eben delves into some unaffected speculative ARM features in the comments, and why they're not vulnerable. His responses in the comments in general are also worth a read, in case folks skip comments as a rule.

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