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Comment by ENOTTY | Hacker News Reader

None of the Cortex M cores seem to be affected, according to ARM. These are the ones mostly used in embedded applications.

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Cortex M and all ARM-developed cores prior to v7 are in-order CPUs, so they do not do branch prediction or speculative execution. Without branch prediction this kind of issue cannot exist.

In-order CPUs (e.g. Pentium 1, older Atom and ARM chips, POWER 6) perform branch prediction and speculative execution. They'll predict the branch and start speculatively decoding and executing instructions from the predicted target, then flush the pipeline if there was a misprediction. What they won't do is execute past an instruction that has an unresolved data dependency.

You're right, I was conflating two things. It looks like no in-order ARM cores have branch prediction though.

‘M’ just stands for microcontroller, as apposed to ‘A’ for application.

Many / most complex or multitasking embedded devices will use ‘A’ series to meet their processing requirements, e.g. network routing + OS + firmware / OS programming and multiuser processsing.

I probably should have better qualified my use of the term ‘embedded’.

But it does affect the processors used on the Beaglebone and older Raspberry Pi devices.

It does not affect Raspberry Pi devices to my knowledge. Please provide source for your claims!

The CPUs in Raspberry Pi 1-3 are not affected.

  ARM11, Cortex-A7, Cortex-A5

Raspberry Pi 2 v1 use a Broadcom BCM2836 SoC with a 900 MHz 32-bit quad-core ARM Cortex-A7 processor.

Raspberry Pi 3 (and Pi 2 v1.2) uses a Broadcom BCM2837 SoC with a 1.2 GHz 64-bit quad-core ARM Cortex-A53 processor.

According to ARM website https://developer.arm.com/support/security-update it especially says

  "*Only affected cores are listed, all other Arm cores are NOT affected.*"

and it lists only

  "Cortex-R7, Cortex-R8, Cortex-A8, Cortex-A9, Cortex-A15, 
  Cortex-A17, Cortex-A57, Cortex-A72, Cortex-A73, Cortex-A75"

    and older Raspberry Pi devices

https://www.raspberrypi.org/products/raspberry-pi-2-model-b/

But it looks like I confused the Cortex-A7 with the Cortex-R7, which is not listed.

Cortex M cores don't have MMUs, so they cannot be affected. Also in embedded applications you would only be running trusted code anyway

The MMU isn't part of this attack, and there are ARM cores with MMUs that are not affected. Cortex M cores are in-order CPUs, so they do not have branch prediction or perform speculative execution which the attacks rely on.

The attack is all about bypassing memory protection, which without an MMU to provide any, is moot --- code has access to all of memory to read normally anyway.

Cortex-M has an MPU, not MMU to provide protection for memory regions.

But as said above, the MPU/MMU has no effect on this bug, this is ab out speculative execution and determining information from a cache side channel attack.

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