Should Linux set the new constant-time mode CPU flags?

[Arnd Bergmann]

On Fri, Aug 26, 2022 at 1:15 AM Eric Biggers <ebiggers at kernel.org> wrote:
>
> For arm64, it's not clear to me whether the DIT flag is privileged or not.  If
> privileged, I expect it would need to be set by the kernel just like the Intel
> flag.  If unprivileged, I expect there will still be work to do in the kernel,
> as the flag will need to be set when running any crypto code in the kernel.

7206dc93a58f ("arm64: Expose Arm v8.4 features") added the feature bit for
Armv8.4+ processors. From what I can tell from the documentation and the
kernel source, I see:

- if the feature is set in HWCAP (or /proc/cpuinfo), then the instruction DIT
  register is available in user space, and sensitive code can set or clear the
  constant-time mode for the local thread.
- On CPUs without the feature (almost all ARMv8 ones), the register should
  not be touched.
- The bit is context switched on kernel entry, so setting the bit in user space
  does not change the behavior inside of a syscall
- If we add a user space interface for setting the bit per thread on x86,
  the same interface could be supported to set the bit on arm64 to save
  user space implementations the trouble of checking the feature bits
- the in-kernel crypto code does not set the bit today but could be easily
  changed to do this for CPUs that support it, if we can decide on a policy
  for when to enable or disable it.

        Arnd