On Fri, Aug 26, 2022 at 1:15 AM Eric Biggers <ebiggers@xxxxxxxxxx> 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
