arm64: KVM: Tighten guest core register access from userspace

arm64: KVM: Tighten guest core register access from userspace

We currently allow userspace to access the core register file
in about any possible way, including straddling multiple
registers and doing unaligned accesses.

This is not the expected use of the ABI, and nobody is actually
using it that way. Let's tighten it by explicitly checking
the size and alignment for each field of the register file.

Cc: <[email protected]>
Fixes: 2f4a07c5f9fe ("arm64: KVM: guest one-reg interface")
Reviewed-by: Christoffer Dall <[email protected]>
Reviewed-by: Mark Rutland <[email protected]>
Signed-off-by: Dave Martin <[email protected]>
[maz: rewrote Dave's initial patch to be more easily backported]
Signed-off-by: Marc Zyngier <[email protected]>
Signed-off-by: Will Deacon <[email protected]>