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: <stable@vger.kernel.org>
Fixes: 2f4a07c5f9fe ("arm64: KVM: guest one-reg interface")
Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
[maz: rewrote Dave's initial patch to be more easily backported]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
					struct kvm_guest_debug *dbg)
{
	int ret = 0;

	trace_kvm_set_guest_debug(vcpu, dbg->control);

	if (dbg->control & ~KVM_GUESTDBG_VALID_MASK) {
		ret = -EINVAL;
		goto out;
	}

	if (dbg->control & KVM_GUESTDBG_ENABLE) {
		vcpu->guest_debug = dbg->control;

		/* Hardware assisted Break and Watch points */
		if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW) {
			vcpu->arch.external_debug_state = dbg->arch;
		}

	} else {
		/* If not enabled clear all flags */
		vcpu->guest_debug = 0;
	}

out:
	return ret;
}

int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
					struct kvm_guest_debug *dbg)
{
	int ret = 0;

	trace_kvm_set_guest_debug(vcpu, dbg->control);

	if (dbg->control & ~KVM_GUESTDBG_VALID_MASK) {
		ret = -EINVAL;
		goto out;
	}

	if (dbg->control & KVM_GUESTDBG_ENABLE) {
		vcpu->guest_debug = dbg->control;

		/* Hardware assisted Break and Watch points */
		if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW) {
			vcpu->arch.external_debug_state = dbg->arch;
		}

	} else {
		/* If not enabled clear all flags */
		vcpu->guest_debug = 0;
	}

out:
	return ret;
}
