s390/uvdevice: Report additional-data length for attestation

Additional data length in the attestation request is an in/out variable.
Software provides the capacity of the buffer. Upon successful request,
firmware reports the actual bytes written to the additional data in that
field. This information is lost, as the length field was not copied back
to userspace before. Attestation might fail, if user space did not
specify the exact amount of needed bytes required, as this length is
part of the attestation measurement.

Signed-off-by: Steffen Eiden <[e-mail]>
Reviewed-by: Claudio Imbrenda <[e-mail]>
Reviewed-by: Janosch Frank <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Message-Id: <[e-mail]>
Signed-off-by: Janosch Frank <[e-mail]>

drivers/s390/char/uvdevice.c

···
 					   struct uvio_attest *uvio_attest)
 {
 	struct uvio_attest __user *user_uvio_attest = (void __user *)uv_ioctl->argument_addr;
+	u32 __user *user_buf_add_len = (u32 __user *)&user_uvio_attest->add_data_len;
 	void __user *user_buf_add = (void __user *)uvio_attest->add_data_addr;
 	void __user *user_buf_meas = (void __user *)uvio_attest->meas_addr;
 	void __user *user_buf_uid = &user_uvio_attest->config_uid;
···
 	if (copy_to_user(user_buf_meas, measurement, uvio_attest->meas_len))
 		return -EFAULT;
 	if (add_data && copy_to_user(user_buf_add, add_data, uvio_attest->add_data_len))
+		return -EFAULT;
+	if (put_user(uvio_attest->add_data_len, user_buf_add_len))
 		return -EFAULT;
 	if (copy_to_user(user_buf_uid, uvcb_attest->config_uid, sizeof(uvcb_attest->config_uid)))
 		return -EFAULT;