Hi Dov,


On 8/31/21 1:59 PM, Dov Murik wrote:

quoted

+
+	/*
+	 * The intermediate response buffer is used while decrypting the
+	 * response payload. Make sure that it has enough space to cover the
+	 * authtag.
+	 */
+	resp_len = sizeof(resp->data) + crypto->a_len;
+	resp = kzalloc(resp_len, GFP_KERNEL_ACCOUNT);

The length of resp->data is 64 bytes; I assume crypto->a_len is not a
lot more (and probably known in advance for AES GCM).  Maybe use a
buffer on the stack instead of allocating and freeing?

The authtag size can be up to 16 bytes, so I guess I can allocate 80 
bytes on stack and avoid the kzalloc().

quoted

+	if (!resp)
+		return -ENOMEM;
+
+	/* Issue the command to get the attestation report */
+	rc = handle_guest_request(snp_dev, req.msg_version, SNP_MSG_KEY_REQ,
+				  &req.data, sizeof(req.data), resp->data, resp_len,
+				  &arg->fw_err);
+	if (rc)
+		goto e_free;
+
+	/* Copy the response payload to userspace */
+	if (copy_to_user((void __user *)arg->resp_data, resp, sizeof(*resp)))
+		rc = -EFAULT;
+
+e_free:
+	kfree(resp);

Since resp contains key material, I think you should explicit_memzero()
it before freeing, so the key bytes don't linger around in unused
memory.  I'm not sure if any copies are made inside the
handle_guest_request call above; maybe zero these as well.

I can do that, but I guess I am trying to find a reason for it. The resp 
buffer is encrypted page, so, the key is protected from the hypervisor 
access. Are you thinking about an attack within the VM guest OS ?

-Brijesh