Re: [PATCH Part1 v5 37/38] virt: sevguest: Add support to derive key

On 01/09/2021 0:04, Brijesh Singh wrote:

Hi Dov,


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

quoted

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

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 ?

Yes, that's the concern, specifically with sensitive buffers (keys).
You don't want many copies floating around in unused memory.

-Dov