On 8/24/22 14:28, Peter Gonda wrote:

On Wed, Aug 24, 2022 at 12:01 PM Dionna Amalie Glaze
[off-list ref] wrote:

quoted

Apologies for the necropost, but I noticed strange behavior testing my
own Golang-based wrapper around the /dev/sev-guest driver.

quoted

+
+static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, int msg_ver,
+                               u8 type, void *req_buf, size_t req_sz, void *resp_buf,
+                               u32 resp_sz, __u64 *fw_err)
+{
+       unsigned long err;
+       u64 seqno;
+       int rc;
+
+       /* Get message sequence and verify that its a non-zero */
+       seqno = snp_get_msg_seqno(snp_dev);
+       if (!seqno)
+               return -EIO;
+
+       memset(snp_dev->response, 0, sizeof(struct snp_guest_msg));
+
+       /* Encrypt the userspace provided payload */
+       rc = enc_payload(snp_dev, seqno, msg_ver, type, req_buf, req_sz);
+       if (rc)
+               return rc;
+
+       /* Call firmware to process the request */
+       rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
+       if (fw_err)
+               *fw_err = err;
+
+       if (rc)
+               return rc;
+

The fw_err is written back regardless of rc, so since err is
uninitialized, you can end up with garbage written back. I've worked
around this by only caring about fw_err when the result is -EIO, but
thought that I should bring this up.

I also noticed that we use a u64 in snp_guest_request_ioctl.fw_err and
u32 in sev_issue_cmd.error when these should be errors from the
sev_ret_code enum IIUC.

The reason for the u64 is that the Extended Guest Request can return a 
firmware error or a hypervisor error. To distinguish between the two, a 
firmware error is contained in the lower 32-bits, while a hypervisor error 
is contained in the upper 32-bits (e.g. when not enough contiguous pages 
of memory have been supplied).

Thanks,
Tom

We can fix snp_issue_guest_request() to set |fw_err| to zero when it
returns 0 but what should we return to userspace if we encounter an
error that prevents the FW from even being called? In ` crypto: ccp -
Ensure psp_ret is always init'd in __sev_platform_init_locked()` we
set the return to -1 so we could continue that convection here and
better codify it in the sev_ret_code enum.

quoted

--
-Dionna Glaze, PhD (she/her)