virt/sev-guest: Return -EIO if certificate buffer is not large enough

Commit

  47894e0f ("virt/sev-guest: Prevent IV reuse in the SNP guest driver")

changed the behavior associated with the return value when the caller
does not supply a large enough certificate buffer. Prior to the commit a
value of -EIO was returned. Now, 0 is returned.  This breaks the
established ABI with the user.

Change the code to detect the buffer size error and return -EIO.

Fixes: 47894e0f ("virt/sev-guest: Prevent IV reuse in the SNP guest driver")
Reported-by: Larry Dewey <larry.dewey@amd.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Tested-by: Larry Dewey <larry.dewey@amd.com>
Cc: <stable@kernel.org>
Link: https://lore.kernel.org/r/2afbcae6daf13f7ad5a4296692e0a0fe1bc1e4ee.1677083979.git.thomas.lendacky@amd.com

drivers/virt/coco/sev-guest/sev-guest.c

@@ -377,9 +377,26 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
 		snp_dev->input.data_npages = certs_npages;
 	}
 
+	/*
+	 * Increment the message sequence number. There is no harm in doing
+	 * this now because decryption uses the value stored in the response
+	 * structure and any failure will wipe the VMPCK, preventing further
+	 * use anyway.
+	 */
+	snp_inc_msg_seqno(snp_dev);
+
 	if (fw_err)
 		*fw_err = err;
 
+	/*
+	 * If an extended guest request was issued and the supplied certificate
+	 * buffer was not large enough, a standard guest request was issued to
+	 * prevent IV reuse. If the standard request was successful, return -EIO
+	 * back to the caller as would have originally been returned.
+	 */
+	if (!rc && err == SNP_GUEST_REQ_INVALID_LEN)
+		return -EIO;
+
 	if (rc) {
 		dev_alert(snp_dev->dev,
 			  "Detected error from ASP request. rc: %d, fw_err: %llu\n",
@@ -395,9 +412,6 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
 		goto disable_vmpck;
 	}
 
-	/* Increment to new message sequence after payload decryption was successful. */
-	snp_inc_msg_seqno(snp_dev);
-
 	return 0;
 
 disable_vmpck: