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

[Tom Lendacky]

Commit 47894e0fa6a5 ("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 return value of -EIO was returned. Now a return value of 0 is
returned. This breaks the established ABI with the user.

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

Fixes: 47894e0fa6a5 ("virt/sev-guest: Prevent IV reuse in the SNP guest driver")
Reported-by: Larry Dewey <larry.dewey@xxxxxxx>
Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
---
 drivers/virt/coco/sev-guest/sev-guest.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
index 4ec4174e05a3..7b4e9009f335 100644
--- a/drivers/virt/coco/sev-guest/sev-guest.c
+++ b/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:
-- 
2.39.1