[PATCH V5] virt: sev: Prevent IV reuse in SNP guest driver

[Peter Gonda]

The AMD Secure Processor (ASP) and an SNP guest use a series of
AES-GCM keys called VMPCKs to communicate securely with each other.
The IV to this scheme is a sequence number that both the ASP and the
guest track. Currently this sequence number in a guest request must
exactly match the sequence number tracked by the ASP. This means that
if the guest sees an error from the host during a request it can only
retry that exact request or disable the VMPCK to prevent an IV reuse.
AES-GCM cannot tolerate IV reuse see: "Authentication Failures in NIST
version of GCM" - Antoine Joux et al.

In order to address this make handle_guest_request() delete the VMPCK
on any non successful return. To allow userspace querying the cert_data
length make handle_guest_request() safe the number of pages required by
the host, then handle_guest_request() retry the request without
requesting the extended data, then return the number of pages required
back to userspace.

Fixes: fce96cf044308 ("virt: Add SEV-SNP guest driver")
Signed-off-by: Peter Gonda <pgonda@xxxxxxxxxx>
Reported-by: Peter Gonda <pgonda@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Michael Roth <michael.roth@xxxxxxx>
Cc: Haowen Bai <baihaowen@xxxxxxxxx>
Cc: Yang Yingliang <yangyingliang@xxxxxxxxxx>
Cc: Marc Orr <marcorr@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Dionna Glaze <dionnaglaze@xxxxxxxxxx>
Cc: Ashish Kalra <Ashish.Kalra@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: kvm@xxxxxxxxxxxxxxx
---
 drivers/virt/coco/sev-guest/sev-guest.c | 83 ++++++++++++++++++++-----
 1 file changed, 69 insertions(+), 14 deletions(-)

diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
index f422f9c58ba79..64b4234c14da8 100644
--- a/drivers/virt/coco/sev-guest/sev-guest.c
+++ b/drivers/virt/coco/sev-guest/sev-guest.c
@@ -67,8 +67,27 @@ static bool is_vmpck_empty(struct snp_guest_dev *snp_dev)
 	return true;
 }
 
+/*
+ * If an error is received from the host or AMD Secure Processor (ASP) there
+ * are two options. Either retry the exact same encrypted request or discontinue
+ * using the VMPCK.
+ *
+ * This is because in the current encryption scheme GHCB v2 uses AES-GCM to
+ * encrypt the requests. The IV for this scheme is the sequence number. GCM
+ * cannot tolerate IV reuse.
+ *
+ * The ASP FW v1.51 only increments the sequence numbers on a successful
+ * guest<->ASP back and forth and only accepts messages at its exact sequence
+ * number.
+ *
+ * So if the sequence number were to be reused the encryption scheme is
+ * vulnerable. If the sequence number were incremented for a fresh IV the ASP
+ * will reject the request.
+ */
 static void snp_disable_vmpck(struct snp_guest_dev *snp_dev)
 {
+	dev_alert(snp_dev->dev, "Disabling vmpck_id: %d to prevent IV reuse.\n",
+		  vmpck_id);
 	memzero_explicit(snp_dev->vmpck, VMPCK_KEY_LEN);
 	snp_dev->vmpck = NULL;
 }
@@ -321,34 +340,70 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
 	if (rc)
 		return rc;
 
-	/* Call firmware to process the request */
+	/*
+	 * Call firmware to process the request. In this function the encrypted
+	 * message enters shared memory with the host. So after this call the
+	 * sequence number must be incremented or the VMPCK must be deleted to
+	 * prevent reuse of the IV.
+	 */
 	rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
+
+	/*
+	 * If the extended guest request fails due to having too small of a
+	 * certificate data buffer retry the same guest request without the
+	 * extended data request in order to not have to reuse the IV.
+	 */
+	if (exit_code == SVM_VMGEXIT_EXT_GUEST_REQUEST &&
+	    err == SNP_GUEST_REQ_INVALID_LEN) {
+		const unsigned int certs_npages = snp_dev->input.data_npages;
+
+		exit_code = SVM_VMGEXIT_GUEST_REQUEST;
+
+		/*
+		 * If this call to the firmware succeeds the sequence number can
+		 * be incremented allowing for continued use of the VMPCK. If
+		 * there is an error reflected in the return value, this value
+		 * is checked further down and the result will be the deletion
+		 * of the VMPCK and the error code being propagated back to the
+		 * user as an IOCLT return code.
+		 */
+		rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
+
+		/*
+		 * Override the error to inform callers the given extended
+		 * request buffer size was too small and give the caller the
+		 * required buffer size.
+		 */
+		err = SNP_GUEST_REQ_INVALID_LEN;
+		snp_dev->input.data_npages = certs_npages;
+	}
+
 	if (fw_err)
 		*fw_err = err;
 
-	if (rc)
-		return rc;
+	if (rc) {
+		dev_alert(snp_dev->dev,
+			  "Detected error from ASP request. rc: %d, fw_err: %llu\n",
+			  rc, *fw_err);
+		goto disable_vmpck;
+	}
 
-	/*
-	 * The verify_and_dec_payload() will fail only if the hypervisor is
-	 * actively modifying the message header or corrupting the encrypted payload.
-	 * This hints that hypervisor is acting in a bad faith. Disable the VMPCK so that
-	 * the key cannot be used for any communication. The key is disabled to ensure
-	 * that AES-GCM does not use the same IV while encrypting the request payload.
-	 */
 	rc = verify_and_dec_payload(snp_dev, resp_buf, resp_sz);
 	if (rc) {
 		dev_alert(snp_dev->dev,
-			  "Detected unexpected decode failure, disabling the vmpck_id %d\n",
-			  vmpck_id);
-		snp_disable_vmpck(snp_dev);
-		return rc;
+			  "Detected unexpected decode failure from ASP. rc: %d\n",
+			  rc);
+		goto disable_vmpck;
 	}
 
 	/* Increment to new message sequence after payload decryption was successful. */
 	snp_inc_msg_seqno(snp_dev);
 
 	return 0;
+
+disable_vmpck:
+	snp_disable_vmpck(snp_dev);
+	return rc;
 }
 
 static int get_report(struct snp_guest_dev *snp_dev, struct snp_guest_request_ioctl *arg)
-- 
2.38.1.493.g58b659f92b-goog