[PATCH V4] virt: sev: Prevent IV reuse in SNP guest driver

[Peter Gonda <pgonda@google.com>]

The 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:
https://csrc.nist.gov/csrc/media/projects/block-cipher-techniques/documents/bcm/comments/800-38-series-drafts/gcm/joux_comments.pdf

To handle userspace querying the cert_data length handle_guest_request()
now: saves the number of pages required by the host, retries the request
without requesting the extended data, then returns the number of pages
required.

Fixes: fce96cf044308 ("virt: Add SEV-SNP guest driver")
Signed-off-by: Peter Gonda <pgonda@google.com>
Reported-by: Peter Gonda <pgonda@google.com>
Cc: Dionna Glaze <dionnaglaze@google.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Michael Roth <michael.roth@amd.com>
Cc: Haowen Bai <baihaowen@meizu.com>
Cc: Yang Yingliang <yangyingliang@huawei.com>
Cc: Marc Orr <marcorr@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Ashish Kalra <Ashish.Kalra@amd.com>
Cc: linux-kernel@vger.kernel.org
Cc: kvm@vger.kernel.org
---
Tested by placing each of the guest requests: attestation quote,
extended attestation quote, and get key. Then tested the extended
attestation quote certificate length querying.

V4
 * As suggested by Dionna moved the extended request retry logic into
   the driver.
 * Due to big change in patch dropped any reviewed-by tags.

---
 drivers/virt/coco/sev-guest/sev-guest.c | 70 +++++++++++++++++++------
 1 file changed, 53 insertions(+), 17 deletions(-)

diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
index f422f9c58ba79..7dd6337ebdd5b 100644
--- a/drivers/virt/coco/sev-guest/sev-guest.c
+++ b/drivers/virt/coco/sev-guest/sev-guest.c
@@ -41,7 +41,7 @@ struct snp_guest_dev {
 	struct device *dev;
 	struct miscdevice misc;
 
-	void *certs_data;
+	u8 (*certs_data)[SEV_FW_BLOB_MAX_SIZE];
 	struct snp_guest_crypto *crypto;
 	struct snp_guest_msg *request, *response;
 	struct snp_secrets_page_layout *layout;
@@ -67,8 +67,27 @@ static bool is_vmpck_empty(struct snp_guest_dev *snp_dev)
 	return true;
 }
 
+/*
+ * If we receive an error from the host or ASP we have two options. We can
+ * either retry the exact same encrypted request or we can 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 we were to reuse the sequence number the encryption scheme is
+ * vulnerable. If we encrypt the sequence number for a fresh IV the ASP will
+ * reject our 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;
 }
@@ -323,32 +342,49 @@ static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, in
 
 	/* Call firmware to process the request */
 	rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
+
+	/*
+	 * If the extended guest request fails due to having to small of a
+	 * certificate data buffer retry the same guest request without the
+	 * extended data request.
+	 */
+	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;
+		rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
+
+		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)
@@ -676,7 +712,7 @@ static int __init sev_guest_probe(struct platform_device *pdev)
 	if (!snp_dev->response)
 		goto e_free_request;
 
-	snp_dev->certs_data = alloc_shared_pages(dev, SEV_FW_BLOB_MAX_SIZE);
+	snp_dev->certs_data = alloc_shared_pages(dev, sizeof(*snp_dev->certs_data));
 	if (!snp_dev->certs_data)
 		goto e_free_response;
 
@@ -703,7 +739,7 @@ static int __init sev_guest_probe(struct platform_device *pdev)
 	return 0;
 
 e_free_cert_data:
-	free_shared_pages(snp_dev->certs_data, SEV_FW_BLOB_MAX_SIZE);
+	free_shared_pages(snp_dev->certs_data, sizeof(*snp_dev->certs_data));
 e_free_response:
 	free_shared_pages(snp_dev->response, sizeof(struct snp_guest_msg));
 e_free_request:
@@ -717,7 +753,7 @@ static int __exit sev_guest_remove(struct platform_device *pdev)
 {
 	struct snp_guest_dev *snp_dev = platform_get_drvdata(pdev);
 
-	free_shared_pages(snp_dev->certs_data, SEV_FW_BLOB_MAX_SIZE);
+	free_shared_pages(snp_dev->certs_data, sizeof(*snp_dev->certs_data));
 	free_shared_pages(snp_dev->response, sizeof(struct snp_guest_msg));
 	free_shared_pages(snp_dev->request, sizeof(struct snp_guest_msg));
 	deinit_crypto(snp_dev->crypto);
-- 
2.38.1.273.g43a17bfeac-goog