virt/sev-guest: Prevent IV reuse in the SNP guest driver

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() save the number of pages required by
the host, then have handle_guest_request() retry the request without
requesting the extended data, then return the number of pages required
back to userspace.

  [ bp: Massage, incorporate Tom's review comments. ]

Fixes: fce96cf044308 ("virt: Add SEV-SNP guest driver")
Reported-by: Peter Gonda <pgonda@google.com>
Signed-off-by: Peter Gonda <pgonda@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Cc: stable@kernel.org
Link: https://lore.kernel.org/r/20221116175558.2373112-1-pgonda@google.com

diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
index f422f9c..1ea6d2e 100644 (file)
--- 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 +340,71 @@ 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 increment the sequence number
+        * and thus avoid IV reuse.
+        */
+       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 ioctl() 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)