Address review: clear crypto callback on error, gate SHA-1 to RSA, cryptocb docs, fix comment

Author: aidangarske

.github/workflows/tpm-ssh.yml

@@ -51,12 +51,17 @@ jobs:
       run: |
         cd wolfssl
         ./autogen.sh
-        # certgen/certreq/certext: generate the X.509 host certificate from the
-        # TPM key. WC_SIG_MIN_HASH_TYPE=SHA: the RSA x509v3-ssh-rsa host cert is
-        # SHA-1; modern wolfSSL otherwise rejects SHA-1 RSA signatures.
+        # certgen/certreq/certext/cryptocb: generate the X.509 host certificate
+        # from the TPM key via the crypto callback.
+        EXTRA_CFLAGS="-DWC_RSA_NO_PADDING"
+        # The RSA x509v3-ssh-rsa host cert is SHA-1; modern wolfSSL otherwise
+        # rejects SHA-1 RSA signatures. Only needed for the RSA cells.
+        if [ "${{ matrix.keytype }}" = "rsa" ]; then
+            EXTRA_CFLAGS="$EXTRA_CFLAGS -DWC_SIG_MIN_HASH_TYPE=WC_HASH_TYPE_SHA"
+        fi
         ./configure --enable-wolftpm --enable-wolfssh --enable-keygen \
-            --enable-certgen --enable-certreq --enable-certext \
-            CFLAGS="-DWC_RSA_NO_PADDING -DWC_SIG_MIN_HASH_TYPE=WC_HASH_TYPE_SHA"
+            --enable-certgen --enable-certreq --enable-certext --enable-cryptocb \
+            CFLAGS="$EXTRA_CFLAGS"
         make
         sudo make install
         sudo ldconfig

README.md

@@ -637,6 +637,7 @@ and wolfSSL/wolfTPM built with certificate generation:
     wolfSSL
         $ ./configure --enable-wolfssh --enable-wolftpm --enable-keygen \
               --enable-certgen --enable-certreq --enable-certext \
+              --enable-cryptocb \
               CFLAGS="-DWC_RSA_NO_PADDING"
     wolfTPM
         $ ./configure --enable-fwtpm --enable-swtpm

examples/tpmcertserver/tpmcertclient.c

@@ -52,8 +52,12 @@ static int TpmCcUserAuth(byte authType, WS_UserAuthData* authData, void* ctx)
 }
 
 
-/* The server host key is an X.509 certificate; it was already verified against
- * the root CA loaded with wolfSSH_CTX_AddRootCert_buffer(). Accept it. */
+/* Host key acceptance callback. wolfSSH verifies the server's X.509 certificate
+ * chain against the root CA loaded with wolfSSH_CTX_AddRootCert_buffer() later,
+ * during the key exchange, when it extracts the public key from the certificate.
+ * Because the client only accepts x509v3 host key algorithms, that CA
+ * verification is always performed. This callback just accepts the presented
+ * host key blob. */
 static int TpmCcHostKeyCheck(const byte* pubKey, word32 pubKeySz, void* ctx)
 {
     WOLFSSH_UNUSED(pubKey);

examples/tpmcertserver/tpmcertserver.c

@@ -146,13 +146,14 @@ static int TpmCsMakeKeyAndCert(WOLFTPM2_DEV* dev, WOLFTPM2_KEY* key,
         rc = 0;
     }
 
-    /* The crypto callback is only needed to self-sign the certificate. Clear
-     * it before wolfSSH runs: host-key signing uses wolfTPM2_SignHashScheme()
-     * directly, and a registered callback would route wolfSSH's certificate
-     * parsing through the TPM. This reset is required, so treat a failure as
-     * fatal. */
-    if (rc == 0 && devId != INVALID_DEVID) {
-        rc = wolfTPM2_ClearCryptoDevCb(dev, devId);
+    /* The crypto callback is only needed to self-sign the certificate. Always
+     * clear it (including on error paths) before wolfSSH runs: host-key signing
+     * uses wolfTPM2_SignHashScheme() directly, and a registered callback would
+     * route wolfSSH's certificate parsing through the TPM. */
+    if (devId != INVALID_DEVID) {
+        int clearRc = wolfTPM2_ClearCryptoDevCb(dev, devId);
+        if (rc == 0)
+            rc = clearRc;
     }
 
     /* Restore a clean password session on the device. The certificate signing