Re: [PATCH 2/2] [crypto] Only verify CA certs in chain of trust

[Henry Kleynhans <hkleynhans@fb.com>]

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Hi Daniel,

This patch tightens the CA verification code to only check the issuer chain of the client cert.  I think this will still not catch expired/invalid certs if the client and server certs have different issuer chains; so maybe this too is not quite the correct fix.  Let me know what you think.

Kind regards, Henry

From: Henry Kleynhans <henry.kleynhans@gmail.com>
Date: Wednesday, 22 December 2021 at 15:06
To: qemu-devel@nongnu.org <qemu-devel@nongnu.org>
Cc: berrange@redhat.com <berrange@redhat.com>, henry.kleynhans@fb.com <henry.kleynhans@fb.com>, Henry Kleynhans <hkleynhans@fb.com>
Subject: [PATCH 2/2] [crypto] Only verify CA certs in chain of trust
From: Henry Kleynhans <hkleynhans@fb.com>

The CA file provided to qemu may contain CA certificates which do not
form part of the chain of trust for the specific certificate we are
sanity checking.

This patch changes the sanity checking from validating every CA
certificate to only checking the CA certificates which are part of the
chain of trust (issuer chain).  Other certificates are ignored.

Signed-off-by: Henry Kleynhans <hkleynhans@fb.com>
---
 crypto/tlscredsx509.c                 | 50 +++++++++++++++++++++++----
 tests/unit/test-crypto-tlscredsx509.c | 25 +++++++++++++-
 2 files changed, 68 insertions(+), 7 deletions(-)

diff --git a/crypto/tlscredsx509.c b/crypto/tlscredsx509.c
index d061c68253..841f80aac6 100644
--- a/crypto/tlscredsx509.c
+++ b/crypto/tlscredsx509.c
@@ -315,6 +315,44 @@ qcrypto_tls_creds_check_cert(QCryptoTLSCredsX509 *creds,
     return 0;
 }

+static int
+qcrypto_tls_creds_check_authority_chain(QCryptoTLSCredsX509 *creds,
+                                        gnutls_x509_crt_t cert,
+                                        gnutls_x509_crt_t *cacerts,
+                                        unsigned int ncacerts,
+                                        const char *cacertFile,
+                                        bool isServer,
+                                        bool isCA,
+                                        Error **errp)
+{
+    gnutls_x509_crt_t *cert_to_check = &cert;
+    int checking_issuer = 1;
+    int retval = 0;
+
+    while (checking_issuer) {
+        checking_issuer = 0;
+
+        if (gnutls_x509_crt_check_issuer(*cert_to_check, *cert_to_check)) {
+            /* The cert is self-signed indicating we have reached the root of trust. */
+            return qcrypto_tls_creds_check_cert(creds, *cert_to_check, cacertFile,
+                                                isServer, isCA, errp);
+        }
+        for (int i = 0; i < ncacerts; i++) {
+            if (gnutls_x509_crt_check_issuer(*cert_to_check, cacerts[i])) {
+                retval = qcrypto_tls_creds_check_cert(creds, cacerts[i], cacertFile,
+                                                      isServer, isCA, errp);
+                if (retval < 0) {
+                    return retval;
+                }
+                cert_to_check = &cacerts[i];
+                checking_issuer = 1;
+                break;
+            }
+        }
+    }
+
+    return -1;
+}

 static int
 qcrypto_tls_creds_check_cert_pair(gnutls_x509_crt_t cert,
@@ -500,12 +538,12 @@ qcrypto_tls_creds_x509_sanity_check(QCryptoTLSCredsX509 *creds,
         goto cleanup;
     }

-    for (i = 0; i < ncacerts; i++) {
-        if (qcrypto_tls_creds_check_cert(creds,
-                                         cacerts[i], cacertFile,
-                                         isServer, true, errp) < 0) {
-            goto cleanup;
-        }
+    if (cert &&
+        qcrypto_tls_creds_check_authority_chain(creds, cert,
+                                                cacerts, ncacerts,
+                                                cacertFile, isServer,
+                                                true, errp) < 0) {
+        goto cleanup;
     }

     if (cert && ncacerts &&
diff --git a/tests/unit/test-crypto-tlscredsx509.c b/tests/unit/test-crypto-tlscredsx509.c
index aab4149b56..e4d657ba61 100644
--- a/tests/unit/test-crypto-tlscredsx509.c
+++ b/tests/unit/test-crypto-tlscredsx509.c
@@ -589,6 +589,12 @@ int main(int argc, char **argv)
                  true, true, GNUTLS_KEY_KEY_CERT_SIGN,
                  false, false, NULL, NULL,
                  0, 0);
+    TLS_CERT_REQ(cacertlevel1creq_invalid, cacertrootreq,
+                 "UK", "qemu level 1c invalid", NULL, NULL, NULL, NULL,
+                 true, true, true,
+                 true, true, GNUTLS_KEY_KEY_CERT_SIGN,
+                 false, false, NULL, NULL,
+                 360, 400);
     TLS_CERT_REQ(cacertlevel2areq, cacertlevel1areq,
                  "UK", "qemu level 2a", NULL, NULL, NULL, NULL,
                  true, true, true,
@@ -617,16 +623,32 @@ int main(int argc, char **argv)
         cacertlevel2areq.crt,
     };

+
     test_tls_write_cert_chain(WORKDIR "cacertchain-ctx.pem",
                               certchain,
                               G_N_ELEMENTS(certchain));

+    gnutls_x509_crt_t certchain_with_invalid[] = {
+        cacertrootreq.crt,
+        cacertlevel1areq.crt,
+        cacertlevel1breq.crt,
+        cacertlevel1creq_invalid.crt,
+        cacertlevel2areq.crt,
+    };
+
+    test_tls_write_cert_chain(WORKDIR "cacertchain-with-invalid-ctx.pem",
+                              certchain_with_invalid,
+                              G_N_ELEMENTS(certchain_with_invalid));
+
     TLS_TEST_REG(chain1, true,
                  WORKDIR "cacertchain-ctx.pem",
                  servercertlevel3areq.filename, false);
     TLS_TEST_REG(chain2, false,
                  WORKDIR "cacertchain-ctx.pem",
                  clientcertlevel2breq.filename, false);
+    TLS_TEST_REG(certchainwithexpiredcert, false,
+                 WORKDIR "cacertchain-with-invalid-ctx.pem",
+                 clientcertlevel2breq.filename, false);

     /* Some missing certs - first two are fatal, the last
      * is ok
@@ -640,7 +662,6 @@ int main(int argc, char **argv)
     TLS_TEST_REG(missingclient, false,
                  cacert1req.filename,
                  "clientcertdoesnotexist.pem", false);
-
     ret = g_test_run();

     test_tls_discard_cert(&cacertreq);
@@ -694,10 +715,12 @@ int main(int argc, char **argv)
     test_tls_discard_cert(&cacertrootreq);
     test_tls_discard_cert(&cacertlevel1areq);
     test_tls_discard_cert(&cacertlevel1breq);
+    test_tls_discard_cert(&cacertlevel1creq_invalid);
     test_tls_discard_cert(&cacertlevel2areq);
     test_tls_discard_cert(&servercertlevel3areq);
     test_tls_discard_cert(&clientcertlevel2breq);
     unlink(WORKDIR "cacertchain-ctx.pem");
+    unlink(WORKDIR "cacertchain-with-invalid-ctx.pem");

     test_tls_cleanup(KEYFILE);
     rmdir(WORKDIR);
--
2.34.1

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