Fix issue 1418 by moving check of KU_KEY_CERT_SIGN and weakening check_issued()

Move check that cert signing is allowed from x509v3_cache_extensions() to
where it belongs: internal_verify(), generalize it for proxy cert signing.
Correct and simplify check_issued(), now checking self-issued (not: self-signed).
Add test case to 25-test_verify.t that demonstrates successful fix.

As prerequisites, this adds the static function check_sig_alg_match()
and the internal functions x509_likely_issued() and x509_signing_allowed().

This is a backport of the core of PR #10587.
Fixes #1418

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/12357)

9 files changed, 123 insertions, 45 deletions

diff --git a/crypto/x509/x509_local.h b/crypto/x509/x509_local.h
index c517a77456..6ac3c7eaa6 100644
--- a/crypto/x509/x509_local.h
+++ b/crypto/x509/x509_local.h
@@ -145,3 +145,5 @@ DEFINE_STACK_OF(STACK_OF_X509_NAME_ENTRY)
 
 void x509_set_signature_info(X509_SIG_INFO *siginf, const X509_ALGOR *alg,
                              const ASN1_STRING *sig);
+int x509_likely_issued(X509 *issuer, X509 *subject);
+int x509_signing_allowed(const X509 *issuer, const X509 *subject);
diff --git a/crypto/x509/x509_vfy.c b/crypto/x509/x509_vfy.c
index 5bd3c4c159..87b51e990d 100644
--- a/crypto/x509/x509_vfy.c
+++ b/crypto/x509/x509_vfy.c
@@ -104,7 +104,12 @@ static int null_callback(int ok, X509_STORE_CTX *e)
     return ok;
 }
 
-/* Return 1 is a certificate is self signed */
+/*
+ * Return 1 if given cert is considered self-signed, 0 if not or on error.
+ * This does not verify self-signedness but relies on x509v3_cache_extensions()
+ * matching issuer and subject names (i.e., the cert being self-issued) and any
+ * present authority key identifier matching the subject key identifier, etc.
+ */
 static int cert_self_signed(X509 *x)
 {
     if (X509_check_purpose(x, -1, 0) != 1)
@@ -325,30 +330,26 @@ static X509 *find_issuer(X509_STORE_CTX *ctx, STACK_OF(X509) *sk, X509 *x)
     return rv;
 }
 
-/* Given a possible certificate and issuer check them */
-
+/*
+ * Check that the given certificate 'x' is issued by the certificate 'issuer'
+ * and the issuer is not yet in ctx->chain, where the exceptional case
+ * that 'x' is self-issued and ctx->chain has just one element is allowed.
+ */
 static int check_issued(X509_STORE_CTX *ctx, X509 *x, X509 *issuer)
 {
-    int ret;
-    if (x == issuer)
-        return cert_self_signed(x);
-    ret = X509_check_issued(issuer, x);
-    if (ret == X509_V_OK) {
+    if (x509_likely_issued(issuer, x) != X509_V_OK)
+        return 0;
+    if ((x->ex_flags & EXFLAG_SI) == 0 || sk_X509_num(ctx->chain) != 1) {
         int i;
         X509 *ch;
-        /* Special case: single self signed certificate */
-        if (cert_self_signed(x) && sk_X509_num(ctx->chain) == 1)
-            return 1;
+
         for (i = 0; i < sk_X509_num(ctx->chain); i++) {
             ch = sk_X509_value(ctx->chain, i);
-            if (ch == issuer || !X509_cmp(ch, issuer)) {
-                ret = X509_V_ERR_PATH_LOOP;
-                break;
-            }
+            if (ch == issuer || X509_cmp(ch, issuer) == 0)
+                return 0;
         }
     }
-
-    return (ret == X509_V_OK);
+    return 1;
 }
 
 /* Alternative lookup method: look from a STACK stored in other_ctx */
@@ -1752,18 +1753,23 @@ static int internal_verify(X509_STORE_CTX *ctx)
      * is allowed to reset errors (at its own peril).
      */
     while (n >= 0) {
-        EVP_PKEY *pkey;
-
         /*
-         * Skip signature check for self signed certificates unless explicitly
-         * asked for.  It doesn't add any security and just wastes time.  If
-         * the issuer's public key is unusable, report the issuer certificate
+         * Skip signature check for self-issued certificates unless explicitly
+         * asked for because it does not add any security and just wastes time.
+         * If the issuer's public key is not available or its key usage does
+         * not support issuing the subject cert, report the issuer certificate
          * and its depth (rather than the depth of the subject).
          */
         if (xs != xi || (ctx->param->flags & X509_V_FLAG_CHECK_SS_SIGNATURE)) {
+            EVP_PKEY *pkey;
+            int issuer_depth = n + (xs == xi ? 0 : 1);
+            int ret = x509_signing_allowed(xi, xs);
+
+            if (ret != X509_V_OK && !verify_cb_cert(ctx, xi, issuer_depth, ret))
+                return 0;
             if ((pkey = X509_get0_pubkey(xi)) == NULL) {
                 if (!verify_cb_cert(ctx, xi, xi != xs ? n+1 : n,
-                        X509_V_ERR_UNABLE_TO_DECODE_ISSUER_PUBLIC_KEY))
+                                    X509_V_ERR_UNABLE_TO_DECODE_ISSUER_PUBLIC_KEY))
                     return 0;
             } else if (X509_verify(xs, pkey) <= 0) {
                 if (!verify_cb_cert(ctx, xs, n,
diff --git a/crypto/x509v3/v3_purp.c b/crypto/x509v3/v3_purp.c
index f023c64895..2b06dba053 100644
--- a/crypto/x509v3/v3_purp.c
+++ b/crypto/x509v3/v3_purp.c
@@ -13,6 +13,7 @@
 #include <openssl/x509v3.h>
 #include <openssl/x509_vfy.h>
 #include "crypto/x509.h"
+#include "../x509/x509_local.h" /* for x509_signing_allowed() */
 #include "internal/tsan_assist.h"
 
 static void x509v3_cache_extensions(X509 *x);
@@ -344,6 +345,21 @@ static int setup_crldp(X509 *x)
     return 1;
 }
 
+/* Check that issuer public key algorithm matches subject signature algorithm */
+static int check_sig_alg_match(const EVP_PKEY *pkey, const X509 *subject)
+{
+    int pkey_nid;
+
+    if (pkey == NULL)
+        return X509_V_ERR_NO_ISSUER_PUBLIC_KEY;
+    if (OBJ_find_sigid_algs(OBJ_obj2nid(subject->cert_info.signature.algorithm),
+                            NULL, &pkey_nid) == 0)
+        return X509_V_ERR_UNSUPPORTED_SIGNATURE_ALGORITHM;
+    if (EVP_PKEY_type(pkey_nid) != EVP_PKEY_base_id(pkey))
+        return X509_V_ERR_SIGNATURE_ALGORITHM_MISMATCH;
+    return X509_V_OK;
+}
+
 #define V1_ROOT (EXFLAG_V1|EXFLAG_SS)
 #define ku_reject(x, usage) \
         (((x)->ex_flags & EXFLAG_KUSAGE) && !((x)->ex_kusage & (usage)))
@@ -496,11 +512,11 @@ static void x509v3_cache_extensions(X509 *x)
         x->ex_flags |= EXFLAG_INVALID;
     /* Does subject name match issuer ? */
     if (!X509_NAME_cmp(X509_get_subject_name(x), X509_get_issuer_name(x))) {
-        x->ex_flags |= EXFLAG_SI;
-        /* If SKID matches AKID also indicate self signed */
-        if (X509_check_akid(x, x->akid) == X509_V_OK &&
-            !ku_reject(x, KU_KEY_CERT_SIGN))
-            x->ex_flags |= EXFLAG_SS;
+        x->ex_flags |= EXFLAG_SI; /* cert is self-issued */
+        if (X509_check_akid(x, x->akid) == X509_V_OK /* SKID matches AKID */
+                /* .. and the signature alg matches the PUBKEY alg: */
+                && check_sig_alg_match(X509_get0_pubkey(x), x) == X509_V_OK)
+            x->ex_flags |= EXFLAG_SS; /* indicate self-signed */
     }
     x->altname = X509_get_ext_d2i(x, NID_subject_alt_name, &i, NULL);
     if (x->altname == NULL && i != -1)
@@ -793,6 +809,23 @@ static int no_check(const X509_PURPOSE *xp, const X509 *x, int ca)
 }
 
 /*-
+ * Check if certificate I<issuer> is allowed to issue certificate I<subject>
+ * according to the B<keyUsage> field of I<issuer> if present
+ * depending on any proxyCertInfo extension of I<subject>.
+ * Returns 0 for OK, or positive for reason for rejection
+ * where reason codes match those for X509_verify_cert().
+ */
+int x509_signing_allowed(const X509 *issuer, const X509 *subject)
+{
+    if (subject->ex_flags & EXFLAG_PROXY) {
+        if (ku_reject(issuer, KU_DIGITAL_SIGNATURE))
+            return X509_V_ERR_KEYUSAGE_NO_DIGITAL_SIGNATURE;
+    } else if (ku_reject(issuer, KU_KEY_CERT_SIGN))
+        return X509_V_ERR_KEYUSAGE_NO_CERTSIGN;
+    return X509_V_OK;
+}
+
+/*-
  * Various checks to see if one certificate issued the second.
  * This can be used to prune a set of possible issuer certificates
  * which have been looked up using some simple method such as by
@@ -800,13 +833,24 @@ static int no_check(const X509_PURPOSE *xp, const X509 *x, int ca)
  * These are:
  * 1. Check issuer_name(subject) == subject_name(issuer)
  * 2. If akid(subject) exists check it matches issuer
- * 3. If key_usage(issuer) exists check it supports certificate signing
+ * 3. Check that issuer public key algorithm matches subject signature algorithm
+ * 4. If key_usage(issuer) exists check it supports certificate signing
  * returns 0 for OK, positive for reason for mismatch, reasons match
  * codes for X509_verify_cert()
  */
 
 int X509_check_issued(X509 *issuer, X509 *subject)
 {
+    int ret;
+
+    if ((ret = x509_likely_issued(issuer, subject)) != X509_V_OK)
+        return ret;
+    return x509_signing_allowed(issuer, subject);
+}
+
+/* do the checks 1., 2., and 3. as described above for X509_check_issued() */
+int x509_likely_issued(X509 *issuer, X509 *subject)
+{
     if (X509_NAME_cmp(X509_get_subject_name(issuer),
                       X509_get_issuer_name(subject)))
         return X509_V_ERR_SUBJECT_ISSUER_MISMATCH;
@@ -824,12 +868,8 @@ int X509_check_issued(X509 *issuer, X509 *subject)
             return ret;
     }
 
-    if (subject->ex_flags & EXFLAG_PROXY) {
-        if (ku_reject(issuer, KU_DIGITAL_SIGNATURE))
-            return X509_V_ERR_KEYUSAGE_NO_DIGITAL_SIGNATURE;
-    } else if (ku_reject(issuer, KU_KEY_CERT_SIGN))
-        return X509_V_ERR_KEYUSAGE_NO_CERTSIGN;
-    return X509_V_OK;
+    /* check if the subject signature alg matches the issuer's PUBKEY alg */
+    return check_sig_alg_match(X509_get0_pubkey(issuer), subject);
 }
 
 int X509_check_akid(X509 *issuer, AUTHORITY_KEYID *akid)
diff --git a/doc/man3/X509_STORE_set_verify_cb_func.pod b/doc/man3/X509_STORE_set_verify_cb_func.pod
index d16881edd8..47de27d1c7 100644
--- a/doc/man3/X509_STORE_set_verify_cb_func.pod
+++ b/doc/man3/X509_STORE_set_verify_cb_func.pod
@@ -137,7 +137,9 @@ I<If no function to get the issuer is provided, the internal default
 function will be used instead.>
 
 X509_STORE_set_check_issued() sets the function to check that a given
-certificate B<x> is issued with the issuer certificate B<issuer>.
+certificate B<x> is issued by the issuer certificate B<issuer> and
+the issuer is not yet in the chain contained in <ctx>, where the exceptional
+case that B<x> is self-issued and ctx->chain has just one element is allowed.
 This function must return 0 on failure (among others if B<x> hasn't
 been issued with B<issuer>) and 1 on success.
 I<If no function to get the issuer is provided, the internal default
diff --git a/doc/man3/X509_check_issued.pod b/doc/man3/X509_check_issued.pod
index f9a541ef71..507198698c 100644
--- a/doc/man3/X509_check_issued.pod
+++ b/doc/man3/X509_check_issued.pod
@@ -2,7 +2,7 @@
 
 =head1 NAME
 
-X509_check_issued - checks if certificate is issued by another
+X509_check_issued - checks if certificate is apparently issued by another
 certificate
 
 =head1 SYNOPSIS
@@ -14,13 +14,14 @@ certificate
 
 =head1 DESCRIPTION
 
-This function checks if certificate I<subject> was issued using CA
-certificate I<issuer>. This function takes into account not only
-matching of issuer field of I<subject> with subject field of I<issuer>,
-but also compares B<authorityKeyIdentifier> extension of I<subject> with
-B<subjectKeyIdentifier> of I<issuer> if B<authorityKeyIdentifier>
-present in the I<subject> certificate and checks B<keyUsage> field of
-I<issuer>.
+X509_check_issued() checks if certificate I<subject> was apparently issued
+using (CA) certificate I<issuer>. This function takes into account not only
+matching of the issuer field of I<subject> with the subject field of I<issuer>,
+but also compares all sub-fields of the B<authorityKeyIdentifier> extension of
+I<subject>, as far as present, with the respective B<subjectKeyIdentifier>,
+serial number, and issuer fields of I<issuer>, as far as present. It also checks
+if the B<keyUsage> field (if present) of I<issuer> allows certificate signing.
+It does not check the certificate signature.
 
 =head1 RETURN VALUES
 
diff --git a/include/openssl/x509_vfy.h b/include/openssl/x509_vfy.h
index adb8bce7cb..0f13739b79 100644
--- a/include/openssl/x509_vfy.h
+++ b/include/openssl/x509_vfy.h
@@ -184,6 +184,9 @@ void X509_STORE_CTX_set_depth(X509_STORE_CTX *ctx, int depth);
 # define         X509_V_ERR_OCSP_VERIFY_NEEDED                   73  /* Need OCSP verification */
 # define         X509_V_ERR_OCSP_VERIFY_FAILED                   74  /* Couldn't verify cert through OCSP */
 # define         X509_V_ERR_OCSP_CERT_UNKNOWN                    75  /* Certificate wasn't recognized by the OCSP responder */
+# define         X509_V_ERR_SIGNATURE_ALGORITHM_MISMATCH         76
+# define         X509_V_ERR_NO_ISSUER_PUBLIC_KEY                 77
+# define         X509_V_ERR_UNSUPPORTED_SIGNATURE_ALGORITHM      78
 
 /* Certificate verify flags */
 
diff --git a/test/certs/ee-self-signed.pem b/test/certs/ee-self-signed.pem
new file mode 100644
index 0000000000..ad1e37ba0e
--- /dev/null
+++ b/test/certs/ee-self-signed.pem
@@ -0,0 +1,18 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
diff --git a/test/certs/setup.sh b/test/certs/setup.sh
index bbe4842a51..7e40f65b68 100755
--- a/test/certs/setup.sh
+++ b/test/certs/setup.sh
@@ -185,6 +185,9 @@ OPENSSL_SIGALG=md5 \
 OPENSSL_KEYBITS=768 \
 ./mkcert.sh genee server.example ee-key-768 ee-cert-768 ca-key ca-cert
 
+# self-signed end-entity cert with explicit keyUsage not including KeyCertSign
+openssl req -new -x509 -key ee-key.pem -subj /CN=ee-self-signed -out ee-self-signed.pem -addext keyUsage=digitalSignature
+
 # Proxy certificates, off of ee-client
 # Start with some good ones
 ./mkcert.sh req pc1-key "0.CN = server.example" "1.CN = proxy 1" | \
diff --git a/test/recipes/25-test_verify.t b/test/recipes/25-test_verify.t
index cf7842cdfd..0c643e583f 100644
--- a/test/recipes/25-test_verify.t
+++ b/test/recipes/25-test_verify.t
@@ -27,7 +27,7 @@ sub verify {
     run(app([@args]));
 }
 
-plan tests => 137;
+plan tests => 138;
 
 # Canonical success
 ok(verify("ee-cert", "sslserver", ["root-cert"], ["ca-cert"]),
@@ -368,6 +368,9 @@ ok(verify("some-names2", "sslserver", ["many-constraints"], ["many-constraints"]
 ok(verify("root-cert-rsa2", "sslserver", ["root-cert-rsa2"], [], "-check_ss_sig"),
     "Public Key Algorithm rsa instead of rsaEncryption");
 
+    ok(verify("ee-self-signed", "sslserver", ["ee-self-signed"], []),
+       "accept trusted self-signed EE cert excluding key usage keyCertSign");
+
 SKIP: {
     skip "Ed25519 is not supported by this OpenSSL build", 1
 	      if disabled("ec");