upstream: ssh-keygen -Y find-principals fixes based on feedback

from Markus:

use "principals" instead of principal, as allowed_signers lines may list
multiple.

When the signing key is a certificate, emit only principals that match
the certificate principal list.

NB. the command -Y name changes: "find-principal" => "find-principals"

ok markus@

OpenBSD-Commit-ID: ab575946ff9a55624cd4e811bfd338bf3b1d0faf

4 files changed, 84 insertions, 33 deletions

diff --git a/ssh-keygen.1 b/ssh-keygen.1
index 5d33902f7..b4a873920 100644
--- a/ssh-keygen.1
+++ b/ssh-keygen.1
@@ -1,4 +1,4 @@
-.\"     $OpenBSD: ssh-keygen.1,v 1.195 2020/01/23 07:16:38 jmc Exp $
+.\"     $OpenBSD: ssh-keygen.1,v 1.196 2020/01/23 23:31:52 djm Exp $
 .\"
 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -138,7 +138,7 @@
 .Fl f Ar krl_file
 .Ar
 .Nm ssh-keygen
-.Fl Y Cm find-principal
+.Fl Y Cm find-principals
 .Fl s Ar signature_file
 .Fl f Ar allowed_signers_file
 .Nm ssh-keygen
@@ -618,8 +618,8 @@ The maximum is 3.
 Specifies a path to a library that will be used when creating
 FIDO authenticator-hosted keys, overriding the default of using
 the internal USB HID support.
-.It Fl Y Cm find-principal
-Find the principal associated with the public key of a signature,
+.It Fl Y Cm find-principals
+Find the principal(s) associated with the public key of a signature,
 provided using the
 .Fl s
 flag in an authorized signers file provided using the
@@ -628,7 +628,8 @@ flag.
 The format of the allowed signers file is documented in the
 .Sx ALLOWED SIGNERS
 section below.
-If a matching principal is found, it is returned on standard output.
+If one or more matching principals are found, they are returned on
+standard output.
 .It Fl Y Cm check-novalidate
 Checks that a signature generated using
 .Nm
diff --git a/ssh-keygen.c b/ssh-keygen.c
index ce94a5ab0..363da70db 100644
--- a/ssh-keygen.c
+++ b/ssh-keygen.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-keygen.c,v 1.387 2020/01/23 07:54:04 djm Exp $ */
+/* $OpenBSD: ssh-keygen.c,v 1.388 2020/01/23 23:31:52 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1994 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -2758,11 +2758,11 @@ done:
 }
 
 static int
-sig_find_principal(const char *signature, const char *allowed_keys) {
+sig_find_principals(const char *signature, const char *allowed_keys) {
         int r, ret = -1, sigfd = -1;
         struct sshbuf *sigbuf = NULL, *abuf = NULL;
         struct sshkey *sign_key = NULL;
-        char *principal = NULL;
+        char *principals = NULL;
 
         if ((abuf = sshbuf_new()) == NULL)
                 fatal("%s: sshbuf_new() failed", __func__);
@@ -2782,12 +2782,11 @@ sig_find_principal(const char *signature, const char *allowed_keys) {
         }
         if ((r = sshsig_get_pubkey(sigbuf, &sign_key)) != 0) {
                 error("%s: sshsig_get_pubkey: %s",
-                      __func__, ssh_err(r));
+                    __func__, ssh_err(r));
                 goto done;
         }
-
-        if ((r = sshsig_find_principal(allowed_keys, sign_key,
-                                      &principal)) != 0) {
+        if ((r = sshsig_find_principals(allowed_keys, sign_key,
+            &principals)) != 0) {
                 error("%s: sshsig_get_principal: %s",
                       __func__, ssh_err(r));
                 goto done;
@@ -2795,7 +2794,7 @@ sig_find_principal(const char *signature, const char *allowed_keys) {
         ret = 0;
 done:
         if (ret == 0 ) {
-                printf("Found matching principal: %s\n", principal);
+                printf("Found matching principal: %s\n", principals);
         } else {
                 printf("Could not find matching principal.\n");
         }
@@ -2804,7 +2803,7 @@ done:
         sshbuf_free(sigbuf);
         sshbuf_free(abuf);
         sshkey_free(sign_key);
-        free(principal);
+        free(principals);
         return ret;
 }
 
@@ -3093,7 +3092,7 @@ usage(void)
             "       ssh-keygen -k -f krl_file [-u] [-s ca_public] [-z version_number]\n"
             "                  file ...\n"
             "       ssh-keygen -Q -f krl_file file ...\n"
-            "       ssh-keygen -Y find-principal -s signature_file -f allowed_signers_file\n"
+            "       ssh-keygen -Y find-principals -s signature_file -f allowed_signers_file\n"
             "       ssh-keygen -Y check-novalidate -n namespace -s signature_file\n"
             "       ssh-keygen -Y sign -f key_file -n namespace file ...\n"
             "       ssh-keygen -Y verify -f allowed_signers_file -I signer_identity\n"
@@ -3357,18 +3356,18 @@ main(int argc, char **argv)
         argc -= optind;
 
         if (sign_op != NULL) {
-                if (strncmp(sign_op, "find-principal", 14) == 0) {
+                if (strncmp(sign_op, "find-principals", 15) == 0) {
                         if (ca_key_path == NULL) {
-                                error("Too few arguments for find-principal:"
+                                error("Too few arguments for find-principals:"
                                       "missing signature file");
                                 exit(1);
                         }
                         if (!have_identity) {
-                                error("Too few arguments for find-principal:"
+                                error("Too few arguments for find-principals:"
                                       "missing allowed keys file");
                                 exit(1);
                         }
-                        return sig_find_principal(ca_key_path, identity_file);
+                        return sig_find_principals(ca_key_path, identity_file);
                 }
                 if (cert_principals == NULL || *cert_principals == '\0') {
                         error("Too few arguments for sign/verify: "
diff --git a/sshsig.c b/sshsig.c
index e9f4baa76..e63a36e1e 100644
--- a/sshsig.c
+++ b/sshsig.c
@@ -868,13 +868,64 @@ sshsig_check_allowed_keys(const char *path, const struct sshkey *sign_key,
 }
 
 static int
-get_matching_principal_from_line(const char *path, u_long linenum, char *line,
+cert_filter_principals(const char *path, u_long linenum,
+    char **principalsp, const struct sshkey *cert)
+{
+        char *cp, *oprincipals, *principals;
+        const char *reason;
+        struct sshbuf *nprincipals;
+        int r = SSH_ERR_INTERNAL_ERROR, success = 0;
+
+        oprincipals = principals = *principalsp;
+        *principalsp = NULL;
+
+        if ((nprincipals = sshbuf_new()) == NULL)
+                return SSH_ERR_ALLOC_FAIL;
+
+        while ((cp = strsep(&principals, ",")) != NULL && *cp != '\0') {
+                if (strcspn(cp, "!?*") != strlen(cp)) {
+                        debug("%s:%lu: principal \"%s\" not authorized: "
+                            "contains wildcards", path, linenum, cp);
+                        continue;
+                }
+                /* Check against principals list in certificate */
+                if ((r = sshkey_cert_check_authority(cert, 0, 1,
+                    cp, &reason)) != 0) {
+                        debug("%s:%lu: principal \"%s\" not authorized: %s",
+                            path, linenum, cp, reason);
+                        continue;
+                }
+                if ((r = sshbuf_putf(nprincipals, "%s%s",
+                    sshbuf_len(nprincipals) != 0 ? "," : "", cp)) != 0) {
+                        error("%s: buffer error", __func__);
+                        goto out;
+                }
+        }
+        if (sshbuf_len(nprincipals) == 0) {
+                error("%s:%lu: no valid principals found", path, linenum);
+                r = SSH_ERR_KEY_CERT_INVALID;
+                goto out;
+        }
+        if ((principals = sshbuf_dup_string(nprincipals)) == NULL) {
+                error("%s: buffer error", __func__);
+                goto out;
+        }
+        /* success */
+        success = 1;
+        *principalsp = principals;
+ out:
+        sshbuf_free(nprincipals);
+        free(oprincipals);
+        return success ? 0 : r;
+}
+
+static int
+get_matching_principals_from_line(const char *path, u_long linenum, char *line,
     const struct sshkey *sign_key, char **principalsp)
 {
         struct sshkey *found_key = NULL;
         char *principals = NULL;
         int r, found = 0;
-        const char *reason = NULL;
         struct sshsigopt *sigopts = NULL;
 
         if (principalsp != NULL)
@@ -894,11 +945,12 @@ get_matching_principal_from_line(const char *path, u_long linenum, char *line,
                 found = 1;
         } else if (sigopts->ca && sshkey_is_cert(sign_key) &&
             sshkey_equal_public(sign_key->cert->signature_key, found_key)) {
-                /* Match of certificate's CA key */
-                if ((r = sshkey_cert_check_authority(sign_key, 0, 1,
-                    principals, &reason)) != 0) {
-                        error("%s:%lu: certificate not authorized: %s",
-                            path, linenum, reason);
+                /* Remove principals listed in file but not allowed by cert */
+                if ((r = cert_filter_principals(path, linenum,
+                    &principals, sign_key)) != 0) {
+                        /* error already displayed */
+                        debug("%s:%lu: cert_filter_principals: %s",
+                            path, linenum, ssh_err(r));
                         goto done;
                 }
                 debug("%s:%lu: matched certificate CA key", path, linenum);
@@ -920,8 +972,8 @@ get_matching_principal_from_line(const char *path, u_long linenum, char *line,
 }
 
 int
-sshsig_find_principal(const char *path, const struct sshkey *sign_key,
-    char **principal)
+sshsig_find_principals(const char *path, const struct sshkey *sign_key,
+    char **principals)
 {
         FILE *f = NULL;
         char *line = NULL;
@@ -939,8 +991,8 @@ sshsig_find_principal(const char *path, const struct sshkey *sign_key,
 
         while (getline(&line, &linesize, f) != -1) {
                 linenum++;
-                r = get_matching_principal_from_line(path, linenum, line,
-                    sign_key, principal);
+                r = get_matching_principals_from_line(path, linenum, line,
+                    sign_key, principals);
                 free(line);
                 line = NULL;
                 if (r == SSH_ERR_KEY_NOT_FOUND)
diff --git a/sshsig.h b/sshsig.h
index 939e3dfe0..63cc1ad1a 100644
--- a/sshsig.h
+++ b/sshsig.h
@@ -93,13 +93,12 @@ struct sshsigopt *sshsigopt_parse(const char *opts,
 void sshsigopt_free(struct sshsigopt *opts);
 
 /* Get public key from signature */
-int
-sshsig_get_pubkey(struct sshbuf *signature, struct sshkey **pubkey);
+int sshsig_get_pubkey(struct sshbuf *signature, struct sshkey **pubkey);
 
 /* Find principal in allowed_keys file, given a sshkey. Returns
  * 0 on success.
  */
-int sshsig_find_principal(const char *path, const struct sshkey *sign_key,
+int sshsig_find_principals(const char *path, const struct sshkey *sign_key,
     char **principal);
 
 #endif /* SSHSIG_H */