Add ssh-keygen -Y match-principals operation to perform matching of

principals names against an allowed signers file.

Requested by and mostly written by Fabian Stelzer, towards a TOFU
model for SSH signatures in git. Some tweaks by me.

"doesn't bother me" deraadt@

4 files changed, 132 insertions, 5 deletions

diff --git a/usr.bin/ssh/ssh-keygen.1 b/usr.bin/ssh/ssh-keygen.1
index f83f515f600..57c106d10a7 100644
--- a/usr.bin/ssh/ssh-keygen.1
+++ b/usr.bin/ssh/ssh-keygen.1
@@ -1,4 +1,4 @@
-.\"	$OpenBSD: ssh-keygen.1,v 1.216 2021/08/11 08:54:17 djm Exp $
+.\"	$OpenBSD: ssh-keygen.1,v 1.217 2021/11/27 07:14:46 djm Exp $
 .\"
 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -35,7 +35,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd $Mdocdate: August 11 2021 $
+.Dd $Mdocdate: November 27 2021 $
 .Dt SSH-KEYGEN 1
 .Os
 .Sh NAME
@@ -151,6 +151,11 @@
 .Fl s Ar signature_file
 .Fl f Ar allowed_signers_file
 .Nm ssh-keygen
+.Fl Y Cm match-principals
+.Op Fl O Ar option
+.Fl I Ar signer_identity
+.Fl f Ar allowed_signers_file
+.Nm ssh-keygen
 .Fl Y Cm check-novalidate
 .Op Fl O Ar option
 .Fl n Ar namespace
@@ -683,6 +688,14 @@ The format of the allowed signers file is documented in the
 section below.
 If one or more matching principals are found, they are returned on
 standard output.
+.It Fl Y Cm match-principals
+Find principal matching the principal name provided using the
+.Fl I
+flag in the authorized signers file specified using the
+.Fl f
+flag.
+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/usr.bin/ssh/ssh-keygen.c b/usr.bin/ssh/ssh-keygen.c
index 0e4d00013b2..ce10e61c442 100644
--- a/usr.bin/ssh/ssh-keygen.c
+++ b/usr.bin/ssh/ssh-keygen.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-keygen.c,v 1.440 2021/10/29 03:20:46 djm Exp $ */
+/* $OpenBSD: ssh-keygen.c,v 1.441 2021/11/27 07:14:46 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1994 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -2826,6 +2826,32 @@ done:
 	return ret;
 }
 
+static int
+sig_match_principals(const char *allowed_keys, char *principal,
+	char * const *opts, size_t nopts)
+{
+	int r;
+	char **principals = NULL;
+	size_t i, nprincipals = 0;
+
+	if ((r = sig_process_opts(opts, nopts, NULL, NULL)) != 0)
+		return r; /* error already logged */
+
+	if ((r = sshsig_match_principals(allowed_keys, principal,
+	    &principals, &nprincipals)) != 0) {
+		debug_f("match: %s", ssh_err(r));
+		fprintf(stderr, "No principal matched.\n");
+		return r;
+	}
+	for (i = 0; i < nprincipals; i++) {
+		printf("%s\n", principals[i]);
+		free(principals[i]);
+	}
+	free(principals);
+
+	return 0;
+}
+
 static void
 do_moduli_gen(const char *out_file, char **opts, size_t nopts)
 {
@@ -3164,6 +3190,7 @@ usage(void)
 	    "                  file ...\n"
 	    "       ssh-keygen -Q [-l] -f krl_file [file ...]\n"
 	    "       ssh-keygen -Y find-principals -s signature_file -f allowed_signers_file\n"
+		"       ssh-keygen -Y match-principals -I signer_identity -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"
@@ -3442,6 +3469,19 @@ main(int argc, char **argv)
 			}
 			return sig_find_principals(ca_key_path, identity_file,
 			    opts, nopts);
+		} else if (strncmp(sign_op, "match-principals", 16) == 0) {
+			if (!have_identity) {
+				error("Too few arguments for match-principals:"
+				    "missing allowed keys file");
+				exit(1);
+			}
+			if (cert_key_id == NULL) {
+				error("Too few arguments for match-principals: "
+				    "missing principal ID");
+				exit(1);
+			}
+			return sig_match_principals(identity_file, cert_key_id,
+			    opts, nopts);
 		} else if (strncmp(sign_op, "sign", 4) == 0) {
 			if (cert_principals == NULL ||
 			    *cert_principals == '\0') {
diff --git a/usr.bin/ssh/sshsig.c b/usr.bin/ssh/sshsig.c
index a3a0ba17561..304c4282173 100644
--- a/usr.bin/ssh/sshsig.c
+++ b/usr.bin/ssh/sshsig.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshsig.c,v 1.23 2021/11/18 03:50:41 djm Exp $ */
+/* $OpenBSD: sshsig.c,v 1.24 2021/11/27 07:14:46 djm Exp $ */
 /*
  * Copyright (c) 2019 Google LLC
  *
@@ -1049,6 +1049,76 @@ sshsig_find_principals(const char *path, const struct sshkey *sign_key,
 }
 
 int
+sshsig_match_principals(const char *path, const char *principal,
+    char ***principalsp, size_t *nprincipalsp)
+{
+	FILE *f = NULL;
+	char *found, *line = NULL, **principals = NULL, **tmp;
+	size_t i, nprincipals = 0, linesize = 0;
+	u_long linenum = 0;
+	int oerrno, r, ret = 0;
+
+	if (principalsp != NULL)
+		*principalsp = NULL;
+	if (nprincipalsp != NULL)
+		*nprincipalsp = 0;
+
+	/* Check key and principal against file */
+	if ((f = fopen(path, "r")) == NULL) {
+		oerrno = errno;
+		error("Unable to open allowed keys file \"%s\": %s",
+		    path, strerror(errno));
+		errno = oerrno;
+		return SSH_ERR_SYSTEM_ERROR;
+	}
+
+	while (getline(&line, &linesize, f) != -1) {
+		linenum++;
+		/* Parse the line */
+		if ((r = parse_principals_key_and_options(path, linenum, line,
+		    principal, &found, NULL, NULL)) != 0) {
+			if (r == SSH_ERR_KEY_NOT_FOUND)
+				continue;
+			ret = r;
+			oerrno = errno;
+			break; /* unexpected error */
+		}
+		if ((tmp = recallocarray(principals, nprincipals,
+		    nprincipals + 1, sizeof(*principals))) == NULL) {
+			ret = SSH_ERR_ALLOC_FAIL;
+			free(found);
+			break;
+		}
+		principals = tmp;
+		principals[nprincipals++] = found; /* transferred */
+		free(line);
+		line = NULL;
+		linesize = 0;
+	}
+	fclose(f);
+
+	if (ret == 0) {
+		if (nprincipals == 0)
+			ret = SSH_ERR_KEY_NOT_FOUND;
+		if (principalsp != NULL) {
+			*principalsp = principals;
+			principals = NULL; /* transferred */
+		}
+		if (nprincipalsp != 0) {
+			*nprincipalsp = nprincipals;
+			nprincipals = 0;
+		}
+	}
+
+	for (i = 0; i < nprincipals; i++)
+		free(principals[i]);
+	free(principals);
+
+	errno = oerrno;
+	return ret;
+}
+
+int
 sshsig_get_pubkey(struct sshbuf *signature, struct sshkey **pubkey)
 {
 	struct sshkey *pk = NULL;
diff --git a/usr.bin/ssh/sshsig.h b/usr.bin/ssh/sshsig.h
index b725c7d7acd..ac5577962fb 100644
--- a/usr.bin/ssh/sshsig.h
+++ b/usr.bin/ssh/sshsig.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshsig.h,v 1.10 2021/07/23 03:37:52 djm Exp $ */
+/* $OpenBSD: sshsig.h,v 1.11 2021/11/27 07:14:46 djm Exp $ */
 /*
  * Copyright (c) 2019 Google LLC
  *
@@ -104,4 +104,8 @@ int sshsig_get_pubkey(struct sshbuf *signature, struct sshkey **pubkey);
 int sshsig_find_principals(const char *path, const struct sshkey *sign_key,
     uint64_t verify_time, char **principal);
 
+/* Find all principals in allowed_keys file matching *principal */
+int sshsig_match_principals(const char *path,
+	const char *principal, char ***principalsp, size_t *nprincipalsp);
+
 #endif /* SSHSIG_H */