[v4,5/9] ssh signing: parse ssh-keygen output and verify signatures

Commit Message

Fabian Stelzer July 19, 2021, 1:33 p.m. UTC

From: Fabian Stelzer <[e-mail]>

to verify a ssh signature we first call ssh-keygen -Y find-principal to
look up the signing principal by their public key from the
allowedSignersFile. If the key is found then we do a verify. Otherwise
we only validate the signature but can not verify the signers identity.

Verification uses the gpg.ssh.allowedSignersFile (see ssh-keygen(1) "ALLOWED
SIGNERS") which contains valid public keys and a principal (usually
user@domain). Depending on the environment this file can be managed by
the individual developer or for example generated by the central
repository server from known ssh keys with push access. If the
repository only allows signed commits / pushes then the file can even be
stored inside it.

To revoke a key put the public key without the principal prefix into
gpg.ssh.revocationKeyring or generate a KRL (see ssh-keygen(1)
"KEY REVOCATION LISTS"). The same considerations about who to trust for
verification as with the allowedSignersFile apply.

Using SSH CA Keys with these files is also possible. Add
"cert-authority" as key option between the principal and the key to mark
it as a CA and all keys signed by it as valid for this CA.

Signed-off-by: Fabian Stelzer <[e-mail]>
---
 builtin/receive-pack.c |   2 +
 gpg-interface.c        | 174 ++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 175 insertions(+), 1 deletion(-)

Patch

diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index a34742513ac..62b11c5f3a4 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -131,6 +131,8 @@  static int receive_pack_config(const char *var, const char *value, void *cb)
 {
 	int status = parse_hide_refs_config(var, value, "receive");
 
+	git_gpg_config(var, value, NULL);
+
 	if (status)
 		return status;
 
diff --git a/gpg-interface.c b/gpg-interface.c
index 2c6eaf47d0f..761aa91d648 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -3,11 +3,13 @@ 
 #include "config.h"
 #include "run-command.h"
 #include "strbuf.h"
+#include "dir.h"
 #include "gpg-interface.h"
 #include "sigchain.h"
 #include "tempfile.h"
 
 static char *configured_signing_key;
+static const char *ssh_allowed_signers, *ssh_revocation_file;
 static enum signature_trust_level configured_min_trust_level = TRUST_UNDEFINED;
 
 struct gpg_format {
@@ -51,6 +53,10 @@  static int verify_gpg_signed_buffer(struct signature_check *sigc,
 				    struct gpg_format *fmt, const char *payload,
 				    size_t payload_size, const char *signature,
 				    size_t signature_size);
+static int verify_ssh_signed_buffer(struct signature_check *sigc,
+				    struct gpg_format *fmt, const char *payload,
+				    size_t payload_size, const char *signature,
+				    size_t signature_size);
 static int sign_buffer_gpg(struct strbuf *buffer, struct strbuf *signature,
 			   const char *signing_key);
 static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
@@ -78,7 +84,7 @@  static struct gpg_format gpg_format[] = {
 		.program = "ssh-keygen",
 		.verify_args = ssh_verify_args,
 		.sigs = ssh_sigs,
-		.verify_signed_buffer = NULL, /* TODO */
+		.verify_signed_buffer = verify_ssh_signed_buffer,
 		.sign_buffer = sign_buffer_ssh
 	},
 };
@@ -343,6 +349,160 @@  static int verify_gpg_signed_buffer(struct signature_check *sigc,
 	return ret;
 }
 
+static void parse_ssh_output(struct signature_check *sigc)
+{
+	const char *line, *principal, *search;
+
+	/*
+	 * ssh-keysign output should be:
+	 * Good "git" signature for PRINCIPAL with RSA key SHA256:FINGERPRINT
+	 * Good "git" signature for PRINCIPAL WITH WHITESPACE with RSA key SHA256:FINGERPRINT
+	 * or for valid but unknown keys:
+	 * Good "git" signature with RSA key SHA256:FINGERPRINT
+	 */
+	sigc->result = 'B';
+	sigc->trust_level = TRUST_NEVER;
+
+	line = xmemdupz(sigc->output, strcspn(sigc->output, "\n"));
+
+	if (skip_prefix(line, "Good \"git\" signature for ", &line)) {
+		/* Valid signature and known principal */
+		sigc->result = 'G';
+		sigc->trust_level = TRUST_FULLY;
+
+		/* Search for the last "with" to get the full principal */
+		principal = line;
+		do {
+			search = strstr(line, " with ");
+			if (search)
+				line = search + 1;
+		} while (search != NULL);
+		sigc->signer = xmemdupz(principal, line - principal - 1);
+		sigc->fingerprint = xstrdup(strstr(line, "key") + 4);
+		sigc->key = xstrdup(sigc->fingerprint);
+	} else if (skip_prefix(line, "Good \"git\" signature with ", &line)) {
+		/* Valid signature, but key unknown */
+		sigc->result = 'G';
+		sigc->trust_level = TRUST_UNDEFINED;
+		sigc->fingerprint = xstrdup(strstr(line, "key") + 4);
+		sigc->key = xstrdup(sigc->fingerprint);
+	}
+}
+
+static const char *get_ssh_allowed_signers(void)
+{
+	if (ssh_allowed_signers)
+		return ssh_allowed_signers;
+
+	die("gpg.ssh.allowedSignersFile needs to be configured and exist for validation");
+}
+
+static int verify_ssh_signed_buffer(struct signature_check *sigc,
+				    struct gpg_format *fmt, const char *payload,
+				    size_t payload_size, const char *signature,
+				    size_t signature_size)
+{
+	struct child_process ssh_keygen = CHILD_PROCESS_INIT;
+	struct tempfile *temp;
+	int ret;
+	const char *line;
+	size_t trust_size;
+	char *principal;
+	struct strbuf ssh_keygen_out = STRBUF_INIT;
+	struct strbuf ssh_keygen_err = STRBUF_INIT;
+
+	temp = mks_tempfile_t(".git_vtag_tmpXXXXXX");
+	if (!temp)
+		return error_errno(_("could not create temporary file"));
+	if (write_in_full(temp->fd, signature, signature_size) < 0 ||
+	    close_tempfile_gently(temp) < 0) {
+		error_errno(_("failed writing detached signature to '%s'"),
+			    temp->filename.buf);
+		delete_tempfile(&temp);
+		return -1;
+	}
+
+	/* Find the principal from the signers */
+	strvec_pushl(&ssh_keygen.args, fmt->program, "-Y", "find-principals",
+		     "-f", get_ssh_allowed_signers(), "-s", temp->filename.buf,
+		     NULL);
+	ret = pipe_command(&ssh_keygen, NULL, 0, &ssh_keygen_out, 0,
+			   &ssh_keygen_err, 0);
+	if (ret && strstr(ssh_keygen_err.buf, "usage:")) {
+		error(_("ssh-keygen -Y find-principals/verify is needed for ssh signature verification (available in openssh version 8.2p1+)"));
+		return ret;
+	}
+	if (ret || !ssh_keygen_out.len) {
+		/* We did not find a matching principal in the allowedSigners - Check
+		 * without validation */
+		child_process_init(&ssh_keygen);
+		strvec_pushl(&ssh_keygen.args, fmt->program, "-Y",
+			     "check-novalidate", "-n", "git", "-s",
+			     temp->filename.buf, NULL);
+		ret = pipe_command(&ssh_keygen, payload, payload_size,
+				   &ssh_keygen_out, 0, &ssh_keygen_err, 0);
+	} else {
+		/* Check every principal we found (one per line) */
+		for (line = ssh_keygen_out.buf; *line;
+		     line = strchrnul(line + 1, '\n')) {
+			while (*line == '\n')
+				line++;
+			if (!*line)
+				break;
+
+			trust_size = strcspn(line, "\n");
+			principal = xmemdupz(line, trust_size);
+
+			child_process_init(&ssh_keygen);
+			strbuf_release(&ssh_keygen_out);
+			strbuf_release(&ssh_keygen_err);
+			strvec_push(&ssh_keygen.args, fmt->program);
+			/* We found principals - Try with each until we find a
+			 * match */
+			strvec_pushl(&ssh_keygen.args, "-Y", "verify", "-n",
+				     "git", "-f", get_ssh_allowed_signers(),
+				     "-I", principal, "-s", temp->filename.buf,
+				     NULL);
+
+			if (ssh_revocation_file) {
+				if (file_exists(ssh_revocation_file)) {
+					strvec_pushl(&ssh_keygen.args, "-r",
+						     ssh_revocation_file, NULL);
+				} else {
+					warning(_("ssh signing revocation file configured but not found: %s"),
+						ssh_revocation_file);
+				}
+			}
+
+			sigchain_push(SIGPIPE, SIG_IGN);
+			ret = pipe_command(&ssh_keygen, payload, payload_size,
+					   &ssh_keygen_out, 0, &ssh_keygen_err, 0);
+			sigchain_pop(SIGPIPE);
+
+			FREE_AND_NULL(principal);
+
+			ret &= starts_with(ssh_keygen_out.buf, "Good");
+			if (ret == 0)
+				break;
+		}
+	}
+
+	sigc->payload = xmemdupz(payload, payload_size);
+	strbuf_stripspace(&ssh_keygen_out, 0);
+	strbuf_stripspace(&ssh_keygen_err, 0);
+	strbuf_add(&ssh_keygen_out, ssh_keygen_err.buf, ssh_keygen_err.len);
+	sigc->output = strbuf_detach(&ssh_keygen_out, NULL);
+	sigc->gpg_status = xstrdup(sigc->output);
+
+	parse_ssh_output(sigc);
+
+	delete_tempfile(&temp);
+	strbuf_release(&ssh_keygen_out);
+	strbuf_release(&ssh_keygen_err);
+
+	return ret;
+}
+
 int check_signature(const char *payload, size_t plen, const char *signature,
 	size_t slen, struct signature_check *sigc)
 {
@@ -453,6 +613,18 @@  int git_gpg_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp(var, "gpg.ssh.allowedsignersfile")) {
+		if (!value)
+			return config_error_nonbool(var);
+		return git_config_string(&ssh_allowed_signers, var, value);
+	}
+
+	if (!strcmp(var, "gpg.ssh.revocationFile")) {
+		if (!value)
+			return config_error_nonbool(var);
+		return git_config_string(&ssh_revocation_file, var, value);
+	}
+
 	if (!strcmp(var, "gpg.program") || !strcmp(var, "gpg.openpgp.program"))
 		fmtname = "openpgp";