[PATCH v6 5/9] ssh signing: parse ssh-keygen output and verify signatures

From

Fabian Stelzer via GitGitGadget <gitgitgadget@gmail.com>

Date

Jul 28, 2021, 19:36 UTC

Message-ID

<725764018ceb5bcecc748cc5169d4305ea9d7d23.1627501009.git.gitgitgadget@gmail.com>

In-Reply-To

<pull.1041.v6.git.git.1627501009.gitgitgadget@gmail.com>

From: Fabian Stelzer <fs@gigacodes.de>

Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
---
 builtin/receive-pack.c |   2 +
 gpg-interface.c        | 179 ++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 180 insertions(+), 1 deletion(-)

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 ec48a37b6cc..703225c3cd3 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 +349,165 @@ 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 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 *buffer_file;
+	int ret = -1;
+	const char *line;
+	size_t trust_size;
+	char *principal;
+	struct strbuf ssh_keygen_out = STRBUF_INIT;
+	struct strbuf ssh_keygen_err = STRBUF_INIT;
+
+	if (!ssh_allowed_signers) {
+		error(_("gpg.ssh.allowedSignersFile needs to be configured and exist for ssh signature verification"));
+		return -1;
+	}
+
+	buffer_file = mks_tempfile_t(".git_vtag_tmpXXXXXX");
+	if (!buffer_file)
+		return error_errno(_("could not create temporary file"));
+	if (write_in_full(buffer_file->fd, signature, signature_size) < 0 ||
+	    close_tempfile_gently(buffer_file) < 0) {
+		error_errno(_("failed writing detached signature to '%s'"),
+			    buffer_file->filename.buf);
+		delete_tempfile(&buffer_file);
+		return -1;
+	}
+
+	/* Find the principal from the signers */
+	strvec_pushl(&ssh_keygen.args, fmt->program,
+		     "-Y", "find-principals",
+		     "-f", ssh_allowed_signers,
+		     "-s", buffer_file->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+)"));
+		goto out;
+	}
+	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", buffer_file->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", ssh_allowed_signers,
+				     "-I", principal,
+				     "-s", buffer_file->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);
+
+out:
+	if (buffer_file)
+		delete_tempfile(&buffer_file);
+	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 +618,18 @@ 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";
 

-- 
gitgitgadget