[PATCH v3 6/9] ssh signing: parse ssh-keygen output and verify signatures

["Fabian Stelzer via GitGitGadget" <gitgitgadget@gmail.com>]

From: Fabian Stelzer <fs@gigacodes.de>

Verification uses the gpg.ssh.keyring file (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 keyring file 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 <fs@gigacodes.de>
---
 builtin/receive-pack.c |   2 +
 gpg-interface.c        | 139 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 141 insertions(+)

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 328af86c272..1be88b87d96 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -3,6 +3,7 @@
 #include "config.h"
 #include "run-command.h"
 #include "strbuf.h"
+#include "dir.h"
 #include "gpg-interface.h"
 #include "sigchain.h"
 #include "tempfile.h"
@@ -156,6 +157,42 @@ static int parse_gpg_trust_level(const char *level,
 	return 1;
 }
 
+static void parse_ssh_output(struct signature_check *sigc)
+{
+	struct string_list parts = STRING_LIST_INIT_DUP;
+	char *line = NULL;
+
+	/*
+	 * ssh-keysign output should be:
+	 * Good "git" signature for PRINCIPAL 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"));
+	string_list_split(&parts, line, ' ', 8);
+	if (parts.nr >= 9 && starts_with(line, "Good \"git\" signature for ")) {
+		/* Valid signature for a trusted signer */
+		sigc->result = 'G';
+		sigc->trust_level = TRUST_FULLY;
+		sigc->signer = xstrdup(parts.items[4].string);
+		sigc->fingerprint = xstrdup(parts.items[8].string);
+		sigc->key = xstrdup(sigc->fingerprint);
+	} else if (parts.nr >= 7 && starts_with(line, "Good \"git\" signature with ")) {
+		/* Valid signature, but key unknown */
+		sigc->result = 'G';
+		sigc->trust_level = TRUST_UNDEFINED;
+		sigc->fingerprint = xstrdup(parts.items[6].string);
+		sigc->key = xstrdup(sigc->fingerprint);
+	}
+	trace_printf("trace: sigc result %c/%d - %s %s %s", sigc->result, sigc->trust_level, sigc->signer, sigc->fingerprint, sigc->key);
+
+	string_list_clear(&parts, 0);
+	FREE_AND_NULL(line);
+}
+
 static void parse_gpg_output(struct signature_check *sigc)
 {
 	const char *buf = sigc->gpg_status;
@@ -269,6 +306,108 @@ error:
 	FREE_AND_NULL(sigc->key);
 }
 
+static int verify_ssh_signature(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 (strstr(ssh_keygen_err.buf, "usage:")) {
+		error(_("openssh version > 8.2p1 is needed for ssh signature verification (ssh-keygen needs -Y find-principals/verify option)"));
+	}
+	if (ret || !ssh_keygen_out.len) {
+		/* We did not find a matching principal in the keyring - 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);
+
+			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;
+}
+
 static int verify_gpg_signature(struct signature_check *sigc, struct gpg_format *fmt,
 	const char *payload, size_t payload_size,
 	const char *signature, size_t signature_size)
-- 
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