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

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

> @@ -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");