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

From: Junio C Hamano <[email-masked]>
Date: Wed, 28 Jul 2021 14:55:00 -0700

"Fabian Stelzer via GitGitGadget" <[email-masked]> writes:

> From: Fabian Stelzer <[email-masked]>
>
> to verify a ssh signature we first call ssh-keygen -Y find-principal to

"to" -> "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 <[email-masked]>
> ---
>  builtin/receive-pack.c |   2 +
>  gpg-interface.c        | 179 ++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 180 insertions(+), 1 deletion(-)

A lot of additions to support a new system, all looking quite
straight-forward.

> @@ -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
>  	},
>  };

Nice.

> @@ -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

A bit unfortunate line that is overly long.  These two are not
mutually exclusive two different choices, but one is a special case
of the other, no?  How about phrasing it like so instead?

	/*
	 * 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
         *
	 * Note that "PRINCIPAL" can contain whitespace, "RSA" and
	 * "SHA256" part could be a different token that names of
	 * the algorithms used, and "FINGERPRINT" is a hexadecimal
         * string.  By finding the last occurence of " with ", we can
         * reliably parse out the PRINCIPAL.
	 */

> +	 * 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);

OK.  This does not care the "RSA" part, which is future resistant.
It assumes the <algo>:<fingerprint> comes after literal " key ",
which I think is a reasonable thing to do.

However, we never checked if the line has "key" in it, so
strstr(line, "key") + 4 may not be pointing at where this code
expects.

> +		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);

Likewise, I guess.

> +	}
> +}
> +
> +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 */

                        /*
                         * Do not forget our multi-line comment
                         * style, please.
                         */

> +			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");

This is somewhat unusual construct in our codebase, I suspect.  And
probably is even wrong.  Didn't you mean

			if (!ret)
				ret = starts_with(...);

instead?  Surely, when pipe_command() failed, it is likely that
ssh_keygen_out may not have anything useful, and checking what the
first up-to-four bytes of it contain unconditionally may be cheap
enough, but the person reading the code would expect you to peek
into the result only when you actually got the result, no?

> +			if (ret == 0)
> +				break;

It's more common to do

			if (!ret)
				break;

in our codebase; in other words, we prefer not to compare with
literal 0, like "if (x == 0)" or "if (y != 0)".

Thanks.