Re: [PATCH v2] Add commit, tag & push signing/verification via SSH keys using ssh-keygen

On 12.07.21 18:55, Ævar Arnfjörð Bjarmason wrote:

I'll change all the whitespace / comments / style issues with the next 
commit. Thanks

quoted

+		sigc->result = 'G';
+		sigc->trust_level = TRUST_FULLY;
+
+		next = strchrnul(output, ' '); // 'principal'
+		replace_cstring(&sigc->signer, output, next);
+		output = next + 1;
+		next = strchrnul(output, ' '); // 'with'
+		output = next + 1;
+		next = strchrnul(output, ' '); // KEY Type
+		output = next + 1;
+		next = strchrnul(output, ' '); // 'key'
+		output = next + 1;

FWIW for new code we'd probably use string_list_split() or
string_list_split_in_place() or strbuf_split_buf() or something, but I
see this is following the existing pattern in the file...

I agree. This is my first patch in the git codebase so it takes a bit 
getting used to all the available utilities.

quoted

+	const char *payload, size_t payload_size,
+	const char *signature, size_t signature_size)
  {
-	struct child_process gpg = CHILD_PROCESS_INIT;
-	struct gpg_format *fmt;
+	struct child_process ssh_keygen = CHILD_PROCESS_INIT;
  	struct tempfile *temp;
  	int ret;
-	struct strbuf buf = STRBUF_INIT;
+	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)

@@ -279,29 +342,125 @@ static int verify_signed_buffer(const char *payload, size_t payload_size,
  		return -1;
  	}
  
-	fmt = get_format_by_sig(signature);
-	if (!fmt)
-		BUG("bad signature '%s'", signature);
+	// 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, "unknown option")) {
+		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')) {

Hrm, can't we use strbuf_getline() here with the underlying io_pump API
that pipe_command() uses, instead of slurping it all up, and then
splitting on '\n' ourselves? (I'm not sure)

Sounds good. I'll give it a try.

quoted

+			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",
+							//TODO: sprintf("-Overify-time=%s", commit->date...),
+							"-n", "git",
+							"-f", get_ssh_allowed_signers(),
+							"-I", principal,
+							"-s", temp->filename.buf,
+							NULL);
+
+			if (ssh_revocation_file && file_exists(ssh_revocation_file)) {
+				strvec_pushl(&ssh_keygen.args, "-r", ssh_revocation_file, NULL);

Do we want to silently ignore missing but configured revocation files?

I'll add a warning

quoted

+			}
+
+			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_output = strbuf_detach(&ssh_keygen_err, NULL); // This flip around is broken...

Broken how? And why the commented-out code as part of the patch?

Sorry, i should have removed it. The original code assigned gpg's stdout 
to gpg_status and stdout to gpg_output which can be a bit confusing.

quoted

-	status = verify_signed_buffer(payload, plen, signature, slen,
-				      &gpg_output, &gpg_status);
-	if (status && !gpg_output.len)
-		goto out;
-	sigc->payload = xmemdupz(payload, plen);
-	sigc->gpg_output = strbuf_detach(&gpg_output, NULL);
-	sigc->gpg_status = strbuf_detach(&gpg_status, NULL);
-	parse_gpg_output(sigc);
+	fmt = get_format_by_sig(signature);
+	if (!fmt)
+		BUG("bad signature '%s'", signature);

So if we run this from receive-pack or whatever we'll BUG() out? I.e. I
think this should be an fsck check or something, but not a BUG(), or
does this not rely on potentially bad object-store state?

The BUG() call is also from the original code. I agree that it should be 
handled differently.
Unfortunately this call is also the reason that when trying to verify a 
new SSH signature with a current git version you'll get a segfault from 
this BUG() :/
I'm not sure if i can do anything about this other than adding a 
completely new tag in the commit itself instead of "gpgsig" which might 
be quite involved. I haven't looked into that too much yet.

quoted

+static char *get_ssh_key_fingerprint(const char *signing_key) {
+	struct child_process ssh_keygen = CHILD_PROCESS_INIT;
+	int ret = -1;
+	struct strbuf fingerprint_stdout = STRBUF_INIT;
+	struct strbuf **fingerprint;
+
+	/* For SSH Signing this can contain a filename or a public key
+	* For textual representation we usually want a fingerprint
+	*/
+	if (istarts_with(signing_key, "ssh-")) {
+		strvec_pushl(&ssh_keygen.args, "ssh-keygen",
+					"-lf", "-",
+					NULL);
+		ret = pipe_command(&ssh_keygen, signing_key, strlen(signing_key), &fingerprint_stdout, 0,  NULL, 0);
+	} else {
+		strvec_pushl(&ssh_keygen.args, "ssh-keygen",
+					"-lf", configured_signing_key,
+					NULL);
+		ret = pipe_command(&ssh_keygen, NULL, 0, &fingerprint_stdout, 0, NULL, 0);
+		if (!!ret)
+			die_errno(_("failed to get the ssh fingerprint for key '%s'"), signing_key);
+		fingerprint = strbuf_split_max(&fingerprint_stdout, ' ', 3);
+		if (fingerprint[1]) {
+			return strbuf_detach(fingerprint[1], NULL);
+		}
+	}
+	die_errno(_("failed to get the ssh fingerprint for key '%s'"), signing_key);
+}

Her you declare a ret that's not used at all in the "istarts_with"
branch, and we fall through to die_errno()?

I'll clean up the logic. Thanks

[I stopped reading mostly at this point]

quoted

[...]
+# test_expect_success GPGSSH 'detect fudged commit with double signature' '
+# 	sed -e "/gpgsig/,/END PGP/d" forged1 >double-base &&
+# 	sed -n -e "/gpgsig/,/END PGP/p" forged1 | \
+# 		sed -e "s/^$(test_oid header)//;s/^ //" | gpg --dearmor >double-sig1.sig &&
+# 	gpg -o double-sig2.sig -u 29472784 --detach-sign double-base &&
+# 	cat double-sig1.sig double-sig2.sig | gpg --enarmor >double-combined.asc &&
+# 	sed -e "s/^\(-.*\)ARMORED FILE/\1SIGNATURE/;1s/^/$(test_oid header) /;2,\$s/^/ /" \
+# 		double-combined.asc > double-gpgsig &&
+# 	sed -e "/committer/r double-gpgsig" double-base >double-commit &&
+# 	git hash-object -w -t commit double-commit >double-commit.commit &&
+# 	test_must_fail git verify-commit $(cat double-commit.commit) &&
+# 	git show --pretty=short --show-signature $(cat double-commit.commit) >double-actual &&
+# 	grep "BAD signature from" double-actual &&
+# 	grep "Good signature from" double-actual
+# '
+
+# test_expect_success GPGSSH 'show double signature with custom format' '
+# 	cat >expect <<-\EOF &&
+# 	E
+
+
+
+
+# 	EOF
+# 	git log -1 --format="%G?%n%GK%n%GS%n%GF%n%GP" $(cat double-commit.commit) >actual &&
+# 	test_cmp expect actual
+# '

Perhaps you're looking for test_expect_failure for TODO tests?

Yes. Although this test explicitly i'm having a hard time to duplicate 
for ssh. I'm still trying to find out if the duplicate signature thing 
is actually an issue with ssh.

I think this patch is *way* past the point of benefitting from being
split into a patch series. It grew from ~200 lines added to ~1k.

Sure, I can easily split the patch into seperate commits. But do i 
create a v3 patch from this or issue a new pull request?
The diff between v2 & v3 would be quite useless otherwise wouldn't it?

And maybe another beginner contribution question:

When i make changes to a patchset do i put new changes from the review 
on top as new commits or do i edit the existing commits?
If so what is the workflow you normally use for this? fixup commits? I 
know about those but haven't worked with them before.

Thanks for your help!