Subject: [PATCH] merge/pull: verify GPG signatures of commits being merged

From: Sebastian Götte <[email-masked]>
Date: Fri, 22 Mar 2013 22:51:40 +0100

git merge/pull:
When --verify-signatures is specified on the command-line of git-merge
or git-pull, check whether the commits being merged have good gpg
signatures and abort the merge in case they do not. This allows e.g.
auto-deployment from untrusted repo hosts.

pretty printing:
Tell about an "untrusted good signature" in addition to the previous
"good signature" and "bad signature". In case of a missing signature,
expand the pretty format string "%G?" to "N" in since this eases the
wirting of anything parsing git-log output.

Signed-off-by: Sebastian Götte <[email-masked]>
---
I moved the commit signature verification code from pretty.c to commit.c
because it is used from pretty.c and builtin/merge.c. I include that pretty
printing change here because I needed to add the good/untrusted check for the
merge --verify-signatures option to really make sense.

Those new %G? options are implicitly tested by t7612-merge-verify-signatures.sh
because %G? is just replaced by the passed-through output of the commit
verification function.

 Documentation/merge-options.txt    |   4 +++
 Documentation/pretty-formats.txt   |   2 +-
 builtin/merge.c                    |  33 +++++++++++++++++-
 commit.c                           |  55 +++++++++++++++++++++++++++++
 commit.h                           |   9 +++++
 git-pull.sh                        |  10 ++++--
 gpg-interface.h                    |   6 ++++
 pretty.c                           |  69 ++++---------------------------------
 t/lib-gpg/pubring.gpg              | Bin 1164 -> 2359 bytes
 t/lib-gpg/random_seed              | Bin 600 -> 600 bytes
 t/lib-gpg/secring.gpg              | Bin 1237 -> 3734 bytes
 t/lib-gpg/trustdb.gpg              | Bin 1280 -> 1360 bytes
 t/t7612-merge-verify-signatures.sh |  61 ++++++++++++++++++++++++++++++++
 13 files changed, 183 insertions(+), 66 deletions(-)

diff --git a/Documentation/merge-options.txt b/Documentation/merge-options.txt
index 0bcbe0a..2f76ab5 100644
--- a/Documentation/merge-options.txt
+++ b/Documentation/merge-options.txt
@@ -83,6 +83,10 @@ option can be used to override --squash.
 	Pass merge strategy specific option through to the merge
 	strategy.
 
+--verify-signatures::
+--no-verify-signatures::
+	Verify that the commits being merged have good trusted GPG signatures
+
 --summary::
 --no-summary::
 	Synonyms to --stat and --no-stat; these are deprecated and will be
diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 105f18a..7297b1b 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -131,7 +131,7 @@ The placeholders are:
 - '%B': raw body (unwrapped subject and body)
 - '%N': commit notes
 - '%GG': raw verification message from GPG for a signed commit
-- '%G?': show either "G" for Good or "B" for Bad for a signed commit
+- '%G?': show "G" for a Good signature, "B" for a Bad signature, "U" for a good, untrusted signature and "N" for no signature
 - '%GS': show the name of the signer for a signed commit
 - '%gD': reflog selector, e.g., `refs/stash@{1}`
 - '%gd': shortened reflog selector, e.g., `stash@{1}`
diff --git a/builtin/merge.c b/builtin/merge.c
index 7c8922c..37ece3d 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -49,7 +49,7 @@ static const char * const builtin_merge_usage[] = {
 static int show_diffstat = 1, shortlog_len = -1, squash;
 static int option_commit = 1, allow_fast_forward = 1;
 static int fast_forward_only, option_edit = -1;
-static int allow_trivial = 1, have_message;
+static int allow_trivial = 1, have_message, verify_signatures = 0;
 static int overwrite_ignore = 1;
 static struct strbuf merge_msg = STRBUF_INIT;
 static struct strategy **use_strategies;
@@ -199,6 +199,8 @@ static struct option builtin_merge_options[] = {
 	OPT_BOOLEAN(0, "ff-only", &fast_forward_only,
 		N_("abort if fast-forward is not possible")),
 	OPT_RERERE_AUTOUPDATE(&allow_rerere_auto),
+	OPT_BOOLEAN(0, "verify-signatures", &verify_signatures,
+		N_("Verify that the named commit has a valid GPG signature")),
 	OPT_CALLBACK('s', "strategy", &use_strategies, N_("strategy"),
 		N_("merge strategy to use"), option_parse_strategy),
 	OPT_CALLBACK('X', "strategy-option", &xopts, N_("option=value"),
@@ -1233,6 +1235,35 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		usage_with_options(builtin_merge_usage,
 			builtin_merge_options);
 
+	if (verify_signatures) {
+		//Verify the commit signatures
+		for (p = remoteheads; p; p = p->next) {
+			struct commit *commit = p->item;
+			struct signature signature;
+			signature.check_result = 0;
+
+			check_commit_signature(commit, &signature);
+			
+			char hex[41];
+			strcpy(hex, find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV));
+			switch(signature.check_result){
+				case 'G':
+					if (verbosity >= 0)
+						printf(_("Commit %s has a good GPG signature by %s\n"), hex, signature.signer);
+					break;
+				case 'U':
+					die(_("Commmit %s has a good GPG signature allegedly by %s, albeit from an untrusted key."), hex, signature.signer);
+				case 'B':
+					die(_("Commmit %s has a bad GPG signature allegedly by %s."), hex, signature.signer);
+				default: //'N'
+					die(_("Commmit %s does not have a good GPG signature. In fact, commit %s does not have a GPG signature at all."), hex, hex);
+			}
+
+			free(signature.gpg_output);
+			free(signature.signer);
+		}
+	}
+
 	strbuf_addstr(&buf, "merge");
 	for (p = remoteheads; p; p = p->next)
 		strbuf_addf(&buf, " %s", merge_remote_util(p->item)->name);
diff --git a/commit.c b/commit.c
index e8eb0ae..300cf98 100644
--- a/commit.c
+++ b/commit.c
@@ -1023,6 +1023,61 @@ free_return:
 	free(buf);
 }
 
+static struct {
+	char result;
+	const char *check;
+} signature_check[] = {
+	{ 'U', ": WARNING: This key is not certified with a trusted signature!" },
+	{ 'G', ": Good signature from " },
+	{ 'B', ": BAD signature from " },
+};
+
+static void parse_signature_lines(struct signature *sig)
+{
+	const char *buf = sig->gpg_output;
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(signature_check); i++) {
+		const char *found = strstr(buf, signature_check[i].check);
+		const char *next;
+		if (!found)
+			continue;
+		sig->check_result = signature_check[i].result;
+		found += strlen(signature_check[i].check);
+		next = strchrnul(found, '\n');
+		sig->signer = xmemdupz(found, next - found);
+		break;
+	}
+}
+
+void check_commit_signature(const struct commit* commit, struct signature *sig)
+{
+	struct strbuf payload = STRBUF_INIT;
+	struct strbuf signature = STRBUF_INIT;
+	struct strbuf gpg_output = STRBUF_INIT;
+	int status;
+
+	sig->check_result = 'N';
+
+	if (parse_signed_commit(commit->object.sha1,
+				&payload, &signature) <= 0)
+		goto out;
+	status = verify_signed_buffer(payload.buf, payload.len,
+				      signature.buf, signature.len,
+				      &gpg_output);
+	if (status && !gpg_output.len)
+		goto out;
+	sig->gpg_output = strbuf_detach(&gpg_output, NULL);
+	parse_signature_lines(sig);
+
+ out:
+	strbuf_release(&gpg_output);
+	strbuf_release(&payload);
+	strbuf_release(&signature);
+}
+
+
+
 void append_merge_tag_headers(struct commit_list *parents,
 			      struct commit_extra_header ***tail)
 {
diff --git a/commit.h b/commit.h
index 4138bb4..2189eb6 100644
--- a/commit.h
+++ b/commit.h
@@ -5,6 +5,7 @@
 #include "tree.h"
 #include "strbuf.h"
 #include "decorate.h"
+#include "gpg-interface.h"
 
 struct commit_list {
 	struct commit *item;
@@ -230,4 +231,12 @@ extern void print_commit_list(struct commit_list *list,
 			      const char *format_cur,
 			      const char *format_last);
 
+/*
+ * Check the signature of the given commit. The result of the check is stored in
+ * sig->check_result, 'G' for a good signature, 'U' for a good signature from an
+ * untrusted signer, 'B' for a bad signature and 'N' for no signature at all.
+ * This may allocate memory for sig->gpg_output and sig->signer.
+ */
+extern void check_commit_signature(const struct commit* commit, struct signature *sig);
+
 #endif /* COMMIT_H */
diff --git a/git-pull.sh b/git-pull.sh
index 266e682..705940d 100755
--- a/git-pull.sh
+++ b/git-pull.sh
@@ -39,7 +39,7 @@ test -z "$(git ls-files -u)" || die_conflict
 test -f "$GIT_DIR/MERGE_HEAD" && die_merge
 
 strategy_args= diffstat= no_commit= squash= no_ff= ff_only=
-log_arg= verbosity= progress= recurse_submodules=
+log_arg= verbosity= progress= recurse_submodules= verify_signatures=
 merge_args= edit=
 curr_branch=$(git symbolic-ref -q HEAD)
 curr_branch_short="${curr_branch#refs/heads/}"
@@ -125,6 +125,12 @@ do
 	--no-recurse-submodules)
 		recurse_submodules=--no-recurse-submodules
 		;;
+	--verify-signatures)
+		verify_signatures=--verify-signatures
+		;;
+	--no-verify-signatures)
+		verify_signatures=--no-verify-signatures
+		;;
 	--d|--dr|--dry|--dry-|--dry-r|--dry-ru|--dry-run)
 		dry_run=--dry-run
 		;;
@@ -283,7 +289,7 @@ true)
 	eval="$eval --onto $merge_head ${oldremoteref:-$merge_head}"
 	;;
 *)
-	eval="git-merge $diffstat $no_commit $edit $squash $no_ff $ff_only"
+	eval="git-merge $diffstat $no_commit $verify_signatures $edit $squash $no_ff $ff_only"
 	eval="$eval  $log_arg $strategy_args $merge_args $verbosity $progress"
 	eval="$eval \"\$merge_name\" HEAD $merge_head"
 	;;
diff --git a/gpg-interface.h b/gpg-interface.h
index b9c3608..3895b2f 100644
--- a/gpg-interface.h
+++ b/gpg-interface.h
@@ -1,6 +1,12 @@
 #ifndef GPG_INTERFACE_H
 #define GPG_INTERFACE_H
 
+struct signature {
+	char *gpg_output;
+	char check_result; // 0 (not checked), N (checked but no further result), U (untrusted, good), G (good) or B (bad)
+	char *signer;
+};
+
 extern int sign_buffer(struct strbuf *buffer, struct strbuf *signature, const char *signing_key);
 extern int verify_signed_buffer(const char *payload, size_t payload_size, const char *signature, size_t signature_size, struct strbuf *gpg_output);
 extern int git_gpg_config(const char *, const char *, void *);
diff --git a/pretty.c b/pretty.c
index eae57ad..875a1db 100644
--- a/pretty.c
+++ b/pretty.c
@@ -756,12 +756,7 @@ struct format_commit_context {
 	const struct pretty_print_context *pretty_ctx;
 	unsigned commit_header_parsed:1;
 	unsigned commit_message_parsed:1;
-	unsigned commit_signature_parsed:1;
-	struct {
-		char *gpg_output;
-		char good_bad;
-		char *signer;
-	} signature;
+	struct signature signature;
 	char *message;
 	size_t width, indent1, indent2;
 
@@ -944,58 +939,6 @@ static void rewrap_message_tail(struct strbuf *sb,
 	c->indent2 = new_indent2;
 }
 
-static struct {
-	char result;
-	const char *check;
-} signature_check[] = {
-	{ 'G', ": Good signature from " },
-	{ 'B', ": BAD signature from " },
-};
-
-static void parse_signature_lines(struct format_commit_context *ctx)
-{
-	const char *buf = ctx->signature.gpg_output;
-	int i;
-
-	for (i = 0; i < ARRAY_SIZE(signature_check); i++) {
-		const char *found = strstr(buf, signature_check[i].check);
-		const char *next;
-		if (!found)
-			continue;
-		ctx->signature.good_bad = signature_check[i].result;
-		found += strlen(signature_check[i].check);
-		next = strchrnul(found, '\n');
-		ctx->signature.signer = xmemdupz(found, next - found);
-		break;
-	}
-}
-
-static void parse_commit_signature(struct format_commit_context *ctx)
-{
-	struct strbuf payload = STRBUF_INIT;
-	struct strbuf signature = STRBUF_INIT;
-	struct strbuf gpg_output = STRBUF_INIT;
-	int status;
-
-	ctx->commit_signature_parsed = 1;
-
-	if (parse_signed_commit(ctx->commit->object.sha1,
-				&payload, &signature) <= 0)
-		goto out;
-	status = verify_signed_buffer(payload.buf, payload.len,
-				      signature.buf, signature.len,
-				      &gpg_output);
-	if (status && !gpg_output.len)
-		goto out;
-	ctx->signature.gpg_output = strbuf_detach(&gpg_output, NULL);
-	parse_signature_lines(ctx);
-
- out:
-	strbuf_release(&gpg_output);
-	strbuf_release(&payload);
-	strbuf_release(&signature);
-}
-
 
 static int format_reflog_person(struct strbuf *sb,
 				char part,
@@ -1182,18 +1125,20 @@ static size_t format_commit_one(struct strbuf *sb, const char *placeholder,
 	}
 
 	if (placeholder[0] == 'G') {
-		if (!c->commit_signature_parsed)
-			parse_commit_signature(c);
+		if (!c->signature.check_result)
+			check_commit_signature(c->commit, &(c->signature));
 		switch (placeholder[1]) {
 		case 'G':
 			if (c->signature.gpg_output)
 				strbuf_addstr(sb, c->signature.gpg_output);
 			break;
 		case '?':
-			switch (c->signature.good_bad) {
+			switch (c->signature.check_result) {
 			case 'G':
 			case 'B':
-				strbuf_addch(sb, c->signature.good_bad);
+			case 'U':
+			case 'N':
+				strbuf_addch(sb, c->signature.check_result);
 			}
 			break;
 		case 'S':
diff --git a/t/lib-gpg/pubring.gpg b/t/lib-gpg/pubring.gpg
index 83855fa4e1c6c37afe550c17afa1e7971042ded5..1a3c2d487c2fda9169751a3068fa51e853a1e519 100644
GIT binary patch
delta 1212
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delta 7
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diff --git a/t/lib-gpg/random_seed b/t/lib-gpg/random_seed
index 8fed1339ed0a744e5663f4a5e6b6ac9bae3d8524..95d249f15fce980f0e8c1a8a18b085b3885708aa 100644
GIT binary patch
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diff --git a/t/lib-gpg/secring.gpg b/t/lib-gpg/secring.gpg
index d831cd9eb3eee613d3c0e1a71093ae01ea7347e3..82dca8f80bf170fde5705862c3eeb9d994725042 100644
GIT binary patch
delta 2524
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delta 7
OcmbOxdzEv;RTcmY+5;N^

diff --git a/t/lib-gpg/trustdb.gpg b/t/lib-gpg/trustdb.gpg
index abace962b8bf84be688a6f27e4ebd0ee7052f210..4879ae9a84650a93a4d15bd6560c5d1b89eb4c2f 100644
GIT binary patch
delta 133
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DE~Fa=

delta 52
zcmcb>)xagdm|l?1%*@Ej$iTqhmVVlAqM`Uk^-atZ9rz~(oS3|U#hLd&3{VON0Ad;p
ADF6Tf

diff --git a/t/t7612-merge-verify-signatures.sh b/t/t7612-merge-verify-signatures.sh
new file mode 100755
index 0000000..47a4f80
--- /dev/null
+++ b/t/t7612-merge-verify-signatures.sh
@@ -0,0 +1,61 @@
+#!/bin/sh
+
+test_description='merge signature verification tests'
+. ./test-lib.sh
+. "$TEST_DIRECTORY/lib-gpg.sh"
+
+test_expect_success GPG 'create signed commits' '
+	echo 1 >file && git add file &&
+	test_tick && git commit -m initial &&
+	git tag initial &&
+
+	git checkout -b side-signed &&
+	echo 3 >elif && git add elif &&
+	test_tick && git commit -S -m "signed on side" &&
+	git checkout initial &&
+
+	git checkout -b side-unsigned &&
+	echo 3 >foo && git add foo &&
+	test_tick && git commit -m "unsigned on side" &&
+	git checkout initial &&
+
+	git checkout -b side-bad &&
+	echo 3 >bar && git add bar &&
+	test_tick && git commit -S -m "bad on side" &&
+	git cat-file commit side-bad >raw &&
+	sed -e "s/bad/forged bad/" raw >forged &&
+	git hash-object -w -t commit forged >forged.commit &&
+	git checkout initial &&
+
+	git checkout -b side-untrusted &&
+	echo 3 >baz && git add baz &&
+	test_tick && git commit -SB7227189 -m "untrusted on side"
+
+	git checkout master
+'
+
+test_expect_success GPG 'merge unsigned commit with verification' '
+	test_must_fail git merge --ff-only --verify-signatures side-unsigned 2> mergeerror &&
+	grep "does not have a GPG signature" mergeerror
+'
+
+test_expect_success GPG 'merge commit with bad signature with verification' '
+	test_must_fail git merge --ff-only --verify-signatures $(cat forged.commit) 2> mergeerror &&
+	grep "has a bad GPG signature" mergeerror
+'
+
+test_expect_success GPG 'merge  commit with untrusted signature with verification' '
+	test_must_fail git merge --ff-only --verify-signatures side-untrusted 2> mergeerror &&
+	grep "from an untrusted key" mergeerror
+'
+
+test_expect_success GPG 'merge signed commit with verification' '
+	git merge -v --ff-only --verify-signatures side-signed > mergeoutput &&
+	grep "has a good GPG signature" mergeoutput
+'
+
+test_expect_success GPG 'merge commit with bad signature without verification' '
+	git merge $(cat forged.commit)
+'
+
+test_done
-- 
1.8.1.5