[PATCH v5 3/5] merge/pull: verify GPG signatures of commits being merged

["Sebastian Götte" <jaseg@physik.tu-berlin.de>]

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.

Signed-off-by: Sebastian Götte <jaseg@physik-pool.tu-berlin.de>
---
 Documentation/merge-options.txt    |  5 ++++
 builtin/merge.c                    | 33 +++++++++++++++++++++++-
 git-pull.sh                        | 10 ++++++--
 t/t7612-merge-verify-signatures.sh | 52 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 97 insertions(+), 3 deletions(-)

diff --git a/Documentation/merge-options.txt b/Documentation/merge-options.txt
index 0bcbe0a..31f1067 100644
--- a/Documentation/merge-options.txt
+++ b/Documentation/merge-options.txt
@@ -83,6 +83,11 @@ 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 GPG signatures and abort the
+	merge in case they do not.
+
 --summary::
 --no-summary::
 	Synonyms to --stat and --no-stat; these are deprecated and will be
diff --git a/builtin/merge.c b/builtin/merge.c
index 7c8922c..cb3e9ea 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;
 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;
+			char hex[41];
+			struct signature_check signature_check;
+			memset(&signature_check, 0, sizeof(signature_check));
+
+			check_commit_signature(commit, &signature_check);
+
+			strcpy(hex, find_unique_abbrev(commit->object.sha1, DEFAULT_ABBREV));
+			switch(signature_check.check_result){
+				case 'G':
+					if (verbosity >= 0)
+						printf(_("Commit %s has a good GPG signature by %s (key fingerprint %s)\n"), hex, signature_check.signer, signature_check.key);
+					break;
+				case 'B':
+					die(_("Commit %s has a bad GPG signature allegedly by %s (key fingerprint %s)."), hex, signature_check.signer, signature_check.key);
+				default: /* 'N' */
+					die(_("Commit %s does not have a good GPG signature. In fact, commit %s does not have a GPG signature at all."), hex, hex);
+			}
+
+			free(signature_check.gpg_output);
+			free(signature_check.gpg_status);
+			free(signature_check.signer);
+			free(signature_check.key);
+		}
+	}
+
 	strbuf_addstr(&buf, "merge");
 	for (p = remoteheads; p; p = p->next)
 		strbuf_addf(&buf, " %s", merge_remote_util(p->item)->name);
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 $merge_head"
 	;;
diff --git a/t/t7612-merge-verify-signatures.sh b/t/t7612-merge-verify-signatures.sh
new file mode 100755
index 0000000..6ccfbf3
--- /dev/null
+++ b/t/t7612-merge-verify-signatures.sh
@@ -0,0 +1,52 @@
+#!/bin/sh
+
+test_description='merge signature verification tests'
+. ./test-lib.sh
+. "$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 master
+'
+
+test_expect_success GPG 'merge unsigned commit with verification' '
+	test_must_fail git merge --ff-only --verify-signatures side-unsigned 2>mergeerror &&
+	test_i18ngrep "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 &&
+	test_i18ngrep "has a bad GPG signature" mergeerror
+'
+
+test_expect_success GPG 'merge signed commit with verification' '
+	git merge --verbose --ff-only --verify-signatures side-signed >mergeoutput &&
+	test_i18ngrep "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