[PATCH v4] gpg-interface.c: detect and reject multiple signatures on commits

["Michał Górny" <mgorny@gentoo.org>]

GnuPG supports creating signatures consisting of multiple signature
packets.  If such a signature is verified, it outputs all the status
messages for each signature separately.  However, git currently does not
account for such scenario and gets terribly confused over getting
multiple *SIG statuses.

For example, if a malicious party alters a signed commit and appends
a new untrusted signature, git is going to ignore the original bad
signature and report untrusted commit instead.  However, %GK and %GS
format strings may still expand to the data corresponding
to the original signature, potentially tricking the scripts into
trusting the malicious commit.

Given that the use of multiple signatures is quite rare, git does not
support creating them without jumping through a few hoops, and finally
supporting them properly would require extensive API improvement, it
seems reasonable to just reject them at the moment.

Signed-off-by: Michał Górny <mgorny@gentoo.org>
---
 gpg-interface.c          | 90 +++++++++++++++++++++++++++-------------
 t/t7510-signed-commit.sh | 26 ++++++++++++
 2 files changed, 87 insertions(+), 29 deletions(-)

Changes in v4:
* switched to using skip_prefix(),
* renamed the variable to seen_exclusive_status,
* made the loop terminate early on first duplicate status seen.

diff --git a/gpg-interface.c b/gpg-interface.c
index db17d65f8..efe2c0d38 100644
--- a/gpg-interface.c
+++ b/gpg-interface.c
@@ -75,48 +75,80 @@ void signature_check_clear(struct signature_check *sigc)
 	FREE_AND_NULL(sigc->key);
 }
 
+/* An exclusive status -- only one of them can appear in output */
+#define GPG_STATUS_EXCLUSIVE	(1<<0)
+
 static struct {
 	char result;
 	const char *check;
+	unsigned int flags;
 } sigcheck_gpg_status[] = {
-	{ 'G', "\n[GNUPG:] GOODSIG " },
-	{ 'B', "\n[GNUPG:] BADSIG " },
-	{ 'U', "\n[GNUPG:] TRUST_NEVER" },
-	{ 'U', "\n[GNUPG:] TRUST_UNDEFINED" },
-	{ 'E', "\n[GNUPG:] ERRSIG "},
-	{ 'X', "\n[GNUPG:] EXPSIG "},
-	{ 'Y', "\n[GNUPG:] EXPKEYSIG "},
-	{ 'R', "\n[GNUPG:] REVKEYSIG "},
+	{ 'G', "GOODSIG ", GPG_STATUS_EXCLUSIVE },
+	{ 'B', "BADSIG ", GPG_STATUS_EXCLUSIVE },
+	{ 'U', "TRUST_NEVER", 0 },
+	{ 'U', "TRUST_UNDEFINED", 0 },
+	{ 'E', "ERRSIG ", GPG_STATUS_EXCLUSIVE },
+	{ 'X', "EXPSIG ", GPG_STATUS_EXCLUSIVE },
+	{ 'Y', "EXPKEYSIG ", GPG_STATUS_EXCLUSIVE },
+	{ 'R', "REVKEYSIG ", GPG_STATUS_EXCLUSIVE },
 };
 
 static void parse_gpg_output(struct signature_check *sigc)
 {
 	const char *buf = sigc->gpg_status;
+	const char *line, *next;
 	int i;
-
-	/* Iterate over all search strings */
-	for (i = 0; i < ARRAY_SIZE(sigcheck_gpg_status); i++) {
-		const char *found, *next;
-
-		if (!skip_prefix(buf, sigcheck_gpg_status[i].check + 1, &found)) {
-			found = strstr(buf, sigcheck_gpg_status[i].check);
-			if (!found)
-				continue;
-			found += strlen(sigcheck_gpg_status[i].check);
-		}
-		sigc->result = sigcheck_gpg_status[i].result;
-		/* The trust messages are not followed by key/signer information */
-		if (sigc->result != 'U') {
-			next = strchrnul(found, ' ');
-			sigc->key = xmemdupz(found, next - found);
-			/* The ERRSIG message is not followed by signer information */
-			if (*next && sigc-> result != 'E') {
-				found = next + 1;
-				next = strchrnul(found, '\n');
-				sigc->signer = xmemdupz(found, next - found);
+	int seen_exclusive_status = 0;
+
+	/* Iterate over all lines */
+	for (line = buf; *line; line = strchrnul(line+1, '\n')) {
+		while (*line == '\n')
+			line++;
+		/* Skip lines that don't start with GNUPG status */
+		if (!skip_prefix(line, "[GNUPG:] ", &line))
+			continue;
+
+		/* Iterate over all search strings */
+		for (i = 0; i < ARRAY_SIZE(sigcheck_gpg_status); i++) {
+			if (skip_prefix(line, sigcheck_gpg_status[i].check, &line)) {
+				if (sigcheck_gpg_status[i].flags & GPG_STATUS_EXCLUSIVE) {
+					if (++seen_exclusive_status > 1)
+						goto found_duplicate_status;
+				}
+
+				sigc->result = sigcheck_gpg_status[i].result;
+				/* The trust messages are not followed by key/signer information */
+				if (sigc->result != 'U') {
+					next = strchrnul(line, ' ');
+					free(sigc->key);
+					sigc->key = xmemdupz(line, next - line);
+					/* The ERRSIG message is not followed by signer information */
+					if (*next && sigc->result != 'E') {
+						line = next + 1;
+						next = strchrnul(line, '\n');
+						free(sigc->signer);
+						sigc->signer = xmemdupz(line, next - line);
+					}
+				}
+
+				break;
 			}
 		}
 	}
+	return;
+
+found_duplicate_status:
+	/*
+	 * GOODSIG, BADSIG etc. can occur only once for each signature.
+	 * Therefore, if we had more than one then we're dealing with multiple
+	 * signatures.  We don't support them currently, and they're rather
+	 * hard to create, so something is likely fishy and we should reject
+	 * them altogether.
+	 */
+	sigc->result = 'E';
+	/* Clear partial data to avoid confusion */
+	FREE_AND_NULL(sigc->signer);
+	FREE_AND_NULL(sigc->key);
 }
 
 int check_signature(const char *payload, size_t plen, const char *signature,
diff --git a/t/t7510-signed-commit.sh b/t/t7510-signed-commit.sh
index 4e37ff8f1..180f0be91 100755
--- a/t/t7510-signed-commit.sh
+++ b/t/t7510-signed-commit.sh
@@ -234,4 +234,30 @@ test_expect_success GPG 'check config gpg.format values' '
 	test_must_fail git commit -S --amend -m "fail"
 '
 
+test_expect_success GPG '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/^gpgsig//;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/^/gpgsig /;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 GPG 'show double signature with custom format' '
+	cat >expect <<-\EOF &&
+	E
+
+
+	EOF
+	git log -1 --format="%G?%n%GK%n%GS" $(cat double-commit.commit) >actual &&
+	test_cmp expect actual
+'
+
 test_done
-- 
2.19.1