Subject: [PATCH v2 2/2] object-name: be more strict in parsing describe-like output

From: "Elijah Newren via GitGitGadget" <[email-masked]>
Date: Sat, 04 Jan 2025 00:17:50 +0000

From: Elijah Newren <[email-masked]>

>From Documentation/revisions.txt:
    '<describeOutput>', e.g. 'v1.7.4.2-679-g3bee7fb'::
      Output from `git describe`; i.e. a closest tag, optionally
      followed by a dash and a number of commits, followed by a dash, a
      'g', and an abbreviated object name.
which means that output of the format
    ${REFNAME}-${INTEGER}-g${HASH}
should parse to fully expand ${HASH}.  This is fine.  However, we
currently don't validate any of ${REFNAME}-${INTEGER}, we only parse
-g${HASH} and assume the rest is valid.  That is problematic, since it
breaks things like

    git cat-file -p branchname:path/to/file/named/i-gaffed

which, when commit affed exists, will not return us information about a
file we are looking for but will instead tell us about commit affed.

Similarly, we should probably not treat
    refs/tags/invalid/./../...../// ~^:/?*\\&[}/busted.lock-g049e0ef6
as a request for commit 050e0ef6 either.

Tighten up the parsing to make sure ${REFNAME} and ${INTEGER} are
present and valid.

Reported-by: Gabriel Amaral <[email-masked]>
Signed-off-by: Elijah Newren <[email-masked]>
---
 object-name.c       | 55 ++++++++++++++++++++++++++++++++++++++++++++-
 t/t6120-describe.sh | 22 ++++++++++++++++++
 2 files changed, 76 insertions(+), 1 deletion(-)

diff --git a/object-name.c b/object-name.c
index e54ef1f621e..71207729f6f 100644
--- a/object-name.c
+++ b/object-name.c
@@ -1271,6 +1271,58 @@ static int peel_onion(struct repository *r, const char *name, int len,
 	return 0;
 }
 
+/*
+ * Documentation/revisions.txt says:
+ *    '<describeOutput>', e.g. 'v1.7.4.2-679-g3bee7fb'::
+ *      Output from `git describe`; i.e. a closest tag, optionally
+ *      followed by a dash and a number of commits, followed by a dash, a
+ *      'g', and an abbreviated object name.
+ *
+ * which means that the stuff before '-g${HASH}' needs to be a valid
+ * refname, a dash, and a non-negative integer.  This function verifies
+ * that.
+ *
+ * In particular, we do not want to treat
+ *   branchname:path/to/file/named/i-gaffed
+ * as a request for commit affed.
+ *
+ * More generally, we should probably not treat
+ *   'refs/heads/./../.../ ~^:/?*[////\\\&}/busted.lock-g050e0ef6ead'
+ * as a request for object 050e0ef6ead either.
+ *
+ * We are called with name[len] == '-' and name[len+1] == 'g', i.e.
+ * we are verifying ${REFNAME}-{INTEGER} part of the name.
+ */
+static int ref_and_count_parts_valid(const char *name, int len)
+{
+	struct strbuf sb;
+	const char *cp;
+	int flags = REFNAME_ALLOW_ONELEVEL;
+	int ret = 1;
+
+	/* Ensure we have at least one digit */
+	if (!isxdigit(name[len-1]))
+		return 0;
+
+	/* Skip over digits backwards until we get to the dash */
+	for (cp = name + len - 2; name < cp; cp--) {
+		if (*cp == '-')
+			break;
+		if (!isxdigit(*cp))
+			return 0;
+	}
+	/* Ensure we found the leading dash */
+	if (*cp != '-')
+		return 0;
+
+	len = cp - name;
+	strbuf_init(&sb, len);
+	strbuf_add(&sb, name, len);
+	ret = !check_refname_format(name, flags);
+	strbuf_release(&sb);
+	return ret;
+}
+
 static int get_describe_name(struct repository *r,
 			     const char *name, int len,
 			     struct object_id *oid)
@@ -1284,7 +1336,8 @@ static int get_describe_name(struct repository *r,
 			/* We must be looking at g in "SOMETHING-g"
 			 * for it to be describe output.
 			 */
-			if (ch == 'g' && cp[-1] == '-') {
+			if (ch == 'g' && cp[-1] == '-' &&
+			    ref_and_count_parts_valid(name, cp - 1 - name)) {
 				cp++;
 				len -= cp - name;
 				return get_short_oid(r,
diff --git a/t/t6120-describe.sh b/t/t6120-describe.sh
index 3f6160d702b..9217bd0fa89 100755
--- a/t/t6120-describe.sh
+++ b/t/t6120-describe.sh
@@ -725,4 +725,26 @@ test_expect_success '--exact-match does not show --always fallback' '
 	test_must_fail git describe --exact-match --always
 '
 
+test_expect_success 'avoid being fooled by describe-like filename' '
+	test_when_finished rm out &&
+
+	git rev-parse --short HEAD >out &&
+	FILENAME=filename-g$(cat out) &&
+	touch $FILENAME &&
+	git add $FILENAME &&
+	git commit -m "Add $FILENAME" &&
+
+	git cat-file -t HEAD:$FILENAME >actual &&
+
+	echo blob >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'do not be fooled by invalid describe format ' '
+	test_when_finished rm out &&
+
+	git rev-parse --short HEAD >out &&
+	test_must_fail git cat-file -t "refs/tags/super-invalid/./../...../ ~^:/?*[////\\\\\\&}/busted.lock-42-g"$(cat out)
+'
+
 test_done
-- 
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