[PATCH v2 2/2] commit: reject overlong UTF-8 sequences

["brian m. carlson" <sandals@crustytoothpaste.net>]

The commit code accepts pseudo-UTF-8 sequences that encode a character with more
bytes than necessary.  Reject such sequences, since they are not valid UTF-8.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
---
 commit.c               | 17 +++++++++++------
 t/t3900-i18n-commit.sh | 11 +++++++++++
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/commit.c b/commit.c
index 2264106..b59c187 100644
--- a/commit.c
+++ b/commit.c
@@ -1240,11 +1240,15 @@ int commit_tree(const struct strbuf *msg, unsigned char *tree,
 static int find_invalid_utf8(const char *buf, int len)
 {
 	int offset = 0;
+	static const unsigned int max_codepoint[] = {
+		0x7f, 0x7ff, 0xffff, 0x10ffff
+	};
 
 	while (len) {
 		unsigned char c = *buf++;
 		int bytes, bad_offset;
 		unsigned int codepoint;
+		unsigned int min_val, max_val;
 
 		len--;
 		offset++;
@@ -1276,8 +1280,12 @@ static int find_invalid_utf8(const char *buf, int len)
 		if (len < bytes)
 			return bad_offset;
 
-		/* Place the encoded bits at the bottom of the value. */
+		/* Place the encoded bits at the bottom of the value and compute the
+		 * valid range.
+		 */
 		codepoint = (c & 0x7f) >> bytes;
+		min_val = max_codepoint[bytes-1] + 1;
+		max_val = max_codepoint[bytes];
 
 		offset += bytes;
 		len -= bytes;
@@ -1290,8 +1298,8 @@ static int find_invalid_utf8(const char *buf, int len)
 				return bad_offset;
 		} while (--bytes);
 
-		/* No codepoints can ever be allocated beyond U+10FFFF. */
-		if (codepoint > 0x10ffff)
+		/* Reject codepoints that are out of range for the sequence length. */
+		if (codepoint < min_val || codepoint > max_val)
 			return bad_offset;
 		/* Surrogates are only for UTF-16 and cannot be encoded in UTF-8. */
 		if ((codepoint & 0x1ff800) == 0xd800)
@@ -1308,9 +1316,6 @@ static int find_invalid_utf8(const char *buf, int len)
  *
  * If it isn't, it assumes any non-utf8 characters are Latin1,
  * and does the conversion.
- *
- * Fixme: we should probably also disallow overlong forms.
- * But we don't do that currently.
  */
 static int verify_utf8(struct strbuf *buf)
 {
diff --git a/t/t3900-i18n-commit.sh b/t/t3900-i18n-commit.sh
index ee8ba6c..94fa1e8 100755
--- a/t/t3900-i18n-commit.sh
+++ b/t/t3900-i18n-commit.sh
@@ -50,6 +50,17 @@ test_expect_success 'UTF-8 invalid characters refused' '
 	grep "did not conform" "$HOME"/stderr
 '
 
+test_expect_success 'UTF-8 overlong sequences rejected' '
+	test_when_finished "rm -f $HOME/stderr $HOME/invalid" &&
+	rm -f "$HOME/stderr" "$HOME/invalid" &&
+	echo "UTF-8 overlong" >F &&
+	printf "\340\202\251ommit message\n\nThis is not a space:\300\240\n" \
+		>"$HOME/invalid" &&
+	git commit -a -F "$HOME/invalid" \
+		2>"$HOME"/stderr &&
+	grep "did not conform" "$HOME"/stderr
+'
+
 rm -f "$HOME/stderr"
 
 for H in ISO8859-1 eucJP ISO-2022-JP
-- 
1.8.3.1