commit: reject invalid UTF-8 codepoints

The commit code already contains code for validating UTF-8, but it does not
check for invalid values, such as guaranteed non-characters and surrogates.  Fix
this by explicitly checking for and rejecting such characters.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -1244,6 +1244,7 @@ static int find_invalid_utf8(const char *buf, int len)
 	while (len) {
 		unsigned char c = *buf++;
 		int bytes, bad_offset;
+		unsigned int codepoint;
 
 		len--;
 		offset++;
@@ -1264,24 +1265,40 @@ static int find_invalid_utf8(const char *buf, int len)
 			bytes++;
 		}
 
-		/* Must be between 1 and 5 more bytes */
-		if (bytes < 1 || bytes > 5)
+		/*
+		 * Must be between 1 and 3 more bytes.  Longer sequences result in
+		 * codepoints beyond U+10FFFF, which are guaranteed never to exist.
+		 */
+		if (bytes < 1 || 3 < bytes)
 			return bad_offset;
 
 		/* Do we *have* that many bytes? */
 		if (len < bytes)
 			return bad_offset;
 
+		/* Place the encoded bits at the bottom of the value. */
+		codepoint = (c & 0x7f) >> bytes;
+
 		offset += bytes;
 		len -= bytes;
 
 		/* And verify that they are good continuation bytes */
 		do {
+			codepoint <<= 6;
+			codepoint |= *buf & 0x3f;
 			if ((*buf++ & 0xc0) != 0x80)
 				return bad_offset;
 		} while (--bytes);
 
-		/* We could/should check the value and length here too */
+		/* No codepoints can ever be allocated beyond U+10FFFF. */
+		if (codepoint > 0x10ffff)
+			return bad_offset;
+		/* Surrogates are only for UTF-16 and cannot be encoded in UTF-8. */
+		if ((codepoint & 0x1ff800) == 0xd800)
+			return bad_offset;
+		/* U+FFFE and U+FFFF are guaranteed non-characters. */
+		if ((codepoint & 0x1ffffe) == 0xfffe)
+			return bad_offset;
 	}
 	return -1;
 }
@@ -1292,8 +1309,8 @@ 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 and
- * invalid characters. But we don't do that currently.
+ * Fixme: we should probably also disallow overlong forms.
+ * But we don't do that currently.
  */
 static int verify_utf8(struct strbuf *buf)
 {

t/t3900-i18n-commit.sh

@@ -39,6 +39,14 @@ test_expect_failure 'UTF-16 refused because of NULs' '
 	git commit -a -F "$TEST_DIRECTORY"/t3900/UTF-16.txt
 '
 
+test_expect_success 'UTF-8 invalid characters refused' '
+	test_when_finished "rm -f $HOME/stderr $HOME/invalid" &&
+	echo "UTF-8 characters" >F &&
+	printf "Commit message\n\nInvalid surrogate:\355\240\200\n" \
+		>"$HOME/invalid" &&
+	git commit -a -F "$HOME/invalid" 2>"$HOME"/stderr &&
+	grep "did not conform" "$HOME"/stderr
+'
 
 for H in ISO8859-1 eucJP ISO-2022-JP
 do