sideband: do allow ANSI color sequences by default

The preceding two commits introduced special handling of the sideband
channel to neutralize ANSI escape sequences before sending the payload
to the terminal, and `sideband.allowControlCharacters` to override that
behavior.

However, some `pre-receive` hooks that are actively used in practice
want to color their messages and therefore rely on the fact that Git
passes them through to the terminal.

In contrast to other ANSI escape sequences, it is highly unlikely that
coloring sequences can be essential tools in attack vectors that mislead
Git users e.g. by hiding crucial information.

Therefore we can have both: Continue to allow ANSI coloring sequences to
be passed to the terminal, and neutralize all other ANSI escape
sequences.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/config/sideband.adoc

@@ -1,5 +1,16 @@
 sideband.allowControlCharacters::
 	By default, control characters that are delivered via the sideband
-	are masked, to prevent potentially unwanted ANSI escape sequences
-	from being sent to the terminal. Use this config setting to override
-	this behavior.
+	are masked, except ANSI color sequences. This prevents potentially
+	unwanted ANSI escape sequences from being sent to the terminal. Use
+	this config setting to override this behavior:
++
+--
+	color::
+		Allow ANSI color sequences, line feeds and horizontal tabs,
+		but mask all other control characters. This is the default.
+	false::
+		Mask all control characters other than line feeds and
+		horizontal tabs.
+	true::
+		Allow all control characters to be sent to the terminal.
+--

sideband.c

@@ -26,7 +26,11 @@ static struct keyword_entry keywords[] = {
 	{ "error",	GIT_COLOR_BOLD_RED },
 };
 
-static int allow_control_characters;
+static enum {
+	ALLOW_NO_CONTROL_CHARACTERS = 0,
+	ALLOW_ALL_CONTROL_CHARACTERS = 1,
+	ALLOW_ANSI_COLOR_SEQUENCES = 2
+} allow_control_characters = ALLOW_ANSI_COLOR_SEQUENCES;
 
 /* Returns a color setting (GIT_COLOR_NEVER, etc). */
 static int use_sideband_colors(void)
@@ -41,8 +45,24 @@ static int use_sideband_colors(void)
 	if (use_sideband_colors_cached >= 0)
 		return use_sideband_colors_cached;
 
-	git_config_get_bool("sideband.allowcontrolcharacters",
-			    &allow_control_characters);
+	switch (git_config_get_maybe_bool("sideband.allowcontrolcharacters", &i)) {
+	case 0: /* Boolean value */
+		allow_control_characters = i ? ALLOW_ALL_CONTROL_CHARACTERS :
+			ALLOW_NO_CONTROL_CHARACTERS;
+		break;
+	case -1: /* non-Boolean value */
+		if (git_config_get_string_tmp("sideband.allowcontrolcharacters",
+					      &value))
+			; /* huh? `get_maybe_bool()` returned -1 */
+		else if (!strcmp(value, "color"))
+			allow_control_characters = ALLOW_ANSI_COLOR_SEQUENCES;
+		else
+			warning(_("unrecognized value for `sideband."
+				  "allowControlCharacters`: '%s'"), value);
+		break;
+	default:
+		break; /* not configured */
+	}
 
 	if (!git_config_get_string_tmp(key, &value))
 		use_sideband_colors_cached = git_config_colorbool(key, value);
@@ -71,9 +91,37 @@ void list_config_color_sideband_slots(struct string_list *list, const char *pref
 		list_config_item(list, prefix, keywords[i].keyword);
 }
 
+static int handle_ansi_color_sequence(struct strbuf *dest, const char *src, int n)
+{
+	int i;
+
+	/*
+	 * Valid ANSI color sequences are of the form
+	 *
+	 * ESC [ [<n> [; <n>]*] m
+	 */
+
+	if (allow_control_characters != ALLOW_ANSI_COLOR_SEQUENCES ||
+	    n < 3 || src[0] != '\x1b' || src[1] != '[')
+		return 0;
+
+	for (i = 2; i < n; i++) {
+		if (src[i] == 'm') {
+			strbuf_add(dest, src, i + 1);
+			return i;
+		}
+		if (!isdigit(src[i]) && src[i] != ';')
+			break;
+	}
+
+	return 0;
+}
+
 static void strbuf_add_sanitized(struct strbuf *dest, const char *src, int n)
 {
-	if (allow_control_characters) {
+	int i;
+
+	if (allow_control_characters == ALLOW_ALL_CONTROL_CHARACTERS) {
 		strbuf_add(dest, src, n);
 		return;
 	}
@@ -82,7 +130,10 @@ static void strbuf_add_sanitized(struct strbuf *dest, const char *src, int n)
 	for (; n && *src; src++, n--) {
 		if (!iscntrl(*src) || *src == '\t' || *src == '\n')
 			strbuf_addch(dest, *src);
-		else {
+		else if ((i = handle_ansi_color_sequence(dest, src, n))) {
+			src += i;
+			n -= i;
+		} else {
 			strbuf_addch(dest, '^');
 			strbuf_addch(dest, 0x40 + *src);
 		}

t/t5409-colorize-remote-messages.sh

@@ -100,7 +100,7 @@ test_expect_success 'fallback to color.ui' '
 
 test_expect_success 'disallow (color) control sequences in sideband' '
 	write_script .git/color-me-surprised <<-\EOF &&
-	printf "error: Have you \\033[31mread\\033[m this?\\n" >&2
+	printf "error: Have you \\033[31mread\\033[m this?\\a\\n" >&2
 	exec "$@"
 	EOF
 	test_config_global uploadPack.packObjectshook ./color-me-surprised &&
@@ -108,12 +108,24 @@ test_expect_success 'disallow (color) control sequences in sideband' '
 
 	git clone --no-local . throw-away 2>stderr &&
 	test_decode_color <stderr >decoded &&
+	test_grep RED decoded &&
+	test_grep "\\^G" stderr &&
+	tr -dc "\\007" <stderr >actual &&
+	test_must_be_empty actual &&
+
+	rm -rf throw-away &&
+	git -c sideband.allowControlCharacters=false \
+		clone --no-local . throw-away 2>stderr &&
+	test_decode_color <stderr >decoded &&
 	test_grep ! RED decoded &&
+	test_grep "\\^G" stderr &&
 
 	rm -rf throw-away &&
 	git -c sideband.allowControlCharacters clone --no-local . throw-away 2>stderr &&
 	test_decode_color <stderr >decoded &&
-	test_grep RED decoded
+	test_grep RED decoded &&
+	tr -dc "\\007" <stderr >actual &&
+	test_file_not_empty actual
 '
 
 test_done