Merge branch 'disallow-control-characters-in-sideband-channel'

This addresses:

- CVE-2024-52005:

	Insufficient neutralization of ANSI escape sequences in sideband
	payload can be used to mislead Git users into believing that
	certain remote-generated messages actually originate from Git.

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

Documentation/config.adoc

@@ -522,6 +522,8 @@ include::config/sequencer.adoc[]
 
 include::config/showbranch.adoc[]
 
+include::config/sideband.adoc[]
+
 include::config/sparse.adoc[]
 
 include::config/splitindex.adoc[]

Documentation/config/sideband.adoc

@@ -0,0 +1,16 @@
+sideband.allowControlCharacters::
+	By default, control characters that are delivered via the sideband
+	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

@@ -26,6 +26,12 @@ static struct keyword_entry keywords[] = {
 	{ "error",	GIT_COLOR_BOLD_RED },
 };
 
+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)
 {
@@ -39,6 +45,25 @@ static int use_sideband_colors(void)
 	if (use_sideband_colors_cached >= 0)
 		return use_sideband_colors_cached;
 
+	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);
 	else if (!git_config_get_string_tmp("color.ui", &value))
@@ -66,6 +91,55 @@ 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)
+{
+	int i;
+
+	if (allow_control_characters == ALLOW_ALL_CONTROL_CHARACTERS) {
+		strbuf_add(dest, src, n);
+		return;
+	}
+
+	strbuf_grow(dest, n);
+	for (; n && *src; src++, n--) {
+		if (!iscntrl(*src) || *src == '\t' || *src == '\n')
+			strbuf_addch(dest, *src);
+		else if ((i = handle_ansi_color_sequence(dest, src, n))) {
+			src += i;
+			n -= i;
+		} else {
+			strbuf_addch(dest, '^');
+			strbuf_addch(dest, 0x40 + *src);
+		}
+	}
+}
+
 /*
  * Optionally highlight one keyword in remote output if it appears at the start
  * of the line. This should be called for a single line only, which is
@@ -81,7 +155,7 @@ static void maybe_colorize_sideband(struct strbuf *dest, const char *src, int n)
 	int i;
 
 	if (!want_color_stderr(use_sideband_colors())) {
-		strbuf_add(dest, src, n);
+		strbuf_add_sanitized(dest, src, n);
 		return;
 	}
 
@@ -114,7 +188,7 @@ static void maybe_colorize_sideband(struct strbuf *dest, const char *src, int n)
 		}
 	}
 
-	strbuf_add(dest, src, n);
+	strbuf_add_sanitized(dest, src, n);
 }
 
 

t/t5409-colorize-remote-messages.sh

@@ -98,4 +98,34 @@ test_expect_success 'fallback to color.ui' '
 	grep "<BOLD;RED>error<RESET>: error" decoded
 '
 
+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?\\a\\n" >&2
+	exec "$@"
+	EOF
+	test_config_global uploadPack.packObjectshook ./color-me-surprised &&
+	test_commit need-at-least-one-commit &&
+
+	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 &&
+	tr -dc "\\007" <stderr >actual &&
+	test_file_not_empty actual
+'
+
 test_done