evdev: strip the device name of format directives

This fixes a format string vulnerabilty.

evdev_log_message() composes a format string consisting of a fixed
prefix (including the rendered device name) and the passed-in format
buffer. This format string is then passed with the arguments to the
actual log handler, which usually and eventually ends up being printf.

If the device name contains a printf-style format directive, these ended
up in the format string and thus get interpreted correctly, e.g. for a
device "Foo%sBar" the log message vs printf invocation ends up being:
  evdev_log_message(device, "some message %s", "some argument");
  printf("event9 - Foo%sBar: some message %s", "some argument");

This can enable an attacker to execute malicious code with the
privileges of the process using libinput.

To exploit this, an attacker needs to be able to create a kernel device
with a malicious name, e.g. through /dev/uinput or a Bluetooth device.

To fix this, convert any potential format directives in the device name
by duplicating percentages.

Pre-rendering the device to avoid the issue altogether would be nicer
but the current log level hooks do not easily allow for this. The device
name is the only user-controlled part of the format string.

A second potential issue is the sysname of the device which is also
sanitized.

This issue was found by Albin Eldstål-Ahrens and Benjamin Svensson from
Assured AB, and independently by Lukas Lamster.

Fixes #752

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit a423d7d326)

meson.build

@@ -733,6 +733,7 @@ if get_option('tests')
 		'test/litest-device-dell-canvas-totem-touch.c',
 		'test/litest-device-elantech-touchpad.c',
 		'test/litest-device-elan-tablet.c',
+		'test/litest-device-format-string.c',
 		'test/litest-device-generic-pressurepad.c',
 		'test/litest-device-generic-singletouch.c',
 		'test/litest-device-gpio-keys.c',

src/evdev.c

@@ -2356,19 +2356,19 @@ evdev_device_create(struct libinput_seat *seat,
 	struct libinput *libinput = seat->libinput;
 	struct evdev_device *device = NULL;
 	int rc;
-	int fd;
+	int fd = -1;
 	int unhandled_device = 0;
 	const char *devnode = udev_device_get_devnode(udev_device);
-	const char *sysname = udev_device_get_sysname(udev_device);
+	char *sysname = str_sanitize(udev_device_get_sysname(udev_device));
 
 	if (!devnode) {
 		log_info(libinput, "%s: no device node associated\n", sysname);
-		return NULL;
+		goto err;
 	}
 
 	if (udev_device_should_be_ignored(udev_device)) {
 		log_debug(libinput, "%s: device is ignored\n", sysname);
-		return NULL;
+		goto err;
 	}
 
 	/* Use non-blocking mode so that we can loop on read on
@@ -2382,13 +2382,15 @@ evdev_device_create(struct libinput_seat *seat,
 			 sysname,
 			 devnode,
 			 strerror(-fd));
-		return NULL;
+		goto err;
 	}
 
 	if (!evdev_device_have_same_syspath(udev_device, fd))
 		goto err;
 
 	device = zalloc(sizeof *device);
+	device->sysname = sysname;
+	sysname = NULL;
 
 	libinput_device_init(&device->base, seat);
 	libinput_seat_ref(seat);
@@ -2411,6 +2413,9 @@ evdev_device_create(struct libinput_seat *seat,
 	device->dispatch = NULL;
 	device->fd = fd;
 	device->devname = libevdev_get_name(device->evdev);
+	/* the log_prefix_name is used as part of a printf format string and
+	 * must not contain % directives, see evdev_log_msg */
+	device->log_prefix_name = str_sanitize(device->devname);
 	device->scroll.threshold = 5.0; /* Default may be overridden */
 	device->scroll.direction_lock_threshold = 5.0; /* Default may be overridden */
 	device->scroll.direction = 0;
@@ -2451,12 +2456,16 @@ evdev_device_create(struct libinput_seat *seat,
 	return device;
 
 err:
-	close_restricted(libinput, fd);
+	if (fd >= 0) {
-	if (device) {
+		close_restricted(libinput, fd);
-		unhandled_device = device->seat_caps == 0;
+		if (device) {
-		evdev_device_destroy(device);
+			unhandled_device = device->seat_caps == 0;
+			evdev_device_destroy(device);
+		}
 	}
 
+	free(sysname);
+
 	return unhandled_device ? EVDEV_UNHANDLED_DEVICE :  NULL;
 }
 
@@ -2469,7 +2478,7 @@ evdev_device_get_output(struct evdev_device *device)
 const char *
 evdev_device_get_sysname(struct evdev_device *device)
 {
-	return udev_device_get_sysname(device->udev_device);
+	return device->sysname;
 }
 
 const char *
@@ -3066,6 +3075,8 @@ evdev_device_destroy(struct evdev_device *device)
 	if (device->base.group)
 		libinput_device_group_unref(device->base.group);
 
+	free(device->log_prefix_name);
+	free(device->sysname);
 	free(device->output_name);
 	filter_destroy(device->pointer.filter);
 	libinput_timer_destroy(&device->scroll.timer);

src/evdev.h

@@ -169,6 +169,8 @@ struct evdev_device {
 	struct udev_device *udev_device;
 	char *output_name;
 	const char *devname;
+	char *log_prefix_name;
+	char *sysname;
 	bool was_removed;
 	int fd;
 	enum evdev_device_seat_capability seat_caps;
@@ -786,7 +788,7 @@ evdev_log_msg(struct evdev_device *device,
 		 sizeof(buf),
 		 "%-7s - %s%s%s",
 		 evdev_device_get_sysname(device),
-		 (priority > LIBINPUT_LOG_PRIORITY_DEBUG) ?  device->devname : "",
+		 (priority > LIBINPUT_LOG_PRIORITY_DEBUG) ?  device->log_prefix_name : "",
 		 (priority > LIBINPUT_LOG_PRIORITY_DEBUG) ?  ": " : "",
 		 format);
 
@@ -824,7 +826,7 @@ evdev_log_msg_ratelimit(struct evdev_device *device,
 		 sizeof(buf),
 		 "%-7s - %s%s%s",
 		 evdev_device_get_sysname(device),
-		 (priority > LIBINPUT_LOG_PRIORITY_DEBUG) ?  device->devname : "",
+		 (priority > LIBINPUT_LOG_PRIORITY_DEBUG) ?  device->log_prefix_name : "",
 		 (priority > LIBINPUT_LOG_PRIORITY_DEBUG) ?  ": " : "",
 		 format);
 

src/util-strings.h

@@ -43,6 +43,8 @@
 #include <xlocale.h>
 #endif
 
+#include "util-macros.h"
+
 static inline bool
 streq(const char *str1, const char *str2)
 {
@@ -398,3 +400,31 @@ safe_basename(const char *filename);
 
 char *
 trunkname(const char *filename);
+
+/**
+ * Return a copy of str with all % converted to %% to make the string
+ * acceptable as printf format.
+ */
+static inline char *
+str_sanitize(const char *str)
+{
+	if (!str)
+		return NULL;
+
+	if (!strchr(str, '%'))
+		return strdup(str);
+
+	size_t slen = min(strlen(str), 512);
+	char *sanitized = zalloc(2 * slen + 1);
+	const char *src = str;
+	char *dst = sanitized;
+
+	for (size_t i = 0; i < slen; i++) {
+		if (*src == '%')
+			*dst++ = '%';
+		*dst++ = *src++;
+	}
+	*dst = '\0';
+
+	return sanitized;
+}

test/litest-device-format-string.c

@@ -0,0 +1,56 @@
+
+/*
+ * Copyright © 2013 Red Hat, Inc.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+#include "config.h"
+
+#include "litest.h"
+#include "litest-int.h"
+
+static struct input_id input_id = {
+	.bustype = 0x3,
+	.vendor = 0x0123,
+	.product = 0x0456,
+};
+
+static int events[] = {
+	EV_KEY, BTN_LEFT,
+	EV_KEY, BTN_RIGHT,
+	EV_KEY, BTN_MIDDLE,
+	EV_REL, REL_X,
+	EV_REL, REL_Y,
+	EV_REL, REL_WHEEL,
+	EV_REL, REL_WHEEL_HI_RES,
+	-1 , -1,
+};
+
+TEST_DEVICE("mouse-format-string",
+	.type = LITEST_MOUSE_FORMAT_STRING,
+	.features = LITEST_RELATIVE | LITEST_BUTTON | LITEST_WHEEL,
+	.interface = NULL,
+
+	.name = "Evil %s %d %x Mouse %p %",
+	.id = &input_id,
+	.absinfo = NULL,
+	.events = events,
+)

test/litest.h

@@ -321,6 +321,7 @@ enum litest_device_type {
 	LITEST_SYNAPTICS_PRESSUREPAD,
 	LITEST_GENERIC_PRESSUREPAD,
 	LITEST_WACOM_ISDV4_524C_PEN,
+	LITEST_MOUSE_FORMAT_STRING,
 };
 
 #define LITEST_DEVICELESS	-2

test/test-utils.c

@@ -1267,6 +1267,31 @@ START_TEST(strstartswith_test)
 }
 END_TEST
 
+START_TEST(strsanitize_test)
+{
+	struct strsanitize_test {
+		const char *string;
+		const char *expected;
+	} tests[] = {
+		{ "foobar", "foobar" },
+		{ "", "" },
+		{ "%", "%%" },
+		{ "%%%%", "%%%%%%%%" },
+		{ "x %s", "x %%s" },
+		{ "x %", "x %%" },
+		{ "%sx", "%%sx" },
+		{ "%s%s", "%%s%%s" },
+		{ NULL, NULL },
+	};
+
+	for (struct strsanitize_test *t = tests; t->string; t++) {
+		char *sanitized = str_sanitize(t->string);
+		ck_assert_str_eq(sanitized, t->expected);
+		free(sanitized);
+	}
+}
+END_TEST
+
 START_TEST(list_test_insert)
 {
 	struct list_test {
@@ -1489,6 +1514,7 @@ litest_utils_suite(void)
 	tcase_add_test(tc, strstrip_test);
 	tcase_add_test(tc, strendswith_test);
 	tcase_add_test(tc, strstartswith_test);
+	tcase_add_test(tc, strsanitize_test);
 	tcase_add_test(tc, time_conversion);
 	tcase_add_test(tc, human_time);