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 a423d7d3269dc32a87384f79e29bb5ac021c83d1)

1 files changed, 1 insertions, 0 deletions

diff --git a/meson.build b/meson.build
index d6b06cca..a909ff19 100644
--- a/meson.build
+++ b/meson.build
@@ -732,6 +732,7 @@ 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',