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evdev: strip the device name of format directives

Peter Hutterer requested to merge whot/libinput:wip/string-format-issue into main

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 (closed)

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