In the Linux kernel, the following vulnerability has been resolved: speakup: keyhelp: guard letter_offsets possible out-of-range indexing

In the Linux kernel, the following vulnerability has been resolved:

speakup: keyhelp: guard letter_offsets possible out-of-range indexing

help_init() builds letter_offsets[] by using the first byte of each
function name as an index via (start & 31) - 1. If function_names are
overridden from sysfs (root) with a name starting outside [a–z], the
index underflows or exceeds the array, leading to OOB write.

Function names can be overridden with the following commands as root:

 1modprobe speakup_soft
 2echo "0 _bad" > /sys/accessibility/speakup/i18n/function_names
 3# then press Insert+2 on /dev/tty

This fix checks the first letter in help_init(), and if it is not in the
[a–z] range the function returns an error to the caller. Eventually this
error is propagated to drivers/accessibility/speakup/main.c:2217, which
causes a bleep sound.