Historically the kernel has allowed TIOCSTI, which will push
characters into a controlling TTY. This continues to be used
as a malicious privilege escalation mechanism, and provides no
meaningful real-world utility any more. Its use is considered
a dangerous legacy operation, and can be disabled on most
systems.

Say Y here only if you have confirmed that your system's
userspace depends on this functionality to continue operating
normally.

Processes which run with CAP_SYS_ADMIN, such as BRLTTY, can
use TIOCSTI even when this is set to N.

This functionality can be changed at runtime with the
dev.tty.legacy_tiocsti sysctl. This configuration option sets
the default value of the sysctl.