[PATCH] sysctl.conf: prevent autoloading of TTY line disciplines

["Peter Müller" <peter.mueller@ipfire.org>]

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Malicious/vulnerable TTY line disciplines have been subject of some
kernel exploits such as CVE-2017-2636, and since - to put it in Greg
Kroah-Hatrman's words - we do not "trust the userspace to do the right
thing", this reduces local kernel attack surface.

Further, there is no legitimate reason why an unprivileged user should
load kernel modules during runtime, anyway.

See also:
- https://lkml.org/lkml/2019/4/15/890
- https://a13xp0p0v.github.io/2017/03/24/CVE-2017-2636.html

Cc: Arne Fitzenreiter <arne.fitzenreiter(a)ipfire.org>
Cc: Michael Tremer <michael.tremer(a)ipfire.org>
Signed-off-by: Peter Müller <peter.mueller(a)ipfire.org>
---
 config/etc/sysctl.conf | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/config/etc/sysctl.conf b/config/etc/sysctl.conf
index d48c7734e..b5ede15ed 100644
--- a/config/etc/sysctl.conf
+++ b/config/etc/sysctl.conf
@@ -39,6 +39,10 @@ net.bridge.bridge-nf-call-ip6tables = 0
 net.bridge.bridge-nf-call-iptables = 0
 net.bridge.bridge-nf-call-arptables = 0
 
+# Restrict loading TTY line disciplines to CAP_SYS_MODULE to prevent unprivileged attackers
+# from loading vulnerable line disciplines with the TIOCSETD ioctl.
+dev.tty.ldisc_autoload = 0
+
 # Try to keep kernel address exposures out of various /proc files (kallsyms, modules, etc).
 kernel.kptr_restrict = 2
 
-- 
2.26.2