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

[Michael Tremer <michael.tremer@ipfire.org>]

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Okay, merged.

> On 2 Apr 2021, at 20:30, Peter Müller <peter.mueller(a)ipfire.org> wrote:
> 
> Hello Michael,
> 
> it seems as the kernel folks backported this into 4.14.x by now:
> 
>> [root(a)maverick ~]# uname -a
>> Linux maverick 4.14.212-ipfire #1 SMP Wed Dec 16 12:01:25 GMT 2020 x86_64 Intel(R) Celeron(R) CPU N3150 @ 1.60GHz GenuineIntel GNU/Linux
>> [root(a)maverick ~]# sysctl dev.tty.ldisc_autoload
>> dev.tty.ldisc_autoload = 1
> 
> Therefore, I would like to see this patch being merged - that is, if it is still applicable. :-)
> 
> Thanks, and best regards,
> Peter Müller
> 
> 
>> Hello Michael,
>> 
>> grmpf, overlooked some patched distribution kernel again. :-/
>> 
>> Sorry for the noise - I will keep the patch queued and wait for kernel 5.9 ...
>> 
>> Thanks, and best regards,
>> Peter Müller
>> 
>> 
>>> This does not exist before kernel 5.1.
>>> 
>>> -Michael
>>> 
>>>> On 5 Oct 2020, at 20:45, Peter Müller <peter.mueller(a)ipfire.org> wrote:
>>>> 
>>>> 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
>>>