Re: [PATCH] Introduce the pkill_on_warn boot parameter

[Alexander Popov <alex.popov@linux.com>]

On 30.09.2021 02:31, Andrew Morton wrote:
> On Wed, 29 Sep 2021 22:01:33 +0300 Alexander Popov <alex.popov@linux.com> wrote:
> 
>> On 29.09.2021 21:58, Alexander Popov wrote:
>>> Currently, the Linux kernel provides two types of reaction to kernel
>>> warnings:
>>>  1. Do nothing (by default),
>>>  2. Call panic() if panic_on_warn is set. That's a very strong reaction,
>>>     so panic_on_warn is usually disabled on production systems.
>>>
>>> From a safety point of view, the Linux kernel misses a middle way of
>>> handling kernel warnings:
>>>  - The kernel should stop the activity that provokes a warning,
>>>  - But the kernel should avoid complete denial of service.
>>>
>>> From a security point of view, kernel warning messages provide a lot of
>>> useful information for attackers. Many GNU/Linux distributions allow
>>> unprivileged users to read the kernel log, so attackers use kernel
>>> warning infoleak in vulnerability exploits. See the examples:
>>>   https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html
>>>   https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html
>>>
>>> Let's introduce the pkill_on_warn boot parameter.
>>> If this parameter is set, the kernel kills all threads in a process
>>> that provoked a kernel warning. This behavior is reasonable from a safety
>>> point of view described above. It is also useful for kernel security
>>> hardening because the system kills an exploit process that hits a
>>> kernel warning.
>>>
>>> Signed-off-by: Alexander Popov <alex.popov@linux.com>
>>
>> This patch was tested using CONFIG_LKDTM.
>> The kernel kills a process that performs this:
>>   echo WARNING > /sys/kernel/debug/provoke-crash/DIRECT
>>
>> If you are fine with this approach, I will prepare a patch adding the
>> pkill_on_warn sysctl.
> 
> Why do we need a boot parameter?  Isn't a sysctl all we need for this
> feature? 

I would say we need both sysctl and boot parameter for pkill_on_warn.
That would be consistent with panic_on_warn, ftrace_dump_on_oops and
oops/panic_on_oops.

> Also, 
> 
> 	if (pkill_on_warn && system_state >= SYSTEM_RUNNING)
> 		do_group_exit(SIGKILL);
> 
> - why do we care about system_state?  An explanatory code comment
>   seems appropriate.
> 
> - do we really want to do this in states > SYSTEM_RUNNING?  If so, why?

A kernel warning may occur at any moment.
I don't have a deep understanding of possible side effects on early boot stages.
So I decided that at least it's safer to avoid interfering before SYSTEM_RUNNING.

Best regards,
Alexander