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Date: Wed, 29 Sep 2021 16:31:43 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: alex.popov@...ux.com
Cc: Jonathan Corbet <corbet@....net>, Paul McKenney <paulmck@...nel.org>,
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Subject: Re: [PATCH] Introduce the pkill_on_warn boot parameter
On Wed, 29 Sep 2021 22:01:33 +0300 Alexander Popov <alex.popov@...ux.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@...ux.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?
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?
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