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Subject: Re: [PATCH] Introduce the pkill_on_warn boot parameter
To:     Jonathan Corbet <corbet@lwn.net>,
        Paul McKenney <paulmck@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
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References: <20210929185823.499268-1-alex.popov@linux.com>
From:   Alexander Popov <alex.popov@linux.com>
Message-ID: <d290202d-a72d-0821-9edf-efbecf6f6cef@linux.com>
Date:   Wed, 29 Sep 2021 22:01:33 +0300
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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.

Best regards,
Alexander

> ---
>  Documentation/admin-guide/kernel-parameters.txt | 4 ++++
>  kernel/panic.c                                  | 5 +++++
>  2 files changed, 9 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 91ba391f9b32..86c748907666 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4112,6 +4112,10 @@
>  	pirq=		[SMP,APIC] Manual mp-table setup
>  			See Documentation/x86/i386/IO-APIC.rst.
>  
> +	pkill_on_warn=	Kill all threads in a process that provoked a
> +			kernel warning.
> +			Format: { "0" | "1" }
> +
>  	plip=		[PPT,NET] Parallel port network link
>  			Format: { parport<nr> | timid | 0 }
>  			See also Documentation/admin-guide/parport.rst.
> diff --git a/kernel/panic.c b/kernel/panic.c
> index cefd7d82366f..47b728bfb1d3 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -53,6 +53,7 @@ static int pause_on_oops_flag;
>  static DEFINE_SPINLOCK(pause_on_oops_lock);
>  bool crash_kexec_post_notifiers;
>  int panic_on_warn __read_mostly;
> +int pkill_on_warn __read_mostly;
>  unsigned long panic_on_taint;
>  bool panic_on_taint_nousertaint = false;
>  
> @@ -610,6 +611,9 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
>  
>  	print_oops_end_marker();
>  
> +	if (pkill_on_warn && system_state >= SYSTEM_RUNNING)
> +		do_group_exit(SIGKILL);
> +
>  	/* Just a warning, don't kill lockdep. */
>  	add_taint(taint, LOCKDEP_STILL_OK);
>  }
> @@ -694,6 +698,7 @@ core_param(panic, panic_timeout, int, 0644);
>  core_param(panic_print, panic_print, ulong, 0644);
>  core_param(pause_on_oops, pause_on_oops, int, 0644);
>  core_param(panic_on_warn, panic_on_warn, int, 0644);
> +core_param(pkill_on_warn, pkill_on_warn, int, 0644);
>  core_param(crash_kexec_post_notifiers, crash_kexec_post_notifiers, bool, 0644);
>  
>  static int __init oops_setup(char *s)
>