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Date:   Fri, 22 Oct 2021 20:30:09 +0300
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Subject: Re: [PATCH] Introduce the pkill_on_warn boot parameter
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To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Petr Mladek <pmladek@suse.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Joerg Roedel <jroedel@suse.de>,
        Maciej Rozycki <macro@orcam.me.uk>,
        Muchun Song <songmuchun@bytedance.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Kees Cook <keescook@chromium.org>,
        Luis Chamberlain <mcgrof@kernel.org>, Wei Liu <wl@xen.org>,
        John Ogness <john.ogness@linutronix.de>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Alexey Kardashevskiy <aik@ozlabs.ru>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Jann Horn <jannh@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Will Deacon <will.deacon@arm.com>,
        David S Miller <davem@davemloft.net>,
        Borislav Petkov <bp@alien8.de>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-hardening@vger.kernel.org,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
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References: <20210929185823.499268-1-alex.popov@linux.com>
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 <20210929194924.GA880162@paulmck-ThinkPad-P17-Gen-1> <YVWAPXSzFNbHz6+U@alley>
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 <0e847d7f-7bf0-cdd4-ba6e-a742ce877a38@linux.com> <87zgrnqmlc.fsf@disp2133>
From:   Alexander Popov <alex.popov@linux.com>
In-Reply-To: <87zgrnqmlc.fsf@disp2133>
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On 05.10.2021 22:48, Eric W. Biederman wrote:
> Especially as calling do_group_exit(SIGKILL) from a random location is
> not a clean way to kill a process.  Strictly speaking it is not even
> killing the process.
> 
> Partly this is just me seeing the introduction of a
> do_group_exit(SIGKILL) call and not likely the maintenance that will be
> needed.  I am still sorting out the problems with other randomly placed
> calls to do_group_exit(SIGKILL) and interactions with ptrace and
> PTRACE_EVENT_EXIT in particular.
> 
> Which is a long winded way of saying if I can predictably trigger a
> warning that calls do_group_exit(SIGKILL), on some architectures I can
> use ptrace and  can convert that warning into a way to manipulate the
> kernel stack to have the contents of my choice.
> 
> If anyone goes forward with this please use the existing oops
> infrastructure so the ptrace interactions and anything else that comes
> up only needs to be fixed once.

Hello Eric, hello everyone.

I learned the oops infrastructure and see that it's arch-specific.
The architectures have separate implementations of the die() function with 
different prototypes. I don't see how to use the oops infrastructure for killing 
all threads in a process that hits a kernel warning.

What do you think about doing the same as the oom_killer (and some other 
subsystems)? It kills all threads in a process this way:
   do_send_sig_info(SIGKILL, SEND_SIG_PRIV, current, PIDTYPE_TGID).

The oom_killer also shows a nice way to avoid killing init and kthreads:
	static bool oom_unkillable_task(struct task_struct *p)
	{
		if (is_global_init(p))
			return true;
		if (p->flags & PF_KTHREAD)
			return true;
		return false;
	}
I want to do something similar.

I would appreciate your comments.
Best regards,
Alexander