Re: [PATCH] Introduce the pkill_on_warn boot parameter

From: Kees Cook <keescook@chromium.org>
To: Petr Mladek <pmladek@suse.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
	Alexander Popov <alex.popov@linux.com>,
	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>,
	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>,
	Thomas Garnier <thgarnie@google.com>,
	Will Deacon <will.deacon@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Laura Abbott <labbott@redhat.com>,
	David S Miller <davem@davemloft.net>,
	Borislav Petkov <bp@alien8.de>,
	kernel-hardening@lists.openwall.com,
	linux-hardening@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, notify@kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] Introduce the pkill_on_warn boot parameter
Date: Thu, 30 Sep 2021 11:28:20 -0700	[thread overview]
Message-ID: <202109301121.7644668F3F@keescook> (raw)
In-Reply-To: <YVWAPXSzFNbHz6+U@alley>

On Thu, Sep 30, 2021 at 11:15:41AM +0200, Petr Mladek wrote:
> On Wed 2021-09-29 12:49:24, Paul E. McKenney wrote:
> > On Wed, Sep 29, 2021 at 10:01:33PM +0300, Alexander Popov 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.
> 
> Honestly, I am not sure if panic_on_warn() or the new pkill_on_warn()
> work as expected. I wonder who uses it in practice and what is
> the experience.

panic_on_warn() gets used by folks with paranoid security concerns.

> The problem is that many developers do not know about this behavior.
> They use WARN() when they are lazy to write more useful message or when
> they want to see all the provided details: task, registry, backtrace.

The documentation[1] on this hopefully clarifies the situation:

  Note that the WARN()-family should only be used for “expected to be
  unreachable” situations. If you want to warn about “reachable but
  undesirable” situations, please use the pr_warn()-family of functions.
  System owners may have set the panic_on_warn sysctl, to make sure their
  systems do not continue running in the face of “unreachable” conditions.

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on

> Also it is inconsistent with pr_warn() behavior. Why a single line
> warning would be innocent and full info WARN() cause panic/pkill?

Because pr_warn() is intended for system admins. WARN() is for
developers and should not be reachable through any known path.

> What about pr_err(), pr_crit(), pr_alert(), pr_emerg()? They inform
> about even more serious problems. Why a warning should cause panic/pkill
> while an alert message is just printed?

Additionally, pr_*() don't include stack traces, etc. WARN() is for
situations that should never happen. pr_warn() is about undesirable but
reachable states.

For example:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d4689846881d160a4d12a514e991a740bcb5d65a

> It somehow reminds me the saga with %pK. We were not able to teach
> developers to use it correctly for years and ended with hashed
> pointers.

And this was pointed out when %pK was introduced, but Linus couldn't be
convinced. He changed his mind, thankfully.

-- 
Kees Cook