linux-hardening - Re: [PATCH] Introduce the pkill_on

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Date:   Thu, 30 Sep 2021 11:28:20 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Alexander Popov <alex.popov@...ux.com>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Joerg Roedel <jroedel@...e.de>,
        Maciej Rozycki <macro@...am.me.uk>,
        Muchun Song <songmuchun@...edance.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Robin Murphy <robin.murphy@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Luis Chamberlain <mcgrof@...nel.org>, Wei Liu <wl@....org>,
        John Ogness <john.ogness@...utronix.de>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Alexey Kardashevskiy <aik@...abs.ru>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Jann Horn <jannh@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Will Deacon <will.deacon@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Laura Abbott <labbott@...hat.com>,
        David S Miller <davem@...emloft.net>,
        Borislav Petkov <bp@...en8.de>,
        kernel-hardening@...ts.openwall.com,
        linux-hardening@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, notify@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] Introduce the pkill_on_warn boot parameter

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