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 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.  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
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