linux-kernel - Re: [RFC PATCH 2/2] x86: Allow paranoid __{get,put}

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Date:   Sat, 4 Nov 2017 00:24:30 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Kees Cook <keescook@...omium.org>
Cc:     Laura Abbott <labbott@...hat.com>,
        kernel-hardening@...ts.openwall.com,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>, X86 ML <x86@...nel.org>
Subject: Re: [RFC PATCH 2/2] x86: Allow paranoid __{get,put}_user

On Fri, Nov 03, 2017 at 05:14:05PM -0700, Kees Cook wrote:
> > x86 turns out to be easier since the safe and unsafe paths are mostly
> > disjoint so we don't have to worry about gcc optimizing out access_ok.
> > I tweaked the Kconfig to someting a bit more generic.
> >
> > The size increase was ~8K in text with a config I tested.
> 
> Specifically, this feature would have caught the waitid() bug in 4.13
> immediately.

You mean, as soon as waitid() was given a kernel address.  At which point
you'd get a shiny way to generate a BUG(), and if something like that
happened under a mutex - it's even more fun...

> > +config PARANOID_UACCESS
> > +       bool "Use paranoid uaccess primitives"
> > +       depends on ARCH_HAS_PARANOID_UACCESS
> > +       help
> > +         Forces access_ok() checks in __get_user(), __put_user(), and other
> > +         low-level uaccess primitives which usually do not have checks. This
> > +         can limit the effect of missing access_ok() checks in higher-level
> > +         primitives, with a runtime performance overhead in some cases and a
> > +         small code size overhead.

IMO that's the wrong way to go - what we need is to reduce the amount of
__get_user()/__put_user(), rather than "instrumenting" them that way.