netdev - Re: [PATCH v5] kptr_restrict for hiding kernel pointers

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Date:	Wed, 22 Dec 2010 16:43:36 -0500
From:	Valdis.Kletnieks@...edu
To:	Dan Rosenberg <drosenberg@...curity.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, linux-security-module@...r.kernel.org,
	jmorris@...ei.org, eric.dumazet@...il.com, tgraf@...radead.org,
	eugeneteo@...nel.org, kees.cook@...onical.com, davem@...emloft.net,
	a.p.zijlstra@...llo.nl, akpm@...ux-foundation.org,
	eparis@...isplace.org
Subject: Re: [PATCH v5] kptr_restrict for hiding kernel pointers

On Wed, 22 Dec 2010 12:17:59 EST, Dan Rosenberg said:
> On Wed, 2010-12-22 at 18:13 +0100, Ingo Molnar wrote:
> > * Dan Rosenberg <drosenberg@...curity.com> wrote:
> > 
> > > +	case 'K':
> > > +		/*
> > > +		 * %pK cannot be used in IRQ context because its test
> > > +		 * for CAP_SYSLOG would be meaningless.
> > > +		 */
> > > +		if (in_irq() || in_serving_softirq() || in_nmi())
> > > +			WARN_ONCE(1, "%%pK used in interrupt context.\n");
> > 
> > Hm, that bit looks possibly broken - some useful warning in irq context could print
> > a pointer into the syslog and this would generate a second warning? That probably
> > would crash as it recurses back into the printk code?

> The double "%%" acts as an escape and simply prints "%" rather than
> treating it as a format specifier.

I think Ingo was more worried about the fact that we're doing a WARN_ONCE which
will generate a call to printk() - while we're in the middle of a printk() already.

So if we hit a 'printk(KERN_INFO "Some blather with a %pK pointer in it",ptr) in irq
context, what we'll get (if we're lucky is:

Some blather with a <50-60 lines of WARN_ONCE output> pointer in it.

If we're unlucky? Well... 


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