Subject: Re: [PATCH v5] kptr_restrict for hiding kernel pointers

From: Dan Rosenberg <[email-masked]>
Date: Wed, 22 Dec 2010 12:35:32 -0500

On Wed, 2010-12-22 at 18:13 +0100, Ingo Molnar wrote:
> * Dan Rosenberg <[email-masked]> 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?
> 

I don't see a reason to ever use %pK to print to the syslog, since
reading it is now optionally protected with dmesg_restrict, and
stripping pointers from the syslog will cripple any post-mortem
debugging for everyone.  I understand the desire to prevent things from
breaking even if it's used incorrectly, but I'm not really convinced
that this would break anything even in this scenario.  The WARN_ONCE
will prevent any unbounded recursion.  I'm just not clear on how this
could cause a crash.

> Instead a warning could be inserted into the generated output instead, for example 
> 'pK-error' (carefully staying within pointer length limits).
> 

If it's used in IRQ context and its output needs to be read by a
userspace utility using %p to parse, this will break it.

-Dan