linux-kernel - Re: [PATCH v3] vsprintf: Check real user/group id for %pK

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Date:	Wed, 09 Oct 2013 15:33:50 -0700
From:	Joe Perches <joe@...ches.com>
To:	Ryan Mallon <rmallon@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, eldad@...refinery.com,
	Jiri Kosina <jkosina@...e.cz>, jgunthorpe@...idianresearch.com,
	Dan Rosenberg <dan.j.rosenberg@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	George Spelvin <linux@...izon.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] vsprintf: Check real user/group id for %pK

On Thu, 2013-10-10 at 09:25 +1100, Ryan Mallon wrote:

> 	if (kptr_restrict && (in_irq() || in_serving_softirq() ||
> 			      in_nmi())) {
> 
> Is making sure that you don't have kernel code doing something like this:
> 
> 	irqreturn_t some_irq_handler(int irq, void *data)
> 	{
> 		struct seq_file *seq = to_seq(data);
> 
> 		seq_printf(seq, "value = %pK\n");
> 		return IRQ_HANDLED;
> 	}
> 
> Because that obviously won't work when kptr_restrict=1 (because the
> CAP_SYSLOG check is meaningless). However, the code is broken regardless
> of the kptr_restrict value.

The only brokenness I see here is that the code doesn't pass
a pointer along with %pK

		seq_printf(seq, "value of seq: %pK\n", seq);

>  Since the default value of kptr_restrict is
> 0, this kind of bug can go over-looked because the seq file will print
> the pointer value correctly when kptr_restrict=0, and it will correctly
> print 0's when kptr_restrict=2, but it will print 'pK-error' when
> kptr_restrict=1. Doing the check in all cases makes it more likely that
> bugs like this get found. In fact, doing something like:
> 
> 	if (WARN_ON(in_irq() || in_serving_softirq() || in_nmi())) {
> 
> Might be better, since that will print a stack-trace showing where the
> offending vsprintf is.

WARN_ON would be potentially _very_ noisy.
Maybe a long period (once a day?) ratelimited dump_stack();



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