Message-ID: <1381358030.2050.36.camel@joe-AO722>
Subject: Re: [PATCH v3] vsprintf: Check real user/group id for %pK
From: Joe Perches <joe@perches.com>
To: Ryan Mallon <rmallon@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, eldad@fogrefinery.com,
        Jiri Kosina <jkosina@suse.cz>, jgunthorpe@obsidianresearch.com,
        Dan Rosenberg <dan.j.rosenberg@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        George Spelvin <linux@horizon.com>,
        "kernel-hardening@lists.openwall.com" 
	<kernel-hardening@lists.openwall.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Date: Wed, 09 Oct 2013 15:33:50 -0700
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	 <1381356014.2050.28.camel@joe-AO722> <5255D2FD.6050705@gmail.com>
	 <1381356861.2050.33.camel@joe-AO722> <5255D7D4.8050204@gmail.com>
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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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