linux-kernel - Re: [PATCH 1/2] vsprintf/sysctl: Bugfix, neaten and document %pK usages

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Message-ID: <52579160.1090800@gmail.com>
Date:	Fri, 11 Oct 2013 16:49:20 +1100
From:	Ryan Mallon <rmallon@...il.com>
To:	Joe Perches <joe@...ches.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Rosenberg <dan.j.rosenberg@...il.com>,
	Rob Landley <rob@...dley.net>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] vsprintf/sysctl: Bugfix, neaten and document %pK
 usages

On 11/10/13 16:38, Joe Perches wrote:
> On Fri, 2013-10-11 at 16:31 +1100, Ryan Mallon wrote:
>> On 11/10/13 16:25, Joe Perches wrote:
>>> Printing kernel pointers via %pK has a minor defect when
>>> kptr_restrict is set to 2:  the pointer may be emitted
>>> as "pK-error" instead of all 0's when in an interrupt.
>>
>> NAK. This is not a defect, as I explained earlier. It is really a defect
>> that it _doesn't_ print 'pK-error' in all cases. 'pK-error' is for
>> finding kernel bugs.
> 
> Not my understanding.
> 
> There is no bug to find when emitting a pointer via %pK.

Yes there is. %pK is not valid from interrupt context, because the
current way it is checked is by checking CAP_SYSLOG. It is irrelevant
that this check is only done when kptr_restrict=1, using %pK from
interrupt handlers is still wrong. We want to notify developers of any
wrong cases.

> 
> The only issue is that has_capability_noaudit can not
> be called from an interrupt.

Right. So any code that uses %pK from interrupt context would be, by
definition, broken. That is what the check is looking for.

It doesn't matter what the value of kptr_restrict happens to be, the
code is still broken. So, with your patch, values 0 and 2 of
kptr_restrict will print a seemingly correct value, but when
kptr_restrict is 1 then it will print 'pK-error'.

That is confusing to users: file prints correct pointer values when
kptr_restrict=0; file prints 'pK-error' when kptr_restrict=1, even
though I am root. Also, because the default is now kptr_restrict=0, less
people will see the 'pK-error' value, so buggy code may go over-looked.

It should print 'pK-error' in all cases, so that any bugs where %pK is
being used from interrupt content are identified regardless of the
setting of kptr_restrict.

Anyway, with the approach that Eric and George suggested, this would
become a non-issue. So probably just best to leave the code as is.

~Ryan

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