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

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Date:	Sun, 29 Sep 2013 20:41:05 -0400
From:	Dan Rosenberg <dan.j.rosenberg@...il.com>
To:	George Spelvin <linux@...izon.com>
CC:	rmallon@...il.com, akpm@...ux-foundation.org,
	eldad@...refinery.com, jkosina@...e.cz,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] printk: Check real user/group id for %pK

On 09/29/2013 07:41 PM, George Spelvin wrote:
>> Right, so the pppd application is actually doing the correct thing.
> And a CAP_SYSLOG setuid binary that *doesn't* DTRT seems like a more
> immediate security hole than leaking kernel addresses.  After all
> kptr_restrict is optional precisely because the benefit is marginal.
>
> The interesting question is what credentials make sense for %pK outside
> of a seq_printf().  Does it even make sense in a generic printk?  In that
> case, it's the permission of the syslogd that matters rather than the
> process generating the message.
>
>> Will wait and see what others have to say.
> Me, too.  Dan in particular.

Firstly, I wouldn't recommend applying %pK's to printk usage. Removing
all addresses from the kernel syslog compromises its usefulness in
debugging basically anything at all. Additionally, many printk calls are
performed from a context where a capability check would yield
unpredictable (or at least meaningless) results. If you want to restrict
access to the kernel syslog by unprivileged users, that should be done
by enabling CONFIG_DMESG_RESTRICT, which was written for this purpose.

With that out of the way, I don't have a strong opinion on how to handle
this case. The proposed patch solves the problem but may break setuid
applications that expect to be able to read /proc/kallsyms contents
based on euid (and implicitly, capabilities) alone. But then again,
these mythical setuid applications are probably broken in some
situations anyway, because what happens if /proc/kallsyms is set to "2"
(unconditionally replace addresses with 0's)? I also can't think of a
better solution.

-Dan
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