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

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Date:	Mon, 30 Sep 2013 10:56:13 +1000
From:	Ryan Mallon <rmallon@...il.com>
To:	Dan Rosenberg <dan.j.rosenberg@...il.com>
CC:	George Spelvin <linux@...izon.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 30/09/13 10:41, Dan Rosenberg wrote:
> 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.

Sorry, the patch description should say 'vsprintf: ' not 'printk: '.
Posting too early in the morning :-).

> 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.

Agreed.

> 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.

Okay, this was just the simplest solution I could come up with that
fixed the issue for me. Is that a tentative acked/reviewed-by? :-).

~Ryan


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