linux-kernel - Re: [PATCH] drm/exynos: Print kernel pointers in a restricted form

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Message-id: <ee50f87d-d79e-d74e-25b3-091c49a5e1d4@samsung.com>
Date:   Wed, 15 Mar 2017 08:32:56 +0100
From:   Andrzej Hajda <a.hajda@...sung.com>
To:     Tobias Jakobi <tjakobi@...h.uni-bielefeld.de>,
        Krzysztof Kozlowski <krzk@...nel.org>
Cc:     Inki Dae <inki.dae@...sung.com>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        David Airlie <airlied@...ux.ie>, Kukjin Kim <kgene@...nel.org>,
        Javier Martinez Canillas <javier@....samsung.com>,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/exynos: Print kernel pointers in a restricted form

Hi Tobias,

On 14.03.2017 21:41, Tobias Jakobi wrote:
> Krzysztof Kozlowski wrote:
>> On Tue, Mar 14, 2017 at 08:17:35PM +0100, Tobias Jakobi wrote:
>>> Krzysztof Kozlowski wrote:
>>>> On Tue, Mar 14, 2017 at 08:01:41PM +0100, Tobias Jakobi wrote:
>>>>> Hello Krzysztof,
>>>>>
>>>>> I was wondering about the benefit of this. From a quick look these are
>>>>> all messages that end up in the kernel log / dmesg.
>>>>>
>>>>> IIRC %pK does nothing there, since dmest_restrict is supposed to be used
>>>>> to deny an unpriviliged user the access to the kernel log.
>>>>>
>>>>> Or am I missing something here?
>>>> These are regular printks so depending on kernel options (e.g. dynamic
>>>> debug, drm.debug) these might be printed also in the console. Of course
>>>> we could argue then if access to one of the consoles is worth
>>>> securing.
>>> This here suggests otherwise.
>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/sysctl/kernel.txt#n388
>>>
>>> I have not tested this, but IIRC %pK is not honored by the kernel
>>> logging infrastucture. That's why dmesg_restrict is there.
>>>
>>> Correct me if I'm wrong.
>> The %pK will not help for dmesg or /proc/kmsg but it will help for
>> console (/dev/ttySACN, ttySN etc) because effectively it uses the same
>> vsprintf()/pointer() functions.
> Thanks for the explanation, I didn't know that there was a difference
> there. In that case, looks good to me.
>
>

Just to clarify %pK:

Documentation/printk-formats.txt:
        %pK     0x01234567 or 0x0123456789abcdef

        For printing kernel pointers which should be hidden from
unprivileged
        users. The behaviour of %pK depends on the kptr_restrict sysctl
- see
        Documentation/sysctl/kernel.txt for more details.

Documentation/sysctl/kernel.txt:

kptr_restrict:

This toggle indicates whether restrictions are placed on
exposing kernel addresses via /proc and other interfaces.

When kptr_restrict is set to (0), the default, there are no restrictions.

When kptr_restrict is set to (1), kernel pointers printed using the %pK
format specifier will be replaced with 0's unless the user has CAP_SYSLOG
and effective user and group ids are equal to the real ids. This is
because %pK checks are done at read() time rather than open() time, so
if permissions are elevated between the open() and the read() (e.g via
a setuid binary) then %pK will not leak kernel pointers to unprivileged
users. Note, this is a temporary solution only. The correct long-term
solution is to do the permission checks at open() time. Consider removing
world read permissions from files that use %pK, and using dmesg_restrict
to protect against uses of %pK in dmesg(8) if leaking kernel pointer
values to unprivileged users is a concern.

When kptr_restrict is set to (2), kernel pointers printed using
%pK will be replaced with 0's regardless of privileges.
---

Regards
Andrzej