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

Tobias Jakobi tjakobi at math.uni-bielefeld.de
Wed Mar 15 05:33:16 PDT 2017

Hello Andrzej,

note that i had already pointed Krzysztof to that documentation in my
previous mail.

- Tobias

Andrzej Hajda wrote:
> 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

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