linux-kernel - Re: selinux vs devtmpfs (vs udev)

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Message-ID: <4C7D1836.8000303@redhat.com>
Date:	Tue, 31 Aug 2010 10:56:54 -0400
From:	Daniel J Walsh <dwalsh@...hat.com>
To:	Harald Hoyer <harald@...hat.com>
CC:	Eric Paris <eparis@...hat.com>, Kay Sievers <kay.sievers@...y.org>,
	linux-kernel@...r.kernel.org, selinux@...ho.nsa.gov,
	greg@...ah.com, sds@...ho.nsa.gov
Subject: Re: selinux vs devtmpfs (vs udev)

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On 08/31/2010 10:39 AM, Harald Hoyer wrote:
> On 08/31/2010 04:11 PM, Daniel J Walsh wrote:
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>> On 08/31/2010 04:44 AM, Harald Hoyer wrote:
>>> On 08/31/2010 01:14 AM, Eric Paris wrote:
>>>> On Sat, 2010-08-28 at 11:57 +0200, Kay Sievers wrote:
>>>>> On Sat, Aug 28, 2010 at 01:00, Eric Paris<eparis@...hat.com>   wrote:
>>>>
>>>>>> In the new new days of devtmpfs things aren't as nice.  The kernel is
>>>>>> magically creating files in /dev.  These are getting created with the
>>>>>> 'default' SELinux context.  So herein lies the problem.
>>>>>>
>>>>>> The first program that tries to access these files get denied by
>>>>>> SELinux.  Now udev actually has logic in it to fix the label on any
>>>>>> closed device file, so udev will at that point swoop in, fix the
>>>>>> label,
>>>>>> and the next program that tries to use the file will work just
>>>>>> fine.  Oh
>>>>>> fun!
>>>>
>>>>> Udev should still label all device nodes, even when they are created
>>>>> by the kernel. Devtmpfs or not should not make a difference here.
>>>>>
>>>>> I guess it's a udev bug introduced with:
>>>>>
>>>>> http://git.kernel.org/?p=linux/hotplug/udev.git;a=commitdiff;h=578cc8a8085a47c963b5940459e475ac5f07219c
>>>>>
>>>>>
>>>>>
>>>>> and we just need to fix that.
>>>>
>>>> Looks like the likely cause.  I see a note in one of the bugzillas that
>>>> says:
>>>>
>>>> Aug 30 14:03:09 pippin udevd-work[347]: preserve file '/dev/dri/card0',
>>>> because it has correct dev_t
>>>>
>>>> Which is certainly the part of code in question.  Do you have a quick
>>>> fix in mind that you plan to push upstream or should I ask the RH udev
>>>> guy to come up with something?
>>>>
>>>> -Eric
>>>>
>>>
>>> The RH udev guy says:
>>>
>>> This patch was introduced, because Red Hat engineers requested, that the
>>> selinux context should not be modified, after they set their own custom
>>> context (virtual machine management).
>>>
>>> So, either we differentiate between "add" and "change" events, or we
>>> should check against the "kernel default" selinux context, before we
>>> call udev_selinux_lsetfilecon().
>>>
>> So the problem is happening because the kernel creates the device rather
>> then udev, and then udev does not change the context because it can not
>> differentiate between this and libvirt putting down a label.
> 
> Is there an easy test to differentiate?
> 
Why is the kernel creating the devices versus telling udev to create the
devices?

You could write a hack that says don't relabel devices with MLS != s0.
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