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

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Message-ID: <4C7D0DAD.9030505@redhat.com>
Date:	Tue, 31 Aug 2010 10:11:57 -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 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.
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