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Date: Fri, 11 Nov 2011 17:20:44 +1100
From: Robert Ancell <>
 Guido Berhoerster <>
Subject: Re: Re: [LightDM] Version 1.0.6 released

On 10/11/11 23:57, Guido Berhoerster wrote:
> * Marc Deslauriers <> [2011-11-09 16:47]:
>> On Wed, 2011-11-02 at 10:40 -0600, Kurt Seifried wrote:
>>> On 11/02/2011 10:31 AM, Yves-Alexis Perez wrote:
>>>> On mer., 2011-11-02 at 10:16 -0600, Kurt Seifried wrote:
>>>>> On 11/02/2011 09:54 AM, Yves-Alexis Perez wrote:
>>>>>> On mer., 2011-11-02 at 11:42 -0400, Robert Ancell wrote:
>>>>>>> Fixes a security issue where using ~/.Xauthority as a symlink would
>>>>>>> cause LightDM to set the destination of the link to user ownership.
>>>>>>> All users of 1.0.4 or 1.0.5 should upgrade immediately.
>>>>>>> Overview of changes in lightdm 1.0.6
>>>>>>>     * Use lchown for correcting ownership of ~/.Xauthority instead of chown
>>>>>> Could a CVE be assigned for this?
>>>>>> Regards,
>>>>> Can you send me the link to this announcement so I can confirm it? Thanks.
>>>> Here's the link to the mailing list mail:
>>>> Regards,
>>> Thanks, confirmed (first hand info is much better). Please use
>>> CVE-2011-4105 for this issue.
>> BTW, the fix that is in 1.0.6 is probably not enough for distros that
>> don't implement hard link restrictions, such as the Yama LSM that is
>> used in Ubuntu.
> Does an incomplete fix in a released version warrant a new CVE?
> I've attached a suggested fix.
Note the attached patch can still be exploited; if the file changes from
a standard file to a hard link / symlink between the lstat and the
fchown then lightdm can be fooled into thinking it's safe when it's
not.  A malicious program could sit there creating a file, deleting it,
then creating a link as fast as possible and eventually it would work. 
We need an atomic operation like lchown, and if that doesn't work the
only safe thing I can think of doing is a) nothing (requiring the user
to manually fix the bug) or b) delete the file (could delete information
set by other programs).

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