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Re: Re: [LightDM] Version 1.0.6 released
From: Robert Ancell <robert.ancell () canonical com>
Date: Fri, 11 Nov 2011 17:20:44 +1100

On 10/11/11 23:57, Guido Berhoerster wrote:
* Marc Deslauriers <marc.deslauriers () canonical com> [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?

Can you send me the link to this announcement so I can confirm it? Thanks.

Here's the link to the mailing list mail:

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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