linux-kernel - Re: SECURITY PROBLEM: filesystem permiossion bypass on FD already opened

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Date:	Sat, 24 Oct 2009 00:42:03 +0200
From:	NiTRo <nitroml@....homelinux.org>
To:	Pavel Machek <pavel@....cz>
CC:	Marcin Slusarz <marcin.slusarz@...il.com>,
	linux-kernel@...r.kernel.org, cve@...re.org,
	Jamie Lokier <jamie@...reable.org>
Subject: Re: SECURITY PROBLEM: filesystem permiossion bypass on FD already
 opened

Pavel Machek ha scritto:
> On Fri 2009-10-23 22:44:44, Marcin Slusarz wrote:
>   
>> On Fri, Oct 23, 2009 at 10:00:47PM +0200, NiTRo wrote:
>>     
>>> Hi to all,
>>>     Sorry for my bad english.
>>>     Just discovered this security problem on my Suse 11 (Linux xxxx
>>>       
>> You did not.
>> http://lkml.org/lkml/2009/10/23/159
>>     
>
> Actually, no, this is something different... and old/known AFAICT.
>   
Marcin, I just saw this, it's quite similar, right... But I don't fall back to /proc/self/fd for reading... I still use the same FD...

>   
>>> 2.6.25.18-0.2-pae #1 SMP 2008-10-21 16:30:26 +0200 i686 i686 i386
>>> GNU/Linux) and my Slackware 10.1.0 (Linux xxxx 2.4.29-ow1 #1 Wed Feb 2
>>> 00:05:42 CET 2005 i586 unknown unknown GNU/Linux) with OpenWall patch.
>>> If a FD is opened on a allowed file and then the permission is changed
>>> the file is still redeable starting from the already read position to
>>> the EOF.
>>>
>>> This is the scenario:
>>>
>>> <root> creates a file /tmp/aaaa with 666 permission an with the "test"
>>> string inside it
>>>         xxx:/tmp # echo test > /tmp/aaaa
>>>         xxx:/tmp # chmod 666 /tmp/aaaa
>>> <sb> opens this file hooking it in a shell as FD number 3
>>>         sb@xxx:~> bash 3< /tmp/aaaa
>>> <sb> read and prints it
>>>         sb@xxx:~> read a <&3
>>>         sb@xxx:~> echo $a
>>>         test
>>>         sb@xxx:~>
>>> ...anythig as expected...
>>> <root> changes the permissions on file to 600 and changes its content
>>> into "test o.o I cannot believe it..."
>>>         xxx:/tmp # chmod 600 /tmp/aaaa
>>>         xxx:/tmp # echo "test o.o I cannot believe it..." > /tmp/aaaa
>>> <sb> continue to try reading the file
>>>         sb@xxx:~> read a <&3
>>>         sb@xxx:~> echo $a
>>>         o.o I cannot believe it...
>>>         sb@...t:~>
>>> ... and this is not expected...
>>>       
>
>
> Really? I'd expect it. I have file open for reading, you wrote
> something new to it, so I can read it back. What is the problem?
> 									Pavel
>   

I sow the second mail to... Shurely you have right... I'm sorry for this
"no-bug"...
I'd expect a read error due to permissions change... Sorry

Thanks a lot
Alessandro
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