Re: SECURITY PROBLEM: filesystem permiossion bypass on FD alreadyopened

[Marcin Slusarz]

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

> 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@test:~>
> ... and this is not expected...
> 
> Writing control seems to be working fine... "bash: echo: write error:
> Bad file descriptor"
> 
> Hope this can help...
> 
> Thanks to all
> Alessandro Soraruf
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