SECURITY PROBLEM: filesystem permiossion bypass on FD already opened

SECURITY PROBLEM: filesystem permiossion bypass on FD already opened

[NiTRo]

Hi to all,
    Sorry for my bad english.
    Just discovered this security problem on my Suse 11 (Linux xxxx
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

Re: SECURITY PROBLEM: filesystem permiossion bypass on FD already opened

[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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Re: SECURITY PROBLEM: filesystem permiossion bypass on FD already opened

[Pavel Machek]

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.

> > 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...


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
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

Re: SECURITY PROBLEM: filesystem permiossion bypass on FD already opened

[NiTRo]

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@test:~>
>>> ... 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

Re: SECURITY PROBLEM: filesystem permiossion bypass on FD already opened

[Pavel Machek]

On Fri 2009-10-23 22:00:47, NiTRo wrote:
> Hi to all,
>     Sorry for my bad english.
>     Just discovered this security problem on my Suse 11 (Linux xxxx
> 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.

There's no reason you could not even seek. And no, it is not a
problem, it is just how unix works.
								Pavel


> 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...

But it should be :-).
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

Re: SECURITY PROBLEM: filesystem permiossion bypass on FD already opened

[David Wagner]

NiTRo  wrote:
>    Just discovered this security problem [...]
>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 not a security problem; this is Unix working as designed.
That's how file descriptors work.  File permissions are checked when
the file is opened, not on every read/write to the file descriptor.
That's a pretty fundamental aspect of how Unix works, and it is
well-documented and well-understood.

On the other hand, the /proc attack that Pavel explains *is* a
security problem.  But that's different.

Re: SECURITY PROBLEM: filesystem permiossion bypass on FD already opened

[Pavel Machek]

> On the other hand, the /proc attack that Pavel explains *is* a
> security problem.  But that's different.

I'm glad I convinced at least someone :-). Thanks,
								Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html