Re: [PATCH 0/3] Document impact of user namespaces and negative permissions

[Alejandro Colomar]

Hi Richard,

On 2023-08-29 23:32, Richard Weinberger wrote:
> ----- Ursprüngliche Mail -----
>> Von: "Alejandro Colomar" <alx@kernel.org>
>> Can you please provide a small shell session where this is exemplified?
> 
> Sure. I sent the following to the shadow maintainers privately on Friday,
> but since the issue is already known for years I don't hesitate to share.
> 
> # On a Debian Bookworm
> # So far no entries are installed.
> $ cat /etc/subuid
> 
> # useradd automatically does so.
> $ useradd -m rw
> $ cat /etc/subuid
> rw:100000:65536
> 
> # Let's create a folder where the group "nogames" has no permissions.
> $ mkdir /games
> $ echo win > /games/game.txt
> $ groupadd nogames
> $ chown -R root:nogames /games
> $ chmod 705 /games
> 
> # User "rw" must not play games
> $ usermod -G nogames rw
> 
> # Works as expected
> rw@localhost:~$ id
> uid=1000(rw) gid=1000(rw) groups=1000(rw),1001(nogames)
> rw@localhost:~$ cat /games/game.txt
> cat: /games/game.txt: Permission denied
> 
> # By using unshare (which utilizes the newuidmap helper) we can get rid of 
> the "nogames" group.
> rw@localhost:~$ unshare -S 0 -G 0 --map-users=100000,0,65536 
> --map-groups=100000,0,65536 id
> uid=0(root) gid=0(root) groups=0(root)
> 
> rw@localhost:~$ unshare -S 0 -G 0 --map-users=100000,0,65536 
> --map-groups=100000,0,65536 cat /games/game.txt
> win
> 
> Thanks,
> //richard

Thanks!

Please include this in the commit message (at least for the Linux man-pages
one).

Cheers,
Alex

-- 
<http://www.alejandro-colomar.es/>
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