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SubjectRe: [PATCH] sysctl: fix permission check while owner isn't GLOBAL_ROOT_UID
On Mon, Jun 28, 2021 at 09:14:10PM +0800, yang.yang29@zte.com.cn wrote:
> > I'm confused about what the actual problem is tbh:
> >
> > root@h3:~# stat -c "%A %a %n" /proc/sys/net/ipv4/ip_forward
> > -rw-r--r-- 644 /proc/sys/net/ipv4/ip_forward
> >
> > root@h3:~# echo 0 > /proc/sys/net/ipv4/ip_forward
> >
> > root@h3:~# cat /proc/sys/net/ipv4/ip_forward
> > 0
> >
> > root@h3:~# cat /proc/self/uid_map
> > 0 100000 1000000000
>
> Sorry to describe too simple. More specific:
> 1.Run dockerd with user namespace enbled
> echo dockremap:296608:65536 > /etc/subuid;echo dockremap:296608:65536 > /etc/subgid
> dockerd ... --userns-remap=default
> 2.Create a container
> docker run -it ... sh
>
> Then root account in the container is 296608 account in the host:
> / # cat /proc/self/uid_map
> 0 296608 65536
>
> In the container, /proc/sys/net/ipv4/ip_forward's owner is root, but root can't modify it:
> / # whoami
> root
> / # ls -l /proc/sys/net/ipv4/ip_forward
> -rw-r--r-- 1 root root 0 Jun 28 12:46 /proc/sys/net/ipv4/ip_forward
> / # cat /proc/sys/net/ipv4/ip_forward
> 1
> / # echo 0 > /proc/sys/net/ipv4/ip_forward
> sh: can't create /proc/sys/net/ipv4/ip_forward: Permission denied
>
> And /proc/sys/net/ipv4/ip_forward has considerd about net namespace,
> see net_ctl_set_ownership() in net\sysctl_net.c.
> So root in container should be able to modify /proc/sys/net/ipv4/ip_forward.
> This doesn't impact /proc/sys/net/ipv4/ip_forward in the host or other container with other net namespace.

Sorry to resort to "It works on my machine" but I just pasted you the
exact same setup and showed you that this works as expected.

So the reason here is likely that you're lacking capabilities
specifically CAP_NET_ADMIN. If I reproduce with your setup I can't even
create a dummy network device:

/ # ip link add bla type dummy
ip: RTNETLINK answers: Operation not permitted

whereas on any unprivileged container that reatins CAP_NET_ADMIN that
just works fine.

So I would think that if you retain CAP_NET_ADMIN things will work out
just fine. Take a look at net_ctl_permissions.

There's really no reason to drop CAP_NET_ADMIN in a network namespace
that is owned by a non-initial user namespace anyway.

Christian

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