Opening /proc/<pid>/net/dev prevents network namespace from expiring

Opening /proc/<pid>/net/dev prevents network namespace from expiring

[Arne Welzel]

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Hello,

as an unprivileged user one is able to keep network namespaces from
expiring by opening /proc/<pid>/net/dev of other processes. I've previously
put this on stackexchange [1] and then bugzilla [2]. That's been a while
though, so posting here for a bit more visibility in case it's something
that's worth fixing.

The reproducer is roughly as follows. As root:
# echo "100" > /proc/sys/user/max_net_namespaces
# while true ; do (unshare -n bash -c 'sleep 0.3 && readlink
/proc/self/ns/net') || sleep 0.5  ; done

As unprivileged user in a second terminal, run the below Python script [3]:
# python3 pin_net_namespaces.py

After about one minute the first terminal will show the following until the
Python process keeping the network namespaces alive is terminated.
...
unshare: unshare failed: No space left on device
unshare: unshare failed: No space left on device

Without the change to max_net_namespaces reproducing just very long, but
then also kernel memory grows fairly large.

Does that seem like problematic behavior? I had attached a patch and tests
to [2], but I fall into the kernel newbie category, so not sure how useful.

Thanks,
   Arne


[1]
https://unix.stackexchange.com/questions/576718/opening-proc-pid-net-dev-prevents-network-namespace-from-expiring-is-this-ex/
[2] https://bugzilla.kernel.org/show_bug.cgi?id=207351

[3] $ cat pin_net_namespaces.py
#!/usr/bin/env python3
import glob
import os
import time

net_namespaces = {}

while True:
    for net_dev in glob.glob("/proc/*/net/dev"):
        try:
            ino = os.stat(net_dev).st_ino
            if ino not in net_namespaces:
                net_namespaces[ino] = open(net_dev)
                print("Have", len(net_namespaces), "namespaces...")
        except FileNotFoundError:
            # not fast enough...
            pass

    time.sleep(0.2)
===

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Re: Opening /proc/<pid>/net/dev prevents network namespace from expiring

[Greg KH]

On Sat, Aug 22, 2020 at 11:05:53PM +0200, Arne Welzel wrote:
> Hello,
> 
> as an unprivileged user one is able to keep network namespaces from
> expiring by opening /proc/<pid>/net/dev of other processes. I've previously
> put this on stackexchange [1] and then bugzilla [2]. That's been a while
> though, so posting here for a bit more visibility in case it's something
> that's worth fixing.

The proper place to send networking bugs/issues is to the networking
developers themselves, at netdev@vger.kernel.org.

Try sending it there, no kernel developers use stackexchange for
development :)

thanks,

greg k-h

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Re: Opening /proc/<pid>/net/dev prevents network namespace from expiring

[Grant Taylor]

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On 8/22/20 3:05 PM, Arne Welzel wrote:
> Hello,

Hi,

> as an unprivileged user one is able to keep network namespaces from 
> expiring by opening /proc/<pid>/net/dev of other processes.

This is what I would expect.

At least based on my understanding of how the various namespaces work. 
If something is using the namespace, it's not removed.  Opening a file / 
device therein would count as using it.

> Does that seem like problematic behavior?

No, not as such.

This seems to me like a permissions issue on the /proc/<pid> directory.

Read:  Set the permissions such that access is restricted appropriately.



-- 
Grant. . . .
unix || die


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