mount + pid namespacing broken ?

From: Frederic Riss
Date: Sat Jun 01 2013 - 17:48:14 EST


I had a little application making use of pid and mount namespaces to
isolate some processes on some machines. This all worked well on 3.7
boxes. A coworker upgraded his machine and noticed that things weren't
working anymore on 3.8. The symptom he noticed is that remounting
/proc inside the namespaced process broke the /proc of the original
namespace. (Remounting /proc is necessary for the pid namespace to be
really effective)

There is a simple way to reproduce the issue if you have a new enough
util-linux where the unshare utility accepts the --pid option. Try

bash-4.2$ unshare --pid --mount
-bash-4.2$ sudo mount -t proc /proc /proc
[sudo] password for friss:
sudo: unable to send audit message: Operation not permitted

[ The audit failure is already a sign of something going wrong more on
that bellow. ]
Then in another terminal running in the root namesapce:

bash-4.2$ ls -l /proc/self
ls: cannot read symbolic link /proc/self: No such file or directory
lrwxrwxrwx 1 root root 0 Jun 1 23:37 /proc/self

As you see, remounting /proc in the private namespace broke the root /proc...

Now the namespace isn't functional anyway even if /proc doesn't get remounted:

bash-4.2$ unshare --pid --mount
-bash-4.2$ id
uid=1001(friss) gid=1001(friss) groups=1001(friss),10(wheel)
-bash-4.2$ id
-bash: fork: Cannot allocate memory

It's able to run one process, but every following invocation will
fail. I suppose the audit issue mentioned above is just another

All of this was working fine on 3.7 kernels. I tried it on latest 3.8
and 3.9 and it fails on both. My application didn't make use of
unshare, but instead forked a new process with the namespacing flags.
The symptoms are identical in both cases.

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