user controllable usermodehelper in br_stp_if.c

[Richard Weinberger]

Hi!

By spawning new network and user namesapces an unprivileged user
is able to execute /sbin/bridge-stp within the initial mount namespace
with global root rights.
While this cannot directly be used to break out of a container or gain
global root rights it could be used by exploit writers as valuable building block.

e.g.
$ unshare -U -r -n /bin/sh
$ brctl addbr br0
$ brctl stp br0 on # this will execute /sbin/bridge-stp

As this mechanism clearly cannot work with containers and seems to be legacy code
I suggest not calling call_usermodehelper() at all if we're not in the initial user namespace.
What do you think?

Thanks,
//richard
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