** Information type changed from Private Security to Public Security
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https://bugs.launchpad.net/bugs/1654676
Title:
lxc-user-nic does not ensure that target netns is caller-owned
Status in lxc package in Ubuntu:
Fix Released
Bug description:
The documentation for lxc-user-nic claims:
It ensures that the calling
user is privileged over the network namespace to which the interface
will be attached.
Actually, the code only verifies that a process of the calling user is
running in the network namespace to which the interface will be
attached. To verify this:
- As root, create a new bridge "lxcbr0".
- As root, add an entry like "user veth lxcbr0 10" to
/etc/lxc/lxc-usernet.
- As the user specified in /etc/lxc/lxc-usernet, run the following
command:
/usr/lib/x86_64-linux-gnu/lxc/lxc-user-nic a b $$ veth lxcbr0 foobar
- Run "ip link show foobar". The output shows that a network interface
with the name "foobar" was created in the init namespace (in which the
user is not privileged).
I suspect that it would also be possible to trick lxc-user-nic into
operating on namespaces the caller can't access by reassigning the PID
while lxc-user-nic is running, between the access check and the
netlink calls - this is probably quite hard to fix, considering that
the netlink interface just uses a PID to identify the target
namespace.
I haven't found any way to actually exploit this; however, it seems
interesting that this can e.g. be used to create network interfaces
whose names contain special characters like dollar, semicolon and
backtick. I'm not sure how dangerous that is - I'm reporting it as a
security bug for now, but if you feel that this has no security
impact, feel free to derestrict it.
One way to fix this might be to temporarily drop privileges
(setresuid(ruid, ruid, 0)) in rename_in_ns() after the first setns()
call. This way, the renaming could only succeed if the caller is
actually privileged in the network namespace.
release: Ubuntu 16.10
version: lxc-common: 2.0.6-0ubuntu1~ubuntu16.10.1
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