Re: [PATCH] kmod: don't load module unless req process has CAP_SYS_MODULE

Greg Kroah-Hartman
2017-05-15T06:11:15Z

On Sun, May 14, 2017 at 08:57:34AM -0500, Eric W. Biederman wrote:
> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
>
> > On Fri, May 12, 2017 at 04:22:59PM -0700, Mahesh Bandewar wrote:
> >> From: Mahesh Bandewar <maheshb@google.com>
> >>
> >> A process inside random user-ns should not load a module, which is
> >> currently possible. As demonstrated in following scenario -
> >>
> >> Create namespaces; especially a user-ns and become root inside.
> >> $ unshare -rfUp -- unshare -unm -- bash
> >>
> >> Try to load the bridge module. It should fail and this is expected!
> >> # modprobe bridge
> >> WARNING: Error inserting stp (/lib/modules/4.11.0-smp-DEV/kernel/net/802/stp.ko): Operation not permitted
> >> FATAL: Error inserting bridge (/lib/modules/4.11.0-smp-DEV/kernel/net/bridge/bridge.ko): Operation not permitted
> >>
> >> Verify bridge module is not loaded.
> >> # lsmod | grep bridge
> >> #
> >>
> >> Now try to create a bridge inside this newly created net-ns which would
> >> mean bridge module need to be loaded.
> >> # ip link add br0 type bridge
> >> # echo $?
> >> 0
> >> # lsmod | grep bridge
> >> bridge 110592 0
> >> stp 16384 1 bridge
> >> llc 16384 2 bridge,stp
> >> #
> >>
> >> After this patch -
> >> # ip link add br0 type bridge
> >> RTNETLINK answers: Operation not supported
> >> # echo $?
> >> 2
> >> # lsmod | grep bridge
> >> #
> >
> > Well, it only loads this because the kernel asked for it to be loaded,
> > right?
> >
> >>
> >> Signed-off-by: Mahesh Bandewar <maheshb@google.com>
> >> ---
> >> kernel/kmod.c | 3 +++
> >> 1 file changed, 3 insertions(+)
> >>
> >> diff --git a/kernel/kmod.c b/kernel/kmod.c
> >> index 563f97e2be36..ac30157169b7 100644
> >> --- a/kernel/kmod.c
> >> +++ b/kernel/kmod.c
> >> @@ -133,6 +133,9 @@ int __request_module(bool wait, const char *fmt, ...)
> >> #define MAX_KMOD_CONCURRENT 50 /* Completely arbitrary value - KAO */
> >> static int kmod_loop_msg;
> >>
> >> + if (!capable(CAP_SYS_MODULE))
> >> + return -EPERM;
> >
> > At first glance this looks right, but I'm worried what this will break
> > that currently relies on this. There might be lots of systems that are
> > used to this being the method that the needed module is requested. What
> > about when userspace asks for a random char device and that module is
> > then loaded? Does this patch break that functionality?
>
> For the specific example give I think we would be better served by
> adding a capability check at the call site. In this case CAP_NET_ADMIN
> as those are the capabilities iproute traditionally has.
>
> We have something similar in dev_load in already in the networking code.
>
> This limits the people who can't load modules to root user in user
> namespaces. I would be fine with any other code paths in a user
> namespace getting a similar treatment.
>
> Eric
>
>
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index bcb0f610ee42..6b72528a4636 100644
> --- a/net/core/rtnetlink.c
> +++ b/net/core/rtnetlink.c
> @@ -2595,7 +2595,7 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
>
> if (!ops) {
> #ifdef CONFIG_MODULES
> - if (kind[0]) {
> + if (kind[0] && capable(CAP_NET_ADMIN)) {
> __rtnl_unlock();
> request_module("rtnl-link-%s", kind);
> rtnl_lock();

I don't object to this if the networking developers don't mind the
change in functionality. They can handle the fallout :)

thanks,

greg k-h