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

Eric W. Biederman
2017-05-14T14:04:09Z

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();