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

Mahesh Bandewar (महेश बंडेवार)
2017-05-15T02:42:32Z

On Sun, May 14, 2017 at 3:45 AM, Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
> On Fri, May 12, 2017 at 04:22:59PM -0700, Mahesh Bandewar wrote:
>> From: Mahesh Bandewar <maheshb@google.com>
>>
[...]
>> 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?
>
Yes, kernel asked for it because of a user action.

>>
>> 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?
>
Any module when loaded gets loaded system-wide as we can't allow
module loading per-ns. To validate the behavior I was comparing it
with insmod/modprobe, if that doesn't allow because of lack of this
capability in default-ns, then this *indirect* method of loading
module should not allow the same action and the behavior should be
consistent. So with that logic if userspace asks for a random
char-device if insmod/modprobe cannot load it, then this method should
not load it either for the consistency, right?

> thanks,
>
> greg k-h