Re: [PATCH v12 0/9] LSM: Multiple concurrent LSMs

Casey Schaufler
2013-01-08T18:17:47Z

On 1/8/2013 9:47 AM, Stephen Smalley wrote:
> On 01/07/2013 08:54 PM, Casey Schaufler wrote:
>> Subject: [PATCH v12 0/9] LSM: Multiple concurrent LSMs
>>
>> Change the infrastructure for Linux Security Modules (LSM)s
>> from a single vector of hook handlers to a list based method
>> for handling multiple concurrent modules.
>>
>> A level of indirection has been introduced in the handling of
>> security blobs. LSMs no longer access ->security fields directly,
>> instead they use an abstraction provided by lsm_[gs]et field
>> functions.
>>
>> The XFRM hooks are only used by SELinux and it is not clear
>> that they can be shared. The First LSM that registers using
>> those hooks gets to use them. Any subsequent LSM that uses
>> those hooks is denied registration.
>>
>> Secids have not been made shareable. Only one LSM that uses
>> secids (SELinux and Smack) can be used at a time. The first
>> to register wins.
>>
>> The "security=" boot option takes a comma separated list of
>> LSMs, registering them in the order presented. The LSM hooks
>> will be executed in the order registered. Hooks that return
>> errors are not short circuited. All hooks are called even
>> if one of the LSM hooks fails. The result returned will be
>> that of the last LSM hook that failed.
>>
>> Some hooks don't fit that model. setprocattr, getprocattr,
>> and a few others are special cased. All behavior from
>> security/capability.c has been moved into the hook handling.
>> The security/commoncap functions used to get called from
>> the LSM specific code. The handling of the capability
>> functions has been moved out of the LSMs and into the
>> hook handling.
>>
>> The /proc/*/attr interfaces are given to one LSM. This
>> can be done by setting CONFIG_SECURITY_PRESENT. Additional
>> interfaces have been created in /proc/*/attr so that
>> each LSM has its own named interfaces.
>>
>> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
>
> Have you run any benchmarks, particularly to compare performance
> overhead in the simple case of a single LSM?

My benchmark work is in the early stages and frankly my
facilities are poorly suited to getting real numbers. One of
the things that someone in the community could do that would
be very helpful would be do some such comparisons. I am
working on getting numbers that are defensible, but it will
take a bit of time.

>
> IIRC, the AppArmor devs indicated that they plan to start using
> secids, which would mean that it would not be possible to stack
> AppArmor with Smack or SELinux using this mechanism. So eventually
> that would have to be addressed in order for this to even support the
> AppArmor+Smack or AppArmor+SELinux use cases.
>
That is my understanding as well. Having groveled around in the
realities of LSM hook usage for some time now I have developed ideas
around the problem. Unfortunately, no one change is going to address
all of the use models. Networking, audit and caching all use secids
in their own clever ways. This is another area where I would be
delighted to entertain suggested implementations.