[PATCH v20 05/23] net: Prepare UDS for security module stacking

[Casey Schaufler]

On 9/4/2020 2:53 PM, Paul Moore wrote:
> On Fri, Sep 4, 2020 at 5:35 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
>> On 9/4/2020 1:08 PM, Paul Moore wrote:
>>> On Wed, Aug 26, 2020 at 11:07 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
>>>> Change the data used in UDS SO_PEERSEC processing from a
>>>> secid to a more general struct lsmblob. Update the
>>>> security_socket_getpeersec_dgram() interface to use the
>>>> lsmblob. There is a small amount of scaffolding code
>>>> that will come out when the security_secid_to_secctx()
>>>> code is brought in line with the lsmblob.
>>>>
>>>> Signed-off-by: Casey Schaufler <casey at schaufler-ca.com>
>>>> ---
>>>>  include/linux/security.h |  7 +++++--
>>>>  include/net/af_unix.h    |  2 +-
>>>>  include/net/scm.h        |  8 +++++---
>>>>  net/ipv4/ip_sockglue.c   |  8 +++++---
>>>>  net/unix/af_unix.c       |  6 +++---
>>>>  security/security.c      | 18 +++++++++++++++---
>>>>  6 files changed, 34 insertions(+), 15 deletions(-)
>>> ...
>>>
>>>> diff --git a/include/net/af_unix.h b/include/net/af_unix.h
>>>> index f42fdddecd41..a86da0cb5ec1 100644
>>>> --- a/include/net/af_unix.h
>>>> +++ b/include/net/af_unix.h
>>>> @@ -36,7 +36,7 @@ struct unix_skb_parms {
>>>>         kgid_t                  gid;
>>>>         struct scm_fp_list      *fp;            /* Passed files         */
>>>>  #ifdef CONFIG_SECURITY_NETWORK
>>>> -       u32                     secid;          /* Security ID          */
>>>> +       struct lsmblob          lsmblob;        /* Security LSM data    */
>>> As mentioned in a previous revision, I remain concerned that this is
>>> going to become a problem due to the size limit on unix_skb_parms.  I
>>> would need to redo the math to be certain, but if I recall correctly
>>> this would limit us to five LSMs assuming both that we don't need to
>>> grow the per-LSM size of lsmblob *and* the netdev folks don't decide
>>> to add more fields to the unix_skb_parms.
>>>
>>> I lost track of that earlier discussion so I'm not sure where it ended
>>> up, but if there is a viable alternative it might be a good idea to
>>> pursue it.
>> Stephen had concerns about the lifecycle management involved. He also
>> pointed out that I had taken a cowards way out when allocations failed.
>> That could result in unexpected behavior when an allocation failed.
>> Fixing that would have required a major re-write of the currently simple
>> UDS attribute code, which I suspect would be as hard a sell to netdev as
>> expanding the secid to a lsmblob. I also thought I'd gotten netdev on the
>> CC: for this patch, but it looks like I missed it.
>>
>> I did start on the UDS attribute re-do, and discovered that I was going
>> to have to introduce new failure paths, and that it might not be possible
>> to maintain compatibility for all cases because of the new possibilities
>> of failure.
> ... and you're hoping to not be responsible for this code by the time
> this becomes a limiting issue? ;)

Well, maybe. More likely that full dementia will have set in by the
time I get the alternative done correctly. It's a _lot_ more complicated.
I'm carefully watching what the BPF people are doing with their
memory management schemes in the hope they will come up with something
useful. 

> I understand the concerns you mention, they are all valid as far as
> I'm concerned, but I think we are going to get burned by this code as
> it currently stands.

Yes, I can see that. We're getting burned by the non-extensibility
of secids. It will take someone smarter than me to figure out how to
fit N secids into 32bits without danger of either failure or memory
allocation.