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

[Paul Moore]

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? ;)

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.

-- 
paul moore
www.paul-moore.com