On Wed, 2022-12-21 at 20:21 -0500, Paul Moore wrote: > On Wed, Dec 21, 2022 at 2:24 PM Paolo Abeni <pabeni@xxxxxxxxxx> wrote: > > I just tested the other option and there is another problem :( > > It's never easy, is it? ;) > > > The first subflow creations happens inside af_inet->create, via the sk- > > > sk_prot->init() hook. The security_socket_post_create() call on the > > owning MPTCP sockets happens after that point. So we copy data from a > > not yet initialized security context (and the test fail badly). > > Hmmm. Let's come back to this later on down this email. > > > There are a few options to cope with that: > > - [ugly hack] call security_socket_post_create() on the mptcp code > > before creating the subflow. I experimented this just to double the > > problem and a possible solution. > > I'm guessing "[ugly hack]" is probably a bit of an understatement. > Let's see if we can do better before we explore this option too much > further. Yup, I compiled the list in "brainstom-mode", trying to include whatever would be possible even if clearly not suitable. [...] > > WDYT? > > Let's go back to the the inet_create() case for a little bit. I'm > thinking we might be able to do something by leveraging the > sk_alloc()->sk_prot_alloc()->security_sk_alloc() code path. As > inet_create() is going to be called from task context here, it seems > like we could do the sock's sid/sclass determination here, cached in > separate fields in the sk_security_struct if necessary, and use those > in a new MPTCP subflow hook. We could also update > selinux_socket_post_create() to take advantage of this as well. We > could also possibly pass the proto struct into security_sk_alloc() if > we needed to identify IPPROTO_MPTCP there as well. > > I'll admit to not chasing down all the details, but I suspect this may > be the cleanest option - thoughts? Thanks, I did not consider such possibility! I think we should be careful to avoid increasing sk_security_struct size. Currently it is 16 bytes, nicely matching a kmalloc slab, any increase will move it on kmalloc-32 bytes slab possibly causing performance and memory regressions). More importantly, I think there is a problem with the sk_clone_lock() -> sk_prot_alloc() -> security_sk_alloc() code path. sk_clone_lock() happens in BH context, if security_transition_sid() needs process context that would be a problem - quickly skimming the code it does not look so, I need to double check. sk_clone_lock() is in a very critical path - socket creation for incoming connections. The sid-related operation there will be unnecessary/discarded by later the selinux_inet_csk_clone(), this will likelly cause performance regressions even for plain TCP sockets. Perhaps the cleanest option could be the one involving the mptcp refactoring, moving subflow creation at a later stage. It could have some minor side benefit for MPTCP, too - solving: https://github.com/multipath-tcp/mptcp_net-next/issues/290 but I'm not fond of that option because it will require quite a bit of time: we need first to have the mptcp refactor in place and then cook the lsm patches. I guess such process will require at least 2 release cycles, due to the needed mptcp(netdev)/lsm trees synchronization. If that would prove to be the most reasonable option, could we consider to transiently merge first something alike: https://lore.kernel.org/mptcp/CAHC9VhSQnhH3UL4gqzu+YiA1Q3YyLLCv88gLJOvw-0+uw5Lvkw@xxxxxxxxxxxxxx/T/#m06c612f84f6b6fe759e670573b2c8092df71607b to have a workable short-term solution, and later revert it when the final solution would be in place? Thanks, Paolo
