On Mon, Sep 30, 2019 at 09:10:18PM +0800, Xin Long wrote:

This patch is to fix a NULL-ptr deref crash in selinux_sctp_bind_connect:

  [...] kasan: GPF could be caused by NULL-ptr deref or user memory access
  [...] RIP: 0010:selinux_sctp_bind_connect+0x16a/0x230
  [...] Call Trace:
  [...]  security_sctp_bind_connect+0x58/0x90
  [...]  sctp_process_asconf+0xa52/0xfd0 [sctp]
  [...]  sctp_sf_do_asconf+0x782/0x980 [sctp]
  [...]  sctp_do_sm+0x139/0x520 [sctp]
  [...]  sctp_assoc_bh_rcv+0x284/0x5c0 [sctp]
  [...]  sctp_backlog_rcv+0x45f/0x880 [sctp]
  [...]  __release_sock+0x120/0x370
  [...]  release_sock+0x4f/0x180
  [...]  sctp_accept+0x3f9/0x5a0 [sctp]
  [...]  inet_accept+0xe7/0x6f0

It was caused by that the 'newsk' sk_socket was not set before going to
security sctp hook when doing accept() on a tcp-type socket:

  inet_accept()->
    sctp_accept():
      lock_sock():
          lock listening 'sk'
                                          do_softirq():
                                            sctp_rcv():  <-- [1]
                                                asconf chunk arrived and
                                                enqueued in 'sk' backlog
      sctp_sock_migrate():
          set asoc's sk to 'newsk'
      release_sock():
          sctp_backlog_rcv():
            lock 'newsk'
            sctp_process_asconf()  <-- [2]
            unlock 'newsk'
    sock_graft():
        set sk_socket  <-- [3]

As it shows, at [1] the asconf chunk would be put into the listening 'sk'
backlog, as accept() was holding its sock lock. Then at [2] asconf would
get processed with 'newsk' as asoc's sk had been set to 'newsk'. However,
'newsk' sk_socket is not set until [3], while selinux_sctp_bind_connect()
would deref it, then kernel crashed.

Note that sctp will migrate such incoming chunks from sk to newsk in
sctp_rcv() if they arrived after the mass-migration performed at
sctp_sock_migrate().

That said, did you explore changing inet_accept() so that
sk1->sk_prot->accept() would return sk2 still/already locked?
That would be enough to block [2] from happening as then it would be
queued on newsk backlog this time and avoid nearly duplicating
inet_accept(). (too bad for this chunk, hit 2 backlogs..)

AFAICT TCP code would be fine with such change. Didn't check other
protocols.