tree 438494755a74c196f0566c55cf2e83e09c7842c0
parent 2c16beeb64054c83dead557ff2ae8ab9eb2b84aa
author Neels Hofmeyr <nhofmeyr@sysmocom.de> 1487463279 +0100
committer Neels Hofmeyr <nhofmeyr@sysmocom.de> 1487512111 +0000

subscr_update_expire_lu(): fix (obscure) segfault

To be paranoid, catch a NULL subscriber and/or bts in
subscr_update_expire_lu(): print an error log and avoid segfault.
(I'm not sure this would really happen in a normal situation.)

During aggressive testing of Paging timeout, I came across this segfault in
msc_release_connection() when conn->expire_timer_stopped is set but
conn->subscr is NULL, at the subscr dereference after:

        if (conn->expire_timer_stopped)
                subscr_update_expire_lu(conn->subscr, conn->bts);

I brought this situation about by a fabricated Paging fault, i.e. in
gsm48_rx_rr_pag_resp() return 0 and don't call gsm48_handle_paging_resp() at
all. Thus conn->subscr is still NULL when expire_timer_stopped is 1.

When looking at CM Service Request handling, the conn->subscr is set before
setting expire_timer_stopped = 1, which is a saner thing to do. But without my
mad 'return 0', there is in fact no way to have a NULL subscriber there.

It looks like all other code paths already do the same, but it's not that
obvious (e.g. _gsm48_rx_mm_serv_req_sec_cb()). So rather catch this case of
NULL conn->subscr, and while at it catch NULL bts as well.

Change-Id: I430dd952b2b928bea7f8360f1e01bb3cccb0a395