Fix heap-use-after-free due to OML link destruction

ipaccess_drop_oml was being called inside an osmo_fd cb context, were
-EBADF must be returned if the structure holding the osmo_fd is freed.
In the middle of the path (see OS#3495 for path tree) it goes through a
signal dispatch, so it's impossible to make sure we return some value to
the osmo_fd cb. As a result, it is required to defer dropping the OML
Link from current code path and do it through a timer.

Fixes following ASan report:
20180822124927913  <0004> abis_nm.c:787 OC=RADIO-CARRIER(02) INST=(00,00,ff): CHANGE ADMINISTRATIVE STATE NACK CAUSE=Message cannot be performed
20180822124927913  <0004> osmo_bsc_main.c:186 Got CHANGE ADMINISTRATIVE STATE NACK going to drop the OML links.
20180822124927913  <0015> bts_ipaccess_nanobts.c:406 (bts=0) Dropping OML link.
...
=================================================================
==17607==ERROR: AddressSanitizer: heap-use-after-free on address 0x62e000060a68 at pc 0x7f5ea8e27086 bp 0x7ffde92b6d80 sp 0x7ffde92b6d78
READ of size 8 at 0x62e000060a68 thread T0
    #0 0x7f5ea8e27085 in handle_ts1_write input/ipaccess.c:371
    #1 0x7f5ea8e27085 in ipaccess_fd_cb input/ipaccess.c:391
    #2 0x7f5ea9147ca8 in osmo_fd_disp_fds libosmocore/src/select.c:217
    #3 0x7f5ea9147ca8 in osmo_select_main libosmocore/src/select.c:257
    #4 0x555813ab79d6 in main osmo-bsc/osmo_bsc_main.c:922
    #5 0x7f5ea76d02e0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202e0)
    #6 0x555813ab84e9 in _start (/bin/osmo-bsc+0x34d4e9)

Fixes: OS#3495
Change-Id: I7c794c763481c28e8c35dc9b11d27969e16feb3c

1 files changed, 2 insertions, 0 deletions

diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index f85887ac7..b827d0a5a 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -923,6 +923,8 @@ struct gsm_bts {
 	struct gsm_e1_subslot oml_e1_link;
 	uint8_t oml_tei;
 	struct e1inp_sign_link *oml_link;
+	/* Timer to use for deferred drop of OML link, see \ref ipaccess_drop_oml_deferred */
+	struct osmo_timer_list oml_drop_link_timer;
 	/* when OML link was established */
 	time_t uptime;