gatmux: take reference to mux object while processing incoming data

When closing down a cmux object, the address sanitizer detects a
use-after-free in gatmux.c (see below).

Avoid this by taking a reference to the mux object during the processing
in received_data().

ofonod[3640549]: ../git/plugins/quectel.c:cfun_disable() 0x610000000b40
ofonod[3640549]: ../git/plugins/quectel.c:close_serial() 0x610000000b40
ofonod[3640549]: ../git/plugins/quectel.c:close_mux() 0x610000000b40
ofonod[3640549]: ../git/examples/emulator.c:powered_watch() Removing modem 0x610000000b40 from the list
ofonod[3640549]: ../git/examples/emulator.c:powered_watch() Removing server watch: 106
ofonod[3640549]: ../git/src/modem.c:modem_change_state() old state: 0, new state: 0

=================================================================
==3640549==ERROR: AddressSanitizer: heap-use-after-free on address 0x62100073dd28 at pc 0x5566b6402a21 bp 0x7ffe7a2db0e0 sp 0x7ffe7a2db0d0
READ of size 8 at 0x62100073dd28 thread T0
    #0 0x5566b6402a20 in debug ../git/gatchat/gatmux.c:109
    #1 0x5566b6404bd7 in channel_close ../git/gatchat/gatmux.c:525
    #2 0x7fa0516e44a6 in g_io_channel_shutdown (/usr/lib/libglib-2.0.so.0+0x774a6)
    #3 0x7fa0516e4644 in g_io_channel_unref (/usr/lib/libglib-2.0.so.0+0x77644)
    #4 0x5566b64048a4 in watch_finalize ../git/gatchat/gatmux.c:474
    #5 0x7fa0516d6f6f  (/usr/lib/libglib-2.0.so.0+0x69f6f)
    #6 0x7fa0516ac6a7 in g_slist_foreach (/usr/lib/libglib-2.0.so.0+0x3f6a7)
    #7 0x7fa0516b277b in g_slist_free_full (/usr/lib/libglib-2.0.so.0+0x4577b)
    #8 0x5566b6403413 in dispatch_sources ../git/gatchat/gatmux.c:224
    #9 0x5566b64039ea in received_data ../git/gatchat/gatmux.c:268
    #10 0x7fa0516d727e in g_main_context_dispatch (/usr/lib/libglib-2.0.so.0+0x6a27e)
    #11 0x7fa0516d91c0  (/usr/lib/libglib-2.0.so.0+0x6c1c0)
    #12 0x7fa0516da0d2 in g_main_loop_run (/usr/lib/libglib-2.0.so.0+0x6d0d2)
    #13 0x5566b6429b1b in main ../git/src/main.c:286
    #14 0x7fa05147fee2 in __libc_start_main (/usr/lib/libc.so.6+0x26ee2)
    #15 0x5566b62531ad in _start (/home/martin/projects/ofono/x86/src/ofonod+0xfc1ad)

0x62100073dd28 is located 40 bytes inside of 4672-byte region [0x62100073dd00,0x62100073ef40)
freed by thread T0 here:
    #0 0x7fa0519256c0 in __interceptor_free /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cc:122
    #1 0x5566b64052d7 in g_at_mux_unref ../git/gatchat/gatmux.c:645
    #2 0x5566b63d6d19 in close_mux ../git/plugins/quectel.c:199
    #3 0x5566b63d7047 in close_serial ../git/plugins/quectel.c:223
    #4 0x5566b63db62a in cfun_disable ../git/plugins/quectel.c:1056
    #5 0x5566b63f6ae1 in at_chat_finish_command ../git/gatchat/gatchat.c:459
    #6 0x5566b63f701b in at_chat_handle_command_response ../git/gatchat/gatchat.c:521
    #7 0x5566b63f785b in have_line ../git/gatchat/gatchat.c:600
    #8 0x5566b63f87f1 in new_bytes ../git/gatchat/gatchat.c:759
    #9 0x5566b640174c in received_data ../git/gatchat/gatio.c:122
    #10 0x5566b64047b4 in watch_dispatch ../git/gatchat/gatmux.c:464
    #11 0x5566b640313b in dispatch_sources ../git/gatchat/gatmux.c:183
    #12 0x5566b64039ea in received_data ../git/gatchat/gatmux.c:268
    #13 0x7fa0516d727e in g_main_context_dispatch (/usr/lib/libglib-2.0.so.0+0x6a27e)

previously allocated by thread T0 here:
    #0 0x7fa051925ce8 in __interceptor_calloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cc:153
    #1 0x5566b6405009 in g_at_mux_new ../git/gatchat/gatmux.c:606
    #2 0x5566b6407f6b in g_at_mux_new_gsm0710_basic ../git/gatchat/gatmux.c:1165
    #3 0x5566b63da9ba in cmux_cb ../git/plugins/quectel.c:882
    #4 0x5566b63f6ae1 in at_chat_finish_command ../git/gatchat/gatchat.c:459
    #5 0x5566b63f701b in at_chat_handle_command_response ../git/gatchat/gatchat.c:521
    #6 0x5566b63f785b in have_line ../git/gatchat/gatchat.c:600
    #7 0x5566b63f87f1 in new_bytes ../git/gatchat/gatchat.c:759
    #8 0x5566b640174c in received_data ../git/gatchat/gatio.c:122
    #9 0x7fa0516d727e in g_main_context_dispatch (/usr/lib/libglib-2.0.so.0+0x6a27e)

SUMMARY: AddressSanitizer: heap-use-after-free ../git/gatchat/gatmux.c:109 in debug
Shadow bytes around the buggy address:
  0x0c42800dfb50: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c42800dfb60: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c42800dfb70: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c42800dfb80: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c42800dfb90: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x0c42800dfba0: fd fd fd fd fd[fd]fd fd fd fd fd fd fd fd fd fd
  0x0c42800dfbb0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c42800dfbc0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c42800dfbd0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c42800dfbe0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c42800dfbf0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==3640549==ABORTING

1 files changed, 8 insertions, 1 deletions

diff --git a/gatchat/gatmux.c b/gatchat/gatmux.c
index 757a5123..d50e5361 100644
--- a/gatchat/gatmux.c
+++ b/gatchat/gatmux.c
@@ -231,6 +231,7 @@ static gboolean received_data(GIOChannel *channel, GIOCondition cond,
 	int i;
 	GIOStatus status;
 	gsize bytes_read;
+	gboolean buffer_full = FALSE;
 
 	if (cond & G_IO_NVAL)
 		return FALSE;
@@ -255,6 +256,8 @@ static gboolean received_data(GIOChannel *channel, GIOCondition cond,
 		if (mux->buf_used > 0)
 			memmove(mux->buf, mux->buf + nread, mux->buf_used);
 
+		g_at_mux_ref(mux);
+
 		for (i = 1; i <= MAX_CHANNELS; i++) {
 			int offset = i / 8;
 			int bit = i % 8;
@@ -267,6 +270,10 @@ static gboolean received_data(GIOChannel *channel, GIOCondition cond,
 
 			dispatch_sources(mux->dlcs[i-1], G_IO_IN);
 		}
+
+		buffer_full = mux->buf_used == sizeof(mux->buf);
+
+		g_at_mux_unref(mux);
 	}
 
 	if (cond & (G_IO_HUP | G_IO_ERR))
@@ -275,7 +282,7 @@ static gboolean received_data(GIOChannel *channel, GIOCondition cond,
 	if (status != G_IO_STATUS_NORMAL && status != G_IO_STATUS_AGAIN)
 		return FALSE;
 
-	if (mux->buf_used == sizeof(mux->buf))
+	if (buffer_full)
 		return FALSE;
 
 	return TRUE;