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Date:   Tue, 13 Jul 2021 17:55:30 +0800
Message-Id: <20210713175519.Bluez.v3.1.Id7aa1152377161d17b442bf258773d9b6c624ca3@changeid>
Mime-Version: 1.0
Subject: [Bluez PATCH v3] a2dp: Fix crash in channel_free while waiting cmd resp
From:   Howard Chung <howardchung@google.com>
To:     linux-bluetooth@vger.kernel.org, luiz.dentz@gmail.com
Cc:     Yun-Hao Chung <howardchung@chromium.org>,
        Archie Pusaka <apusaka@chromium.org>
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Precedence: bulk

From: Yun-Hao Chung <howardchung@chromium.org>

When channel_free is called and we are waiting for a command response
from the peer, bluez NULL the setup->session but would not free its
setup_cb. Since setup_cb holds a ref of setup, the setup wouldn't be
freed and if service_removed is called after channel_free, a2dp_cancel
tries to abort the ongoing avdtp commands, which accesses the
setup->session and triggers a crash.

This change finalizes all avdtp commands before assigning setup->session
to NULL in channel_free.

Crash stack trace:
0x000059f01943e688	(bluetoothd -avdtp.c:3690)
avdtp_abort
0x000059f01943928a	(bluetoothd -a2dp.c:3069)
a2dp_cancel
0x000059f0194377fa	(bluetoothd -sink.c:324)
sink_unregister
0x000059f01948715a	(bluetoothd -service.c:177)
service_remove
0x000059f01948d77c	(bluetoothd -device.c:5346)
device_remove
0x000059f019476d14	(bluetoothd -adapter.c:7202)
adapter_remove
0x000059f019476c3e	(bluetoothd -adapter.c:10827)
adapter_cleanup
0x000059f01949d8d7	(bluetoothd -main.c:1114)		main
0x0000787b36185d74	(libc.so.6 -libc-start.c:308)
__libc_start_main
0x000059f019433e39	(bluetoothd + 0x00026e39)		_start
0x00007fff2d2c0127

Reviewed-by: Archie Pusaka <apusaka@chromium.org>
---
There are two other options to fix this crash.
1. add a NULL check in a2dp_cancel before calling avdtp_abort.
2. call setup_cb_free to every setup_cb in setup->cb in channel_free.

Since each setup_cb needs setup->session, I think there is no need to
keep the setup_cb after assigning setup->session to NULL. So the first
option is not ideal. If the second option is adopted, there would be
some time that sink/source->connect_id/disconnect_id is not zero, but
there is no corresponding setup_cb.

Test steps:
Reproduce the crash with the following steps. Verify the crash is
no longer observed after this change.
1. ignore AVDTP_SET_CONFIGURATION resp by modifying avdtp.c
2. turn on a paired headset
3. check the bluetooth.log, while bluez is waiting for
   AVDTP_SET_CONFIGURATION resp, stop bluetoothd immediately.
   This will trigger:
   session_cb (I/O error) -> connection_lost
   -> avdtp_set_state (AVDTP_SESSION_STATE_DISCONNECTED)
   -> avdtp_state_cb -> channel_remove -> channel_free
   then:
   adapter_cleanup -> adapter_remove -> device_remove -> service_remove
   -> a2dp_sink_remove -> sink_unregister -> sink_free -> a2dp_cancel
4. check if bluetoothd crash
The above steps can trigger the crash 100%.

Changes in v3:
- Remove the duplicated finalize_all

Changes in v2:
- Implement helper function finalize_all
- add setup_ref before finalize_all, in case the setup is freed during
  finalize

 profiles/audio/a2dp.c | 37 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/profiles/audio/a2dp.c b/profiles/audio/a2dp.c
index d31ed845cbe7..39d5e3ef736c 100644
--- a/profiles/audio/a2dp.c
+++ b/profiles/audio/a2dp.c
@@ -404,6 +404,38 @@ static void finalize_discover(struct a2dp_setup *s)
 	}
 }
 
+static void finalize_all(struct a2dp_setup *s)
+{
+	GSList *l;
+	struct avdtp_stream *stream = s->err ? NULL : s->stream;
+
+	for (l = s->cb; l != NULL; ) {
+		struct a2dp_setup_cb *cb = l->data;
+
+		l = l->next;
+
+		if (cb->discover_cb) {
+			cb->discover_cb(s->session, s->seps,
+					error_to_errno(s->err), cb->user_data);
+		} else if (cb->select_cb) {
+			cb->select_cb(s->session, s->seps, s->caps,
+					error_to_errno(s->err), cb->user_data);
+		} else if (cb->suspend_cb) {
+			cb->suspend_cb(s->session,
+					error_to_errno(s->err), cb->user_data);
+		} else if (cb->resume_cb) {
+			cb->resume_cb(s->session,
+					error_to_errno(s->err), cb->user_data);
+		} else if (cb->config_cb) {
+			cb->config_cb(s->session, s->seps, stream,
+					error_to_errno(s->err), cb->user_data);
+		} else
+			warn("setup_cb doesn't have any callback function");
+
+		setup_cb_free(cb);
+	}
+}
+
 static struct a2dp_setup *find_setup_by_session(struct avdtp *session)
 {
 	GSList *l;
@@ -1540,9 +1572,12 @@ static void channel_free(void *data)
 	setup = find_setup_by_session(chan->session);
 	if (setup) {
 		setup->chan = NULL;
+		setup_ref(setup);
+		/* Finalize pending commands before we NULL setup->session */
+		finalize_setup_errno(setup, -ENOTCONN, finalize_all, NULL);
 		avdtp_unref(setup->session);
 		setup->session = NULL;
-		finalize_setup_errno(setup, -ENOTCONN, NULL);
+		setup_unref(setup);
 	}
 
 	g_free(chan);
-- 
2.32.0.93.g670b81a890-goog