Aiden Luo created GUACAMOLE-428:
-----------------------------------
Summary: segfault in guac_vnc_user_leave_handler
Key: GUACAMOLE-428
URL: https://issues.apache.org/jira/browse/GUACAMOLE-428
Project: Guacamole
Issue Type: Bug
Components: VNC
Affects Versions: 0.9.13-incubating
Environment: guacd with container
Reporter: Aiden Luo
Priority: Major
1. core dump
{code:c}
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x00007f932c1763dc in guac_vnc_user_leave_handler (user=0x7f9328004980) at
user.c:116
116 guac_common_cursor_remove_user(vnc_client->display->cursor, user);
(gdb) bt
#0 0x00007f932c1763dc in guac_vnc_user_leave_handler (user=0x7f9328004980) at
user.c:116
#1 0x00007f93322d1f7f in guac_client_remove_user (client=0x7f92b800ad90,
user=0x7f9328004980) at client.c:339
#2 0x0000000000405c60 in guacd_handle_user (user=0x7f9328004980) at user.c:304
#3 0x0000000000404ca2 in guacd_user_thread (data=0x7f92b801bac0) at proc.c:95
#4 0x00007f9331922064 in start_thread (arg=0x7f92b37fe700) at
pthread_create.c:309
#5 0x00007f9330b0562d in clone () at
../sysdeps/unix/sysv/linux/x86_64/clone.S:111
(gdb) l
111 int guac_vnc_user_leave_handler(guac_user* user) {
112
113 guac_vnc_client* vnc_client = (guac_vnc_client*) user->client->data;
114
115 /* Update shared cursor state */
116 guac_common_cursor_remove_user(vnc_client->display->cursor, user);
117
118 /* Free settings if not owner (owner settings will be freed with
client) */
119 if (!user->owner) {
120 guac_vnc_settings* settings = (guac_vnc_settings*) user->data;
(gdb) p *vnc_client
$1 = {client_thread = 140268061849344, rfb_client = 0x0, rfb_MallocFrameBuffer
= 0x7f931f5f5030, copy_rect_used = 0,
settings = 0x7f9328006890, display = 0x0, clipboard = 0x7f92b8035b60, audio =
0x0, sftp_user = 0x0, sftp_session = 0x0,
sftp_filesystem = 0x0, clipboard_reader = 0x7f932c1782de
<GUAC_READ_ISO8859_1>,
clipboard_writer = 0x7f932c178414 <GUAC_WRITE_ISO8859_1>}
(gdb) p *user
$2 = {client = 0x7f92b800ad90, socket = 0x7f932800af30, user_id =
0x7f9328004c30 "@b4e03b65-8317-4049-9f73-61c9a92272f9",
owner = 1, active = 0, __prev = 0x0, __next = 0x0, last_received_timestamp =
1586735641, last_frame_duration = 0,
processing_lag = 0, info = {optimal_width = 977, optimal_height = 668,
audio_mimetypes = 0x7f932800b000,
video_mimetypes = 0x7f932800b020, image_mimetypes = 0x7f9328004ae0,
optimal_resolution = 96},
__stream_pool = 0x7f9328004a90, __output_streams = 0x7f9328005670,
__input_streams = 0x7f9328004c60,
__object_pool = 0x7f932800afb0, __objects = 0x7f9328006080, data =
0x7f9328006890,
mouse_handler = 0x7f932c17592e <guac_vnc_user_mouse_handler>, key_handler =
0x7f932c1759a7 <guac_vnc_user_key_handler>,
clipboard_handler = 0x7f932c174ea1 <guac_vnc_clipboard_handler>, size_handler
= 0x0, file_handler = 0x0,
pipe_handler = 0x0, ack_handler = 0x0, blob_handler = 0x0, end_handler = 0x0,
sync_handler = 0x0, leave_handler = 0x0,
get_handler = 0x0, put_handler = 0x0, audio_handler = 0x0}
{code}
2. analysis
When user connect to guacd, guacd will create a new process to handle the
connection. In new process then create `static void* guacd_user_thread(void*
data)` thread to do real work.
In guacd_user_thread thread, will create a `void*
guac_vnc_client_thread(void* data)` thread to read frame from VNC server and
write to guac_socket. `vnc_client->clipboard_reader`,
`vnc_client->rfb_client` and `vnc_client->display` will be setup in this thread.
Also guacd_user_thread will create another `void* guacd_user_input_thread(void*
data)` thread to receive data from client and send to VNC sever.
guacd_user_thread will wait guacd_user_input_thread finished, then call `void
guac_client_remove_user(guac_client* client, guac_user* user)` function to
remove user which will call `int guac_vnc_user_leave_handler(guac_user*
user)`.
The segfault error caused by line `
guac_common_cursor_remove_user(vnc_client->display->cursor, user)` in function
`guac_vnc_user_leave_handler`，because `vnc_client->display` is NLL.
Why this segfault will happen?
My guess is that there exist concurrently problem. If guacd_user_input_thread
exit before `vnc_client->display` gets initialized in guac_vnc_client_thread,
the `guac_common_cursor_remove_user` function will do null pointer dereference.
We should check `vnc_client->display` if is null in function `void
guac_client_remove_user(guac_client* client, guac_user* user)`.
--
This message was sent by Atlassian JIRA
(v6.4.14#64029)
