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Posted to commits@guacamole.apache.org by "Nick Couchman (JIRA)" <ji...@apache.org> on 2017/11/02 14:36:00 UTC
[jira] [Closed] (GUACAMOLE-428) segfault in
guac_vnc_user_leave_handler caused by concurrent problem
[ https://issues.apache.org/jira/browse/GUACAMOLE-428?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Nick Couchman closed GUACAMOLE-428.
-----------------------------------
Resolution: Duplicate
Looks like this issue is a duplicate of 424, which already has a pull request in process.
> segfault in guac_vnc_user_leave_handler caused by concurrent problem
> --------------------------------------------------------------------
>
> 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
> Attachments: guacamole-server-call-flow.png
>
>
> 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 concurrent 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)+.
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