[jira] [Updated] (GUACAMOLE-1053) Segfault in gaucd (race between RDP client and user threads)

["Grigory Trenin (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/GUACAMOLE-1053?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Grigory Trenin updated GUACAMOLE-1053:
--------------------------------------
    Summary: Segfault in gaucd (race between RDP client and user threads)  (was: Segfault in gaucd (race between RDP client user threads))

> Segfault in gaucd (race between RDP client and user threads)
> ------------------------------------------------------------
>
>                 Key: GUACAMOLE-1053
>                 URL: https://issues.apache.org/jira/browse/GUACAMOLE-1053
>             Project: Guacamole
>          Issue Type: Bug
>          Components: guacd
>    Affects Versions: 1.1.0
>         Environment: RHEL 7.7 (x86_64)
>            Reporter: Grigory Trenin
>            Priority: Major
>
> {noformat}
> Program terminated with signal 11, Segmentation fault.
> #0  guac_rdp_keyboard_update_keysym (keyboard=0x7faa4c364010, keysym=65288, pressed=0) at keyboard.c:444
> 444         if (!keyboard->synchronized) {
> (gdb) p keyboard->synchronized
> Cannot access memory at address 0x7faa4c36401c
> (gdb) bt
> #0  guac_rdp_keyboard_update_keysym (keyboard=0x7faa4c364010, keysym=65288, pressed=0) at keyboard.c:444
> #1  0x00007faae2f50bf0 in guac_user_input_thread (data=<optimized out>) at user-handshake.c:165
> #2  0x00007faae2372ea5 in start_thread () from /lib64/libpthread.so.0
> #3  0x00007faae0c678cd in clone () from /lib64/libc.so.6
> (gdb) info threads
>   Id   Target Id         Frame
>   3    Thread 0x7fa9eb7fe700 (LWP 7379) 0x00007faae2374017 in pthread_join () from /lib64/libpthread.so.0
>   2    Thread 0x7fa9ebfff700 (LWP 7376) (Exiting) 0x00007faae2379bad in recvmsg () from /lib64/libpthread.so.0
> * 1    Thread 0x7fa9f8ff9700 (LWP 7381) guac_rdp_keyboard_update_keysym (keyboard=0x7faa4c364010, keysym=65288, pressed=0)
>     at keyboard.c:444
> {noformat}
> "info threads" shows that there are only 3 threads running:
>  # guac_user_input_thread (the thread segfault occured in)
>  # guacd_connection_thread
>  # guacd_user_thread waiting for guac_user_input_thread
> Note that there is no client thread running.
>  So here is what happenned: *guac_rdp_client_thread* exited and freed *rdp_client->keyboard* structure, while user input thread was still running and tried to access *rdp_client->keyboard* after it was freed.
> This is an old bug. Found a 2-year old GUACAMOLE-433 which looks exactly the same, but it was closed as "Cannot reproduce". It does not happen often indeed. We have 120 users actively using Guacamole (since COVID-19) and have seen this segfault only twice within the last month.
> As a relief, a NULL value can be assigned to *rdp_client->keyboard* immediately after freeing it. It will decrease segfault probability, but of course, the race condition between a client and user input threads remains.
> To fix it reliably we probably need to introduce a mutex that will not allow to run a user input handlers concurrently with RDP connection setup/cleanup code.



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