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https://issues.apache.org/jira/browse/GUACAMOLE-1053?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17092412#comment-17092412
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Grigory Trenin commented on GUACAMOLE-1053:
-------------------------------------------
Yes, it happened only twice in a production environment, and I was unable to
reproduce it in test, though I tried hard. This lead me to source code
analysis, which I presented in this issue. I have also raised a pull request
[#272|https://github.com/apache/guacamole-server/pull/272].
> 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: Minor
>
> {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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