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[Launchpad Bug Tracker <1895757@xxxxxxxxxxxxxxxxxx>]

You have been subscribed to a public bug by Heitor Alves de Siqueira (halves):

[Impact]
sudo commands can hang when IO logging is enabled

[Description]
When doing cleanup in pty_close(), sudo can leave file descriptors and events
behind that would later cause poll() to wait on a "dead" pty. This can cause
sudo to hang when IO logging is enabled, due to the poll() timeouts.

The issue has been fixed upstream by the commit below:
- In pty_close() close the slave and remove any events associated (4df454310dae)

$ rmadison sudo
 sudo | 1.8.16-0ubuntu1     | xenial           | source, ...
 sudo | 1.8.16-0ubuntu1.9   | xenial-security  | source, ...
 sudo | 1.8.16-0ubuntu1.9   | xenial-updates   | source, ...
 sudo | 1.8.21p2-3ubuntu1   | bionic           | source, ...
 sudo | 1.8.21p2-3ubuntu1.2 | bionic-security  | source, ...
 sudo | 1.8.21p2-3ubuntu1.2 | bionic-updates   | source, ... <----
 sudo | 1.8.31-1ubuntu1     | focal            | source, ...
 sudo | 1.8.31-1ubuntu1.1   | focal-updates    | source, ...
 sudo | 1.9.1-1ubuntu1      | groovy           | source, ...

Xenial doesn't exhibit this behaviour, so fixes are only needed for Bionic
(Focal onwards already have the fix by default due to sudo version).

[Test Case]

1. Ensure /etc/sudoers contains 'Defaults use_pty'
2. Execute the following test command:
$ for i in {1..10}; do sudo -- cat /var/log/syslog; done

The terminal will hang during syslog output.

[Regression Potential]
The fix introduces additional cleaning when closing/flushing pty devices, so the
regression potential should be low. It has been present upstream since
sudo-1.8.23, so it has seen thorough testing in most Linux distributions
including Ubuntu.

A regression could possibly cause issues when switching back out from sudo
sessions, as the changes only touch the pty_close path, but seems unlikely
considering the patch has been present in other Ubuntu releases as well.

--
An SSH terminal into an Ubuntu server (tested on 18.04.5) hangs running a command using 'sudo' when 'use_pty' is set in /etc/sudoers.

Steps to reproduce ('sudo' version --> 1.8.21p2-3ubuntu1.2):

1) Log in into an Ubuntu server (tested on 18.04.5 using SSH)
2) Ensure that /etc/sudoers has the following line (add this line if not present)
Defaults	use_pty
3) Execute the following (test 'sudo' command):
for i in {1..10}; do sudo -- cat /var/log/syslog; done

The terminal hangs and the following backtrace is obtained:

(gdb) bt
#0  0x00007f751d5c8cc4 in __GI___poll (fds=0x55d0159917b0, nfds=1, timeout=-1) at ../sysdeps/unix/sysv/linux/poll.c:29
#1  0x00007f751d8b146a in poll (__timeout=<optimized out>, __nfds=<optimized out>, __fds=<optimized out>) at /usr/include/x86_64-linux-gnu/bits/poll2.h:46
#2  sudo_ev_scan_impl (base=base@entry=0x55d015990dc0, flags=flags@entry=0) at ../../../lib/util/event_poll.c:155
#3  0x00007f751d8aa74d in sudo_ev_loop_v1 (base=base@entry=0x55d015990dc0, flags=flags@entry=0) at ../../../lib/util/event.c:617
#4  0x000055d01570597a in del_io_events (nonblocking=nonblocking@entry=false) at ../../src/exec_pty.c:1537
#5  0x000055d015707b97 in pty_close (cstat=0x7ffd074d6110) at ../../src/exec_pty.c:697
#6  exec_pty (details=details@entry=0x55d01591e0e0 <command_details>, cstat=cstat@entry=0x7ffd074d6110) at ../../src/exec_pty.c:1412
#7  0x000055d015701178 in sudo_execute (details=0x55d01591e0e0 <command_details>, cstat=0x7ffd074d6110) at ../../src/exec.c:391
#8  0x000055d01570e15b in run_command (details=0x55d01591e0e0 <command_details>) at ../../src/sudo.c:968
#9  0x000055d0156ff9a0 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at ../../src/sudo.c:294

A similar (most likely the same) bug has been reported here
https://access.redhat.com/solutions/3404401.

** Affects: sudo (Ubuntu)
     Importance: Undecided
         Status: Fix Released

** Affects: sudo (Ubuntu Bionic)
     Importance: High
     Assignee: Heitor Alves de Siqueira (halves)
         Status: In Progress


** Tags: sts sts-sponsor
-- 
Terminal hangs running sudo when "use_pty" is set in /etc/sudoers
https://bugs.launchpad.net/bugs/1895757
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