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-- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to openssh in Ubuntu. https://bugs.launchpad.net/bugs/1877454 Title: openssh-server hangs with AuthorizedKeysCommand Status in openssh package in Ubuntu: Fix Released Status in openssh source package in Xenial: Fix Released Bug description: [Impact] On a default Xenial install, when sshd is configured to obtain the list of allowed keys using AuthorizedKeysCommand (or the list of allowed certificate principals using AuthorizedPrincipalsCommand), and if the command pointed by Authorized{Keys,Principals}Command generates a lot of output, sshd will hang while reading this same output. In a nutshell, the problem happens when the subprocess generates enough data to fill the pipe's buffer; in this scenario, sshd will wait(2) on the subprocess, which will be blocked trying to write the rest of the output. [Test Case] In order to reproduce the bug, one can: $ lxc launch ubuntu-daily:xenial openssh-server-bug1877454 $ lxc shell openssh-server-bug1877454 # ssh-keygen (no need to choose a passphrase for the key, just hit ENTER on all prompts) # cat > authkeyscommand.sh << __EOF__ #!/bin/bash cat /root/.ssh/id_rsa.pub echo head -c 1M < /dev/urandom __EOF__ # chmod +x authkeyscommand.sh # cat >> /etc/ssh/sshd_config << __EOF__ AuthorizedKeysCommand /root/authkeyscommand.sh AuthorizedKeysCommandUser root __EOF__ # systemctl reload sshd.service # ssh root@127.0.0.1 You will notice that ssh will stay there waiting for sshd's reply, which won't come. The expected result would be for ssh to succeed. [Regression Potential] Since the affected code deals with executing a subprocess, reading its output through a pipe, and then relying on wait(2) to determine whether the subprocess exited correctly; and considering that this code is written in C without the help of features like RAII and with the use of goto statements, we are not able to disconsider the chances of making a mistake and forgetting to free a resource or to properly read/write from/to pipes, for example. This is, after all, the reason the bug happened in the first place. Having said that, openssh contains extensive tests and the code is well organized and relatively easy to follow. Upon close inspection, there doesn't seem to be an evident problem with the backported fixes. As usual when dealing with a somewhat older distribution, there is always the possibility of encountering problems because we will be recompiling openssh using the most recent versions of its build dependencies. [Original Description] Please consider applying this change to openssh-server distributed in Xenial (16.04). Without it, sshd can sporadically hang when making use of the `AuthorizedKeysCommand` option. https://github.com/openssh/openssh- portable/commit/ddd3d34e5c7979ca6f4a3a98a7d219a4ed3d98c2 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/1877454/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : touch-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp
