** Description changed: [Impact] - In certain edge cases where the terminal goes away while an ssh process is running, ssh can be left consuming 100% CPU. This increases processing costs for cloud users and wastes energy. + In certain edge cases where the terminal goes away while an ssh process is running, ssh can be left consuming 100% CPU. This increases processing costs for cloud users and wastes energy. While this is an uncommon error, googling indicates many people have run into it in several different ways. It seems important to get this fixed in stable releases. This is a regression in jammy presumably due to change from select() to poll() (see OpennSSH 8.9 Release Announcement [1] ), fixed by upstream commit d6556de1db0822c76ba2745cf5c097d9472adf7c "upstream: fix poll() spin when a channel's output fd closes..." [2]. 1: https://lwn.net/Articles/885886/ 2. https://github.com/openssh/openssh-portable/commit/d6556de1db0822c76ba2745cf5c097d9472adf7c [Test Case] $ lxc launch ubuntu-daily:jammy ssh-cpu $ lxc shell ssh-cpu # passwd -d root # ssh-keygen -t rsa -N '' -f /root/.ssh/id_rsa # cat << EOF >>/etc/ssh/ssh_config StrictHostKeyChecking accept-new EOF # sed -ri 's/^PasswordAuthentication/#PasswordAuthentication/' /etc/ssh/sshd_config # cat << EOF >>/etc/ssh/sshd_config PermitRootLogin yes PubkeyAuthentication yes PermitEmptyPasswords yes PasswordAuthentication yes ChallengeResponseAuthentication no UsePAM no EOF # systemctl restart sshd # ssh localhost 2> >({exec 1>&2}) You can shell into the container from a second terminal and use "htop" to verify that ssh is using 100% of one of the CPU cores: $ lxc shell ssh-cpu # htop This should show one CPU pegged at 100% due to the 'ssh localhost' process Next, return to the first terminal, exit out of the sub-ssh session and install the fix: # logout # add-apt-repository -yus ppa:bryce/openssh-sru-lp1986521 # apt-get full-upgrade -y Now repeat the test in the first terminal window, while viewing htop in the second terminal: # ssh localhost 2> >({exec 1>&2}) [Where Problems Could Occur] While the patch in question is well tested upstream, it has a relatively high line count and as such is difficult to assure correctness by visual code checking. However, it's not clear that the line count could be significantly reduced without risking loss of correctness. Thus this relies more on testing to assure robustness, than on code review. The code involves polling behavior, so issues to watch for would more likely involve process handling, i.e. problems with socket polling. Beyond that, the usual generic issues to watch for - build issues, dependency issues during build or on upgrade, and service restarting. - [Original Report] The OpenSSH package 8.9p1 as shipped with U22.04 (8.9p1-3) suffers from the bug described at https://bugzilla.mindrot.org/show_bug.cgi?id=3411 and https://bugzilla.mindrot.org/show_bug.cgi?id=3405 A command such as "xterm -e 'ssh -f remote.host sleep 60'" will pop up an xterm, ask for whatever authentication is needed, close the xterm, and leave the ssh client spinning consuming CPU time for 60 seconds before it exits. It should leave the ssh client idle for 60 seconds. Many uses of ssh to launch graphical applications will be caught by this bug. This is fixed in OpenSSH 9.0p1 as the first bugfix listed in its release notes at https://www.openssh.com/txt/release-9.0
-- 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/1986521 Title: ssh client spins if output fd closed Status in portable OpenSSH: Unknown Status in openssh package in Ubuntu: Fix Released Status in openssh source package in Jammy: Triaged Bug description: [Impact] In certain edge cases where the terminal goes away while an ssh process is running, ssh can be left consuming 100% CPU. This increases processing costs for cloud users and wastes energy. While this is an uncommon error, googling indicates many people have run into it in several different ways. It seems important to get this fixed in stable releases. This is a regression in jammy presumably due to change from select() to poll() (see OpennSSH 8.9 Release Announcement [1] ), fixed by upstream commit d6556de1db0822c76ba2745cf5c097d9472adf7c "upstream: fix poll() spin when a channel's output fd closes..." [2]. 1: https://lwn.net/Articles/885886/ 2. https://github.com/openssh/openssh-portable/commit/d6556de1db0822c76ba2745cf5c097d9472adf7c [Test Case] $ lxc launch ubuntu-daily:jammy ssh-cpu $ lxc shell ssh-cpu # passwd -d root # ssh-keygen -t rsa -N '' -f /root/.ssh/id_rsa # cat << EOF >>/etc/ssh/ssh_config StrictHostKeyChecking accept-new EOF # sed -ri 's/^PasswordAuthentication/#PasswordAuthentication/' /etc/ssh/sshd_config # cat << EOF >>/etc/ssh/sshd_config PermitRootLogin yes PubkeyAuthentication yes PermitEmptyPasswords yes PasswordAuthentication yes ChallengeResponseAuthentication no UsePAM no EOF # systemctl restart sshd # ssh localhost 2> >({exec 1>&2}) You can shell into the container from a second terminal and use "htop" to verify that ssh is using 100% of one of the CPU cores: $ lxc shell ssh-cpu # htop This should show one CPU pegged at 100% due to the 'ssh localhost' process Next, return to the first terminal, exit out of the sub-ssh session and install the fix: # logout # add-apt-repository -yus ppa:bryce/openssh-sru-lp1986521 # apt-get full-upgrade -y Now repeat the test in the first terminal window, while viewing htop in the second terminal: # ssh localhost 2> >({exec 1>&2}) [Where Problems Could Occur] While the patch in question is well tested upstream, it has a relatively high line count and as such is difficult to assure correctness by visual code checking. However, it's not clear that the line count could be significantly reduced without risking loss of correctness. Thus this relies more on testing to assure robustness, than on code review. The code involves polling behavior, so issues to watch for would more likely involve process handling, i.e. problems with socket polling. Beyond that, the usual generic issues to watch for - build issues, dependency issues during build or on upgrade, and service restarting. [Original Report] The OpenSSH package 8.9p1 as shipped with U22.04 (8.9p1-3) suffers from the bug described at https://bugzilla.mindrot.org/show_bug.cgi?id=3411 and https://bugzilla.mindrot.org/show_bug.cgi?id=3405 A command such as "xterm -e 'ssh -f remote.host sleep 60'" will pop up an xterm, ask for whatever authentication is needed, close the xterm, and leave the ssh client spinning consuming CPU time for 60 seconds before it exits. It should leave the ssh client idle for 60 seconds. Many uses of ssh to launch graphical applications will be caught by this bug. This is fixed in OpenSSH 9.0p1 as the first bugfix listed in its release notes at https://www.openssh.com/txt/release-9.0 To manage notifications about this bug go to: https://bugs.launchpad.net/openssh/+bug/1986521/+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
