[Bug 1927745] Re: Non-thread-safe functions used in multi-threaded rpc.gssd

[Andreas Hasenack]

I filed https://bugs.launchpad.net/ubuntu/+source/nfs-utils/+bug/1928259
for the restart issue.

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Title:
  Non-thread-safe functions used in multi-threaded rpc.gssd

Status in nfs-utils package in Ubuntu:
  Fix Released
Status in nfs-utils source package in Bionic:
  Fix Committed
Status in nfs-utils package in Debian:
  Fix Released
Status in nfs-utils package in Fedora:
  Fix Released

Bug description:
  [Impact]
  rpc-gssd can hang or crash when a kerberos nfsv4 mount point is accessed by multiple users simultaneously. The problem happens because the daemon uses the strtok() function which is not thread safe.

  The fix from upstream removes strtok() and uses strsep() instead.
  These patches are already applied in focal and later, via merges from
  debian.

  [Test Plan]
  As with all race conditions, this test case may take a while to reproduce the problem.

  # Create a bionic VM. It seems to help if it's created with multiple cpus/cores. I had more success with 4 cores and 1GiB of RAM.
  # if using lxd to launch a VM, you can run this before: "lxc config set vm-name limits.cpu=4". Just don't forget to undo it, or set to your normal number of CPUs, after the test
  # Login and get its ip, and take note of it:
  export IP=$(ip route get default 8.8.8.8 | grep ^8 | awk '{print $7}')
  echo $IP

  # adjust /etc/hosts:
  echo "$IP $(hostname).example.com" | sudo tee -a /etc/hosts

  # adjust /etc/resolv.conf:
  echo "search example.com" | sudo tee -a /etc/resolv.conf

  # verify hostname -f returns the fqdn of the vm, i.e., a name with the .example.com domain:
  hostname -f

  # If you still don't get the correct FQDN, try the below, adjusting for your hostname if $(hostname) isn't working properly:
  sudo hostnamectl set-hostname $(hostname).example.com

  # Run these commands, and when asked:
  # - for realm: EXAMPLE.COM
  # - for kdc and admin server: use the vm's IP

  sudo apt update && sudo apt install nfs-server krb5-kdc krb5-admin-
  server krb5-user gcc

  # create a kerberos realm. When prompted, use any password you want:
  sudo krb5_newrealm

  # create an nfs service ticket, and store it in the keytab
  sudo kadmin.local -q "addprinc -randkey nfs/$(hostname -f)"
  sudo kadmin.local -q "ktadd nfs/$(hostname -f)"

  # create test directories
  sudo mkdir -p /mnt/test_krb5/ /export
  sudo touch /export/foo

  # adjust nfs config and restart the nfs server:
  sudo sed -r -i "s,^NEED_SVCGSSD=.*,NEED_SVCGSSD=\"yes\"," /etc/default/nfs-kernel-server
  sudo sed -r -i "s,^NEED_GSSD=.*,NEED_GSSD=\"yes\"," /etc/default/nfs-common
  sudo systemctl restart nfs-server

  # configure an nfs export:
  echo "/export *(sec=krb5,rw,sync,no_subtree_check)" | sudo tee -a /etc/exports
  sudo exportfs -rva

  # confirm it's available
  sudo showmount -e localhost

  # mount it
  sudo mount $(hostname -f):/export /mnt/test_krb5/
  sudo ls -la /mnt/test_krb5

  # download bug attachments
  wget -ct0 https://bugs.launchpad.net/ubuntu/+source/nfs-utils/+bug/1927745/+attachment/5496166/+files/stat_as.c https://bugs.launchpad.net/ubuntu/+source/nfs-utils/+bug/1927745/+attachment/5496167/+files/bz1419280_test_threads
  chmod +x bz1419280_test_threads

  # build reproducer
  gcc stat_as.c -o stat_as

  # run test script as root. It may take a few minutes to trigger the bug
  sudo ./bz1419280_test_threads

  # wait
  # Once you get the confirmation:
  calling stat on '/mnt/test_krb5/foo' with uids 9995 through 10035
  reproduced the bug after 114 iterations

  # Check for a "stat_as" D state process:
  $ ps axw|grep stat_as
  17814 pts/1    D      0:00 ./stat_as /mnt/test_krb5/foo 9995 10035

  # To restore functionality, restart rpc-gssd:
  sudo systemctl restart rpc-gssd.service

  With the updated packages, the script will not detect the bug and
  never stop.

  [Where problems could occur]
  NFS v4 services are more complex than earlier versions, and are comprised of several services/daemons. It's possible for the restart done after the automatic package upgrade to show up as a regression due to several factors:
  - not all needed services were restarted (bug, but not introduced by this change)
  - depending on mount options, client mount points may appear as hung and take a while to recover
  - configuration errors on the server which were up until now not noticed, and only manifest themselves after a restart
  - some sites, due to the lack of configuration options in /etc/default/nfs-*, might have overriden systemd service files and hardcoded other command line options there. If not done properly (i.e., not done in /etc/systemd via overrides), these local changes will be lost after the package upgrade. I know of at least rpc-gssd, which has no command-line options available in /etc/default/nfs-*, and I know of users who have tweaked this service in many different ways to add things like -v or -n to its command line option.

  [Other Info]
  The upstream patches have been applied since February 2017 and have not been changed or reverted. They are also applied in Debian and Fedora, and ubuntu since focal at least.

  There is an additional patch, but part of the fix, which dupes the
  string for appropriate logging. Its memory is also freed.

  It may be hard to reproduce this bug in a test environment. I've
  gotten to the error in as little as a few seconds, but other times it
  took hundreds of attempts. YMMV.

  [Original Description]

  Fixed in focal and later, due to sync from debian

  Bionic affected.

  I'll add a proper description in a moment.

  RH: https://bugzilla.redhat.com/show_bug.cgi?id=1419280
  Debian BTS: https://bugs.debian.org/895381
  ML: http://www.spinics.net/lists/linux-nfs/msg62111.html
  ML: http://www.spinics.net/lists/linux-nfs/msg62099.html

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