[Bug 1646813] Re: ssh (client) stucks in an endless loop with 100% CPU

[Colin Watson]

SIGTTOU happens when a background process tries to send output to the
terminal.  If you're getting this then it generally indicates a
configuration error.  Try using the -n option, and of course making sure
that you have suitable public-key authentication arrangements so that
ssh doesn't need to prompt for a passphrase?

     -n      Redirects stdin from /dev/null (actually, prevents reading from
             stdin).  This must be used when ssh is run in the background.  A
             common trick is to use this to run X11 programs on a remote
             machine.  For example, ssh -n shadows.cs.hut.fi emacs & will
             start an emacs on shadows.cs.hut.fi, and the X11 connection will
             be automatically forwarded over an encrypted channel.  The ssh
             program will be put in the background.  (This does not work if
             ssh needs to ask for a password or passphrase; see also the -f
             option.)

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https://bugs.launchpad.net/bugs/1646813

Title:
  ssh (client) stucks in an endless loop with 100% CPU

Status in openssh package in Ubuntu:
  New

Bug description:
  I have automation suite, which uses ssh to connect to servers.

  Under high speed conditions (I was never able to reproduce this
  manually, but many times in the script), ssh can stuck in an endless
  loop, causing 100% CPU usage.

  strace on it looks like this:

  ioctl(5, TCGETS, {B38400 opost isig icanon echo ...}) = 0
  ioctl(5, SNDCTL_TMR_CONTINUE or TCSETSF, {B38400 opost isig icanon echo ...}) = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  --- SIGTTOU {si_signo=SIGTTOU, si_code=SI_KERNEL} ---
  rt_sigreturn({mask=[]})                 = -1 EINTR (Interrupted system call)
  ioctl(5, TCGETS, {B38400 opost isig icanon echo ...}) = 0
  ioctl(5, SNDCTL_TMR_CONTINUE or TCSETSF, {B38400 opost isig icanon echo ...}) = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  (repeated endlessly)

  Command line during call:

  timeout 120 ssh -o StrictHostKeyChecking=no -o
  UserKnownHostsFile=/dev/null -o UpdateHostKeys=no -i
  /tmp/DIBCTL_ssh_keylN8SGlprivate cloud-user at 188.42.217.229 uname -a

  (IP address and a private key are changed on every run).

  This ssh script is called from a shell script, which is in turn called
  by python supbrocess.check_call() function, with no special settings
  for stdout/stderr/stdin (all connected to console).

  backtrace:

  #0  0x00007fe7ad8f4ed8 in tcsetattr () from /lib/x86_64-linux-gnu/libc.so.6
  #1  0x000055cbfd2544d6 in ?? ()
  #2  0x000055cbfd22cf57 in ?? ()
  #3  0x000055cbfd2064b0 in ?? ()
  #4  0x000055cbfd208bdd in ?? ()
  #5  0x000055cbfd208d28 in ?? ()
  #6  0x000055cbfd230c59 in ?? ()
  #7  0x000055cbfd2088a4 in ?? ()
  #8  0x000055cbfd204b44 in ?? ()
  #9  0x000055cbfd1f3f26 in ?? ()
  #10 0x00007fe7ad8183f1 in __libc_start_main () from /lib/x86_64-linux-gnu/libc.so.6
  #11 0x000055cbfd1f5b0a in ?? ()

  Package versions:
  ii  libc6:amd64                           2.24-3ubuntu1                               amd64        GNU C Library: Shared libraries
  ii  openssh-client                        1:7.3p1-3build1                             amd64        secure shell (SSH) client, for secure access to remote machines

  uname:
  Linux 4.8.0-26-generic #28-Ubuntu SMP Tue Oct 18 14:39:52 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

  ProblemType: Bug
  DistroRelease: Ubuntu 17.04
  Package: ssh (not installed)
  ProcVersionSignature: Ubuntu 4.8.0-26.28-generic 4.8.0
  Uname: Linux 4.8.0-26-generic x86_64
  ApportVersion: 2.20.3-0ubuntu8
  Architecture: amd64
  CurrentDesktop: X-Cinnamon
  Date: Fri Dec  2 13:29:50 2016
  EcryptfsInUse: Yes
  InstallationDate: Installed on 2013-07-19 (1231 days ago)
  InstallationMedia: Xubuntu 13.04 "Raring Ringtail" - Release i386 (20130423.1)
  SourcePackage: openssh
  UpgradeStatus: Upgraded to zesty on 2016-11-22 (9 days ago)

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