[Bug 2043114] Re: sshd segmentation fault on 20.04.6 (focal)

[Ivaylo Markov]

Yes, seems pretty consistent. I just tried it again before I write out
the reproduction steps, and it took maybe a couple of minutes.

The configuration is basically just:

sed -i.bak 's/#LogLevel INFO/LogLevel DEBUG/g' /etc/ssh/sshd_config
systemctl restart sshd

As for making enough connections to cause the issue, I use something
along the lines of:

parallel -j 99 -N0 "ssh root at example 'mount; sleep 1; cat /proc/cpuinfo;
free -h;  dd if=/dev/zero of=/dev/null bs=1 count=8192; mount -av; sleep
$(($RANDOM % 5)); lshw'"  ::: {1..5000}

It is essentially running random commands to simulate some sort of
activity on the machine. The amount of parallel connections (`-j 99`)
might need to be adjusted - too little connections won't trigger the
bug, too many can cause an OOM situation.

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https://bugs.launchpad.net/bugs/2043114

Title:
  sshd segmentation fault on 20.04.6 (focal)

Status in openssh package in Ubuntu:
  Confirmed

Bug description:
  We have a physical server running Ubuntu 20.04.6 LTS (amd64) and openssh-server 1:8.2p1-4ubuntu0.9. Sometimes sshd crashes with a segmentation fault on remote login with key authentication:
  [193107.651745] sshd[1229630]: segfault at 5557eba6a008 ip 00007f2326a2ca53 sp 00007ffcba40c510 error 4 in libc-2.31.so[7f23269b8000+178000]

  We’ve changed only the following values in the stock sshd_config file:

  LogLevel DEBUG
  PasswordAuthentication no
  MaxStartups 100:30:100

  The server is used for automated software testing, and sometimes our test suite might make a large amount of SSH connections in a short period of time, which seems to be correlated with the crashes. But at the same time, I have to note that the connection count was not near the MaxStartups limit, and we’ve had crashes before adding that setting.
  Since the backtrace shows the debug logging function in the stack, we’re currently experimenting with using `LogLevel INFO` to try and isolate the issue.

  I am attaching the backtrace. I could provide the full dump file,
  although I am hesitant due to the possibility of private keys or other
  sensitive information leaking.

  # apt-cache policy openssh-server
  openssh-server:
    Installed: 1:8.2p1-4ubuntu0.9
    Candidate: 1:8.2p1-4ubuntu0.9
    Version table:
   *** 1:8.2p1-4ubuntu0.9 500
          500 http://mirrors.storpool.com/ubuntu/archive focal-updates/main amd64 Packages
          500 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages
          100 /var/lib/dpkg/status
       1:8.2p1-4 500
          500 http://mirrors.storpool.com/ubuntu/archive focal/main amd64 Packages
  --- 
  ProblemType: Bug
  ApportVersion: 2.20.11-0ubuntu27.27
  Architecture: amd64
  CasperMD5CheckResult: skip
  DistroRelease: Ubuntu 20.04
  Package: openssh-server 1:8.2p1-4ubuntu0.9
  PackageArchitecture: amd64
  ProcVersionSignature: Ubuntu 5.4.0-128.144-generic 5.4.210
  Tags:  focal
  Uname: Linux 5.4.0-128-generic x86_64
  UpgradeStatus: Upgraded to focal on 2021-01-13 (1030 days ago)
  UserGroups: N/A
  _MarkForUpload: True

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