MSYS2 sshd fails with "privsep_preauth: preauth child terminated by signal 11" if running in MINGW64 shell

[sdbbs]

Description / Steps to reproduce the issue

There is nothing I hate more than doing a procedure without problems for years, then not doing that for a couple of weeks, and then when doing it "now", it fails. So, in this case, what I've been doing for years is:

• on my desktop PC, open a MINGW64 terminal, and run sshd:

  $(which sshd) -o LogLevel=DEBUG1 -De -p 222
  

• open another MINGW64 terminal, and run the tunnel command:

  ssh -C -NT -R 2222:127.0.0.1:222 REMOTEUSER@remoteserver.com
  

• and finally, on a remote computer, use ssh -p 2222 DESKTOPUSER@remoteserver.com to access desktop PC.

Well, this time, when I tried this, I got:

$ ssh -p 2222 DESKTOPUSER@remoteserver.com
DESKTOPUSER@remoteserver.com's password:
Connection to remoteserver.com closed by remote host.
Connection to remoteserver.com closed.

Great, just the best, when things shut down, and there is no error message. Well, the sshd then writes this in its log:

...
debug1: rekey in after 134217728 blocks [preauth]
debug1: KEX done [preauth]
debug1: userauth-request for user DESKTOPUSER service ssh-connection method none [preauth]
debug1: attempt 0 failures 0 [preauth]
debug1: userauth-request for user DESKTOPUSER service ssh-connection method keyboard-interactive [preauth]
debug1: attempt 1 failures 0 [preauth]
debug1: keyboard-interactive devs  [preauth]
debug1: auth2_challenge: user=DESKTOPUSER devs= [preauth]
debug1: kbdint_alloc: devices '' [preauth]
debug1: userauth-request for user DESKTOPUSER service ssh-connection method password [preauth]
debug1: attempt 2 failures 1 [preauth]
Accepted password for DESKTOPUSER from 127.0.0.1 port 48400 ssh2
debug1: monitor_child_preauth: user DESKTOPUSER authenticated by privileged process
debug1: monitor_read_log: child log fd closed
privsep_preauth: preauth child terminated by signal 11
debug1: do_cleanup
...

So, password got accepted, and then for apparently no reason at all, the "preauth child" got "terminated by signal 11". So something in there, a child process of sshd crashed with signal 11 - but what? gdb did not give me an answer (it did not even stop when this message is printed), but then I guessed, maybe this "child" is actually the shell we'd try to start upon successful auth, which in this case would be the MINGW64 bash, as I'm running the above in a MINGW64 terminal.

However, I had forgotten that openssh is actually a MSYS2 packaage:

$ pacman -F sshd.exe
msys/openssh 8.9p1-3 (net-utils) [installed: 9.7p1-1]
    usr/bin/sshd.exe

$ grep -i openssh /var/log/pacman.log  | tail -1  # last updated on:
[2024-04-02T21:00:47+0200] [ALPM] upgraded openssh (9.6p1-1 -> 9.7p1-1)

So, I simply stopped the previous sshd server, and opened a MSYS2 terminal, and ran $(which sshd) -o LogLevel=DEBUG1 -De -p 222 there; and lo and behold - the ssh -p 2222 DESKTOPUSER@remoteserver.com on the remote computer now works!

Since I had previously used MSYS2 in a MINGW64 shell without problems, I'd say a regression happened somewhere, though not sure exactly where. Still, I'd like this functionality back, if possible ...

Expected behavior

Well, if I ran $(which sshd) -o LogLevel=DEBUG1 -De -p 222 in a MINGW64 terminal on desktop PC and tunnelled it, and then executed ssh -p 2222 DESKTOPUSER@remoteserver.com on a remote PC, I'd expect the ssh command on the remote server to succeed and a connection to be established, whereby sshd typically prints something like this:

...
debug1: userauth-request for user DESKTOPUSER service ssh-connection method password [preauth]
debug1: attempt 2 failures 1 [preauth]
Accepted password for DESKTOPUSER from 127.0.0.1 port 48305 ssh2
debug1: monitor_child_preauth: user DESKTOPUSER authenticated by privileged process
debug1: monitor_read_log: child log fd closed
debug1: rekey in after 134217728 blocks
debug1: rekey out after 134217728 blocks
debug1: ssh_packet_set_postauth: called
debug1: active: key options: agent-forwarding port-forwarding pty user-rc x11-forwarding
debug1: Entering interactive session for SSH2.
debug1: server_init_dispatch
debug1: server_input_channel_open: ctype session rchan 0 win 1048576 max 16384
debug1: input_session_request
debug1: channel 0: new session [server-session] (inactive timeout: 0)
debug1: session_new: session 0
...

Actual behavior

As described above, the ssh on remote PC fails, whereby sshd on local PC outputs privsep_preauth: preauth child terminated by signal 11

Verification

• I have verified that my MSYS2 is up-to-date before submitting the report (see https://www.msys2.org/docs/updating/)

Windows Version

MINGW64_NT-10.0-19045

Are you willing to submit a PR?

No response