[Bug 3639] New: server thread aborts during client login after receiving SSH2_MSG_KEXINIT

Sun Dec 3 07:45:02 AEDT 2023

https://bugzilla.mindrot.org/show_bug.cgi?id=3639

            Bug ID: 3639
           Summary: server thread aborts during client login after
                    receiving SSH2_MSG_KEXINIT
           Product: Portable OpenSSH
           Version: 9.2p1
          Hardware: ARM
                OS: Linux
            Status: NEW
          Severity: critical
          Priority: P5
         Component: sshd
          Assignee: unassigned-bugs at mindrot.org
          Reporter: jtm.moon.forum.user+mindrot at gmail.com

tl;dr I downloaded and compiled openssh-9.2p1.tar.gz . When an openssh
client attempts to login it sends SSH2_MSG_KEXINIT, the server
immediately resets the connection

### Reproduction Steps

Using Raspbian 11 (based on Debian 11 Bullseye) on a Raspberry Pi 4
(ARM aarch64), I downloaded
https://mirror.edgecast.com/pub/OpenBSD/OpenSSH/portable/openssh-9.2p1.tar.gz

I compiled and installed it.

First, make sure necessary build packages are available

    apt install \
        libssl-dev \
        gcc g++ gdb cpp \
        make cmake \
        libtool \
        libc6 \
        autoconf automake pkg-config \
        build-essential \
        gettext \
        libzstd1 zlib1g \
        libssh-4 libssh-dev libssl3 \
        libc6-dev libc6 \
        libcrypt-dev

Download, build, install

    cd /tmp
    wget
https://mirror.edgecast.com/pub/OpenBSD/OpenSSH/portable/openssh-9.2p1.tar.gz
    tar -xvf openssh-9.2p1.tar.gz
    cd openssh-9.2p1
    ./configure --prefix=/opt/openssh-9.2p1
    make
    make install

Adjust sshd_config

    vim /opt/openssh-9.2p1/etc/sshd_config

Add lines for a unique port, 2232, increase the log level

    Port 2232
    LogLevel DEBUG3

Otherwise, the `sshd_config` is used as-is.

Manually start `sshd`

    /opt/openssh-9.2p1/sbin/sshd -D

Tail the logs

    tail -f /var/log/auth.log

On a different host, attempt to login using the openssh client

    PS> ssh.exe root at 192.168.1.2 -p 2232 -vvvv

The tail of the output shows

    ...
    debug3: send packet: type 20
    debug1: SSH2_MSG_KEXINIT sent
    debug3: recv - from CB ERROR:10054, io:000002E46F4CB690
    Connection reset by 192.168.1.2 port 2232

That output is from Windows ssh.exe (OpenSSH_for_Windows_8.6p1,
LibreSSL 3.4.3).

Using Ubuntu 22 x64 ssh (OpenSSH_8.9p1 Ubuntu-3ubuntu0.4, OpenSSL 3.0.2
15 Mar 2022) the ssh client output looks like:

    $ ssh root at 192.168.1.2 -p 2232 -vvvv
    ...
    debug3: send packet: type 20
    debug1: SSH2_MSG_KEXINIT sent
    Connection reset by 192.168.1.2 port 2232

Using Debian 11 ARM ssh client compiled from the same compilation
(OpenSSH_9.2p1, OpenSSL 1.1.1w  11 Sep 2023) the same error occurs.

    $ /opt/openssh-9.2p1/bin/ssh -p 2232 root at 192.168.1.2 -vvvvv
    ...
    debug3: send packet: type 20
    debug1: SSH2_MSG_KEXINIT sent
    Connection reset by 192.168.1.2 port 2232

The server log messages from `/var/log/auth.log` are

     2023-12-02T12:28:41.051665-08:00 host1 sshd[3790]: Connection from
192.168.1.3 port 62155 on 192.168.1.2 port 2232 rdomain ""
2023-12-02T12:28:41.050817-08:00 host1 sshd[3790]: Connection from
192.168.1.3 port 62155 on 192.168.1.2 port 2232 rdomain ""
2023-12-02T12:28:41.053381-08:00 host1 audit[3791]: SECCOMP
auid=4294967295 uid=107 gid=65534 ses=4294967295 pid=3791 comm="sshd"
exe="/opt/openssh-9.2p1/sbin/sshd" sig=31 arch=40000028 syscall=20
compat=1 ip=0xf787080c code=0x0

### Notes

This error does not occur using release 9.1p1.
This error does occur for release 9.2p1 up to 9.5p1 (I tried all of
them).

I attempted to reproduce this on a Ubuntu 22 x64 Virtual Machine as the
server. The error did not occur (logins succeeded).

Various information about the host on which the error occurs

    $ lscpu
    Architecture:                    aarch64
    Byte Order:                      Little Endian
    CPU(s):                          4
    On-line CPU(s) list:             0-3
    Thread(s) per core:              1
    Core(s) per socket:              4
    Socket(s):                       1
    Vendor ID:                       ARM
    Model:                           3
    Model name:                      Cortex-A72
    Stepping:                        r0p3
    CPU max MHz:                     1500.0000
    CPU min MHz:                     600.0000
    BogoMIPS:                        108.00
    L1d cache:                       128 KiB
    L1i cache:                       192 KiB
    L2 cache:                        1 MiB
    Vulnerability Itlb multihit:     Not affected
    Vulnerability L1tf:              Not affected
    Vulnerability Mds:               Not affected
    Vulnerability Meltdown:          Not affected
    Vulnerability Mmio stale data:   Not affected
    Vulnerability Retbleed:          Not affected
    Vulnerability Spec store bypass: Vulnerable
    Vulnerability Spectre v1:        Mitigation; __user pointer
sanitization
    Vulnerability Spectre v2:        Vulnerable
    Vulnerability Srbds:             Not affected
    Vulnerability Tsx async abort:   Not affected
    Flags:                           fp asimd evtstrm crc32 cpuid

    $ uname -a
    Linux host1 6.1.21-v8+ #1642 SMP PREEMPT Mon Apr  3 17:24:16 BST
2023 aarch64 GNU/Linux

The host `apt` packages (dpkg) are up-to-date.

I suspect this bug is specific to this Debian-derived distribution
and/or ARM architecture.

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