[Bug 273566] stray characters contaminate serial console
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Date: Mon, 04 Sep 2023 18:00:37 UTC
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=273566
Bug ID: 273566
Summary: stray characters contaminate serial console
Product: Base System
Version: CURRENT
Hardware: arm
OS: Any
Status: New
Severity: Affects Only Me
Priority: ---
Component: arm
Assignee: freebsd-arm@FreeBSD.org
Reporter: fbsd@www.zefox.net
From time to time serial console connections on raspberry Pi (2, 3 and 4)
systems allow stray characters to enter serial console commands. Since the
console is often used by root, this seems potentially insecure.
The setup consists of a pair of Pi's. One holds a usb-serial adapter,
with the serial end connected to a second Pi's console uart. Connection
is established by ssh into the first Pi, starting tip or cu, and then
logging into the second Pi into the usual way. Often, login is to root.
Often, the ssh connection to the first Pi drops spontaneously. It can be
re-established by repeating the connection procedure, if the original
shell running on the second Pi's serial console is still running the old
session remains up and prior commands can be run from history.
When that is done, sometimes the apparent "last words" to the second Pi
appear to be things that were never intentionally entered before the
disconnect. Here's one example, in this case the terminal session is
running on a Pi4 running RasPiOS, the ssh connection is to a Pi2 running
FreeBSD 14.0-ALPHA4 armv7 1400097 (GENERIC) #1, the Pi running the serial
console is running 15-current:client_loop: send disconnect: Broken pipe
bob@raspberrypi:~ $ ssh 192.168.1.13
Password for bob@generic:
Last login: Mon Sep 4 08:14:21 2023 from 192.168.1.10
FreeBSD 14.0-ALPHA4 armv7 1400097 (GENERIC) #1 stable/14-n265029-d6fec2dacf80:
Mon Sep 4 02:12:35 PDT 2023
Welcome to FreeBSD!
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-- Dru <genesis@istar.ca>
bob@generic:~ % su
Password:
# tip ucom
Stale lock on cuaU0 PID=1181... overriding.
connected
Y\HHMHLN\247\261́\315m\335u違\311\351͡}}¥ѥѥ½
YHHMHLN��́�m�u違��͡}}�ѥѥ�: Command not found.
bob@pelorus:/usr/src % ½¹¹ѥ½¹±½͕åɕµ½ѕ¡½ÍÑ5)5)5)ÿ
Too many )'s.
bob@pelorus:/usr/src
If one types up-arrow to recall history, the first few old "commands"
are rubbish, though they are not un-displayable as would be expected
from line noise (the copy-paste from FreeBSD to RasPiOS is misleading).
In this example an FTDI-232 was in use, but PL2303 has behaved similarly.
The origin of the disconnect is unclear. I think it's a problem with
ssh between the RasPiOS terminal and the Pi holding the USB-serial
adapter, but don't know how to tell for sure. It seems to be load-related,
with a busy FreeBSD Pi more inclined to drop connections even if the
connection itself is entirely idle.
FWIW, stable/12 does not display this behavior, with ssh-usb-serial-console
connections staying up for days or weeks.
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