Bug#768314: systemd: encrypted disk passphrase prompt nearly unusable without plymouth

[Maximilian Gaukler]

found 768314 systemd/215-10
fixed 768314 systemd/218-8
retitle 768314 systemd: encrypted disk passphrase prompt nearly unusable 
without plymouth
severity important 768314
thanks


Hi everyone,


after half a day of testing and reproducing I would like to summarize 
this bug and add my own explanation:


Summary:

The way in which systemd queries for cryptsetup passwords is badly 
usable in multiple ways. There are two main problems:
- Other output is not suppressed, which may overwrite or hide the 
password prompt (except when plymouth io multiplexing is active)
- a 90 second startup timeout kills the password prompt after inactivity

For one disk, this is a slight usability problem. For multiple encrypted 
volumes with longer passwords, it is a nightmare that makes using the 
system annoying or impossible, often dropping to a rescue shell.



Steps to reproduce it without any special hardware:

- setup a simple debian jessie installation, unencrypted root+swap, no 
LVM or fancy stuff
    (e.g. in virtualbox, use snapshots for quickly rolling back after 
testing something)
- apt-get install cryptsetup
- run the attached reproducer script as root. It sets up 3 crypto disks 
from loopback files.
- reboot



test scenarios:

a) try to enter the password - it is "test" for each of the 3 disks
  -> If you enter your password correctly, it works most of the time.
screen shot is attached, please note that the last two password prompts 
are immediately overwritten by other messages.

b) just press enter randomly instead of typing a password
  -> even the next password prompts will not be visible, but overwritten 
by other messages!

c) just wait for >90sec
-> an emergency shell will be started and stopped, mixed with some 
password prompts and "A start job is running for..."
-> if you wait even longer, the system will perform a weird dance 
between "Give root password" and "Please enter passphrase for disk".


Test results:




Possible solutions and workarounds:

a) installing plymouth *and* enabling it by adding "splash" to the 
default kernel commandline.

Just installing plymouth will not help here due to bug #768329 in 
plymouth. If this were fixed we could add a dependency (recommends?) for 
plymouth to systemd/jessie or cryptsetup/jessie as a dirty workaround.

b) systemd 218-8 from experimental
solves the underlying problems good enough so that I consider the bug as 
fixed there:
- suppressing unnecessary output
- not having a timeout on password entry

For comparison, two screenshots are attached. In both I entered the 
first two passwords and then made a screenshot at the password prompt 
for the third disk. found-*.png is the problematic systemd version 
currently in jessie, notfound-*.png the one in experimental.


Thanks

Max

-----
(Since I am not very familiar with the debian bug tracking system, 
please remind me if I do something wrong.)
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#!/bin/bash
set -e
echo -n test > /root/cryptPassphrase

num_disks=3
for i in `seq 1 $num_disks`; do
	echo "cryptTest$i /root/cryptTest$i none luks" >> /etc/crypttab
	echo "/dev/mapper/cryptTest$i /mnt/cryptTest$i ext4 defaults 0 0" >> /etc/fstab
	mkdir -p /mnt/cryptTest$i
	fallocate /root/cryptTest$i -l 42M
	cryptsetup --batch-mode luksFormat /root/cryptTest$i /root/cryptPassphrase 
	cryptsetup luksOpen /root/cryptTest$i cryptTest$i --key-file=/root/cryptPassphrase 
	mkfs.ext3 /dev/mapper/cryptTest$i
	cryptsetup luksClose cryptTest$i
done

echo "now reboot. The crypto password is: test"
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