Dropbear initramfs hook creates authorized_keys file in an invalid folder

Bug #1645555 reported by Ibrahim Karahan on 2016-11-29
This bug affects 3 people
Affects Status Importance Assigned to Milestone
dropbear (Ubuntu)

Bug Description

Dropbear initramfs hook copies `/etc/initramfs-tools/root/.ssh/authorized_keys` into initramfs for passwordless login during boot.

The script responsible is `/usr/share/initramfs-tools/hooks/dropbear`. The destination folder is set at line 37:

`home=$(mktemp -d "$DESTDIR/root-XXXXXX")`

which ends up creating a temp directory in initramfs file, similar to `/root-hZQ48a`, instead of `/root`. Dropbear fails to find `/root/.ssh/authorized_keys` at boot and does not allow remote logins.

See the contents of a generated initrd with:
`cd `mktemp -d` && gzip -dc /boot/initrd.img-`uname -r` | cpio -ivd`

Setting the $home variable with the following lines should solve the issue:
mkdir -p $home

Launchpad Janitor (janitor) wrote :

Status changed to 'Confirmed' because the bug affects multiple users.

Changed in dropbear (Ubuntu):
status: New → Confirmed
Jamie Lokier (jamie-shareable) wrote :

The temp directory is fine, because the script also generates /etc/passwd setting root's home directory to be the temp directory.

Jérôme Drouet (jerome-drouet) wrote :

not so fine to me because it changes ~root directory in a way you can not predict/reuse in any other hook

for instance :
/ # cat /etc/passwd
/ # cd
~# pwd

how can we guess the name in other hooks, let's say to put some files in ~root ?

this change seems to be related to https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=558115 , but other hooks should not use /root as a mountpoint as described (haven't check if it is still the case)

Still exists on Ubuntu 18.04:

$ sudo lsinitramfs /boot/initrd.img-4.15.0-29-generic | grep root

The temp folder created with "$DESTDIR/root-XXXXXX" scheme is garbage. It's never renamed to /root folder, thus Dropbear fails to find the authorized_keys file.

The workaround in #1 still works.

Everything looks OK in /boot/initrd.img-4.15.0-*


NB: root-xxxxxx/.ssh/autorized_keys comes from /etc/dropbear-initramfs/authorized_keys must be in 'dropbear' format, not 'openssh' format, then use dropbearkey or dropbearconvert, and update-initramfs to recreate your initrd image.

But it seems that root-xxxxxx/.ssh/authorized_keys isn't expanded properly from initrd !!!???


First, I worked around with /usr/share/initramfs-tools/hooks/dropbear script to create several copies of this authorized_keys in $home and $DESTDIR, then in BusyBox, on (initramfs) prompt, I can find one in / from $DESTDIR, one in /root-xxxxxx/ from $home, but none in /root-xxxxxx/.ssh/ !!!???

Second, I'm booting thru PXE, and iPXE, with an HTTP server, then I have the ability to download authorized_keys, or to move other copies that I stressed above, into /root-xxxxxx/.ssh/, and can, then, log in...

Why is not expanded properly even it is well located in the initrd image????

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