meta-phosphor: fix openssh key generation on read-only-rootfs types

Some of our file system layouts enable the `read-only-rootfs` feature,
which happens to trigger some code in rootfs-postcommands.bbclass that
moves the SSH key location from `/etc` to `/var`.  For Dropbear, the
default was to move it to `/var/lib`, which we happen to put into an
overlay, but for OpenSSH it moved it to `/var/run`.  The result of this
is that the SSH key is regenerated on each reboot.

In order to bypass this code that expects the SSH key to be in a
volatile file system, Yocto provides the `overlayfs-etc` IMAGE_FEATURE
as well.  We need to enable this, but this feature as a side-effect
generates an alternative `/sbin/init` similar to what we do for
pre-mounting the overlay.  We need to disable this aspect so I've set
some variables and appends to cause `overlay-etc.bbclass` to have no
effect.

Lastly, the result of all of this is that the location for the dropbear
key moves from `/var/lib` to `/etc` (which is what the default is on
the jffs2-based layouts already).  Add some migration services that
will move existing keys in the old location over to `/etc` so that
users do not notice a host key change as part of this.

Tested: Tested on Bletchley (OpenSSH) and Witherspoon (Dropbear).
Bletchley no longer regenerates the SSH key on each reboot.
Witherspoon has the key location in `/etc/dropbear` as expected and
the migration service successfully runs before the
`dropbearkey.service`.

```
May 05 21:46:40 witherspoon systemd[1]: Starting SSH Key Generation...
May 05 21:46:41 witherspoon sh[268]: Generating 2048 bit rsa key, this may take a while...
May 05 21:47:13 witherspoon sh[268]: Public key portion is:
May 05 21:47:13 witherspoon sh[268]: ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCgiywAIF3RleqNphZZuUjNCXDI10ChEAoPI02/g9F8CiXI2Pc55nFHh/hrTn7niawydpEc8FH62rf1WpoA5hYkKrj/j6i2Iv1UrGFZX4q9IwlFcd3...
May 05 21:47:13 witherspoon sh[268]: Fingerprint: SHA256:tsjx4PBtcaiLnUCFh4XESPRnTXoGsgujVrbdJD4INMY
May 05 21:47:13 witherspoon systemd[1]: Finished SSH Key Generation.
```

Manually moved the key to `/var/lib` and rebooted and observed the same
key moved back to `/etc` (on Witherspoon).

```
May 05 21:49:01 witherspoon systemd[1]: Starting Migrate dropbear keys from /var/lib to /etc...
May 05 21:49:02 witherspoon migrate-key-location[194]: Migrating Dropbear key from /var/lib to /etc.
May 05 21:49:11 witherspoon systemd[1]: Finished Migrate dropbear keys from /var/lib to /etc.
May 05 21:49:14 witherspoon systemd[1]: Starting SSH Key Generation...
May 05 21:49:18 witherspoon systemd[1]: Finished SSH Key Generation.
```

After one last reboot, the key in `/etc` is reused:

```
May 05 21:51:44 witherspoon systemd[1]: Starting SSH Key Generation...
May 05 21:51:45 witherspoon systemd[1]: Finished SSH Key Generation.
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I68b6c383f91931995e3d3203e5aafd8fdc23f750

3 files changed, 36 insertions, 0 deletions

diff --git a/meta-phosphor/recipes-core/dropbear/dropbear/dropbear-migrate-key-location.service b/meta-phosphor/recipes-core/dropbear/dropbear/dropbear-migrate-key-location.service
new file mode 100644
index 0000000000..f8c12bf70c
--- /dev/null
+++ b/meta-phosphor/recipes-core/dropbear/dropbear/dropbear-migrate-key-location.service
@@ -0,0 +1,12 @@
+[Unit]
+Description=Migrate dropbear keys from /var/lib to /etc
+Before=dropbearkey.service
+ConditionPathExists=/var/lib/dropbear/dropbear_rsa_host_key
+
+[Service]
+RemainAfterExit=yes
+Type=oneshot
+ExecStart=/usr/libexec/dropbear/migrate-key-location
+
+[Install]
+WantedBy=dropbearkey.service
diff --git a/meta-phosphor/recipes-core/dropbear/dropbear/migrate-key-location b/meta-phosphor/recipes-core/dropbear/dropbear/migrate-key-location
new file mode 100644
index 0000000000..ce968946c2
--- /dev/null
+++ b/meta-phosphor/recipes-core/dropbear/dropbear/migrate-key-location
@@ -0,0 +1,11 @@
+#!/bin/sh
+
+if [ ! -e /etc/dropbear/dropbear_rsa_host_key ]; then
+    if [ -e /var/lib/dropbear/dropbear_rsa_host_key ]; then
+        echo "Migrating Dropbear key from /var/lib to /etc."
+        mkdir -p /etc/dropbear
+        mv /var/lib/dropbear/dropbear_rsa_host_key /etc/dropbear
+    else
+        echo "No Dropbear key found in /var/lib."
+    fi
+fi
diff --git a/meta-phosphor/recipes-core/dropbear/dropbear_%.bbappend b/meta-phosphor/recipes-core/dropbear/dropbear_%.bbappend
index a3cbd1b69d..6448fbfdb2 100644
--- a/meta-phosphor/recipes-core/dropbear/dropbear_%.bbappend
+++ b/meta-phosphor/recipes-core/dropbear/dropbear_%.bbappend
@@ -6,8 +6,21 @@ FILESEXTRAPATHS:prepend := "${THISDIR}/${PN}:"
 SRC_URI += "file://dropbearkey.service \
             file://localoptions.h \
             file://dropbear.default \
+            file://dropbear-migrate-key-location.service \
+            file://migrate-key-location \
            "
 
 # pull in OpenSSH's /usr/libexec/sftp-server so we don't have to rely
 # on the crufty old scp protocol for file transfer
 RDEPENDS:${PN} += "openssh-sftp-server"
+
+# Add service to migrate the dropbear keys from /var/lib to /etc.
+do_install:append() {
+    install -d ${D}${base_libdir}/systemd/system
+    install -m 0644 ${WORKDIR}/dropbear-migrate-key-location.service \
+        ${D}${base_libdir}/systemd/system
+
+    install -d ${D}${libexecdir}/${BPN}
+    install -m 0755 ${WORKDIR}/migrate-key-location ${D}${libexecdir}/${BPN}
+}
+SYSTEMD_SERVICE:${PN}:append = " dropbear-migrate-key-location.service"