[meta-oe][PATCH v3] openssh: Atomically generate host keys

[Joshua Watt <jpewhacker@gmail.com>]

Generating the host keys atomically prevents power interruptions during
the first boot from leaving the key files incomplete, which often
prevents users from being able to ssh into the device.
---
 meta/recipes-connectivity/openssh/openssh/init     | 21 +++----------
 .../openssh/openssh/sshd-check-key                 | 36 ++++++++++++++++++++++
 .../openssh/openssh/sshdgenkeys.service            | 24 +++++++--------
 meta/recipes-connectivity/openssh/openssh_7.5p1.bb |  8 +++++
 4 files changed, 60 insertions(+), 29 deletions(-)
 create mode 100644 meta/recipes-connectivity/openssh/openssh/sshd-check-key

diff --git a/meta/recipes-connectivity/openssh/openssh/init b/meta/recipes-connectivity/openssh/openssh/init
index 1f63725..22124a9 100644
--- a/meta/recipes-connectivity/openssh/openssh/init
+++ b/meta/recipes-connectivity/openssh/openssh/init
@@ -45,23 +45,10 @@ check_config() {
 }
 
 check_keys() {
-	# create keys if necessary
-	if [ ! -f $HOST_KEY_RSA ]; then
-		echo "  generating ssh RSA key..."
-		ssh-keygen -q -f $HOST_KEY_RSA -N '' -t rsa
-	fi
-	if [ ! -f $HOST_KEY_ECDSA ]; then
-		echo "  generating ssh ECDSA key..."
-		ssh-keygen -q -f $HOST_KEY_ECDSA -N '' -t ecdsa
-	fi
-	if [ ! -f $HOST_KEY_DSA ]; then
-		echo "  generating ssh DSA key..."
-		ssh-keygen -q -f $HOST_KEY_DSA -N '' -t dsa
-	fi
-	if [ ! -f $HOST_KEY_ED25519 ]; then
-		echo "  generating ssh ED25519 key..."
-		ssh-keygen -q -f $HOST_KEY_ED25519 -N '' -t ed25519
-	fi
+    @LIBEXECDIR@/sshd-check-key $HOST_KEY_RSA rsa
+    @LIBEXECDIR@/sshd-check-key $HOST_KEY_ECDSA ecdsa
+    @LIBEXECDIR@/sshd-check-key $HOST_KEY_DSA dsa
+    @LIBEXECDIR@/sshd-check-key $HOST_KEY_ED25519 ed25519
 }
 
 export PATH="${PATH:+$PATH:}/usr/sbin:/sbin"
diff --git a/meta/recipes-connectivity/openssh/openssh/sshd-check-key b/meta/recipes-connectivity/openssh/openssh/sshd-check-key
new file mode 100644
index 0000000..d2613af
--- /dev/null
+++ b/meta/recipes-connectivity/openssh/openssh/sshd-check-key
@@ -0,0 +1,36 @@
+#! /bin/sh
+set -e
+
+NAME="$1"
+TYPE="$2"
+
+if [ -z "$NAME" ] || [ -z "$TYPE" ]; then
+    echo "Usage: $0 NAME TYPE"
+    exit 1;
+fi
+
+DIR="$(dirname "$NAME")"
+
+if [ ! -f "$NAME" ]; then
+    echo "  generating ssh $TYPE key..."
+    ssh-keygen -q -f "${NAME}.tmp" -N '' -t $TYPE
+
+    # Move (Atomically rename) files
+    mv -f "${NAME}.tmp.pub" "${NAME}.pub"
+
+    # This sync does double duty: Ensuring that the data in the temporary
+    # private key file is on disk before the rename, and ensuring that the
+    # public key rename is completed before the private key rename, since we
+    # switch on the existence of the private key to trigger key generation.
+    # This does mean it is possible for the public key to exist, but be garbage
+    # but this is OK because in that case the private key won't exist and the
+    # keys will be regenerated.
+    #
+    # In the event that sync understands arguments that limit what it tries to
+    # fsync(), we provided them. If it does not, it will simply call sync()
+    # which is just as well
+    sync "${NAME}.pub" "$DIR" "${NAME}.tmp"
+
+    mv -f "${NAME}.tmp" "${NAME}"
+fi
+
diff --git a/meta/recipes-connectivity/openssh/openssh/sshdgenkeys.service b/meta/recipes-connectivity/openssh/openssh/sshdgenkeys.service
index 148e6ad..5d08b53 100644
--- a/meta/recipes-connectivity/openssh/openssh/sshdgenkeys.service
+++ b/meta/recipes-connectivity/openssh/openssh/sshdgenkeys.service
@@ -1,22 +1,22 @@
 [Unit]
 Description=OpenSSH Key Generation
 RequiresMountsFor=/var /run
-ConditionPathExists=!/var/run/ssh/ssh_host_rsa_key
-ConditionPathExists=!/var/run/ssh/ssh_host_dsa_key
-ConditionPathExists=!/var/run/ssh/ssh_host_ecdsa_key
-ConditionPathExists=!/var/run/ssh/ssh_host_ed25519_key
-ConditionPathExists=!/etc/ssh/ssh_host_rsa_key
-ConditionPathExists=!/etc/ssh/ssh_host_dsa_key
-ConditionPathExists=!/etc/ssh/ssh_host_ecdsa_key
-ConditionPathExists=!/etc/ssh/ssh_host_ed25519_key
+ConditionPathExists=|!/var/run/ssh/ssh_host_rsa_key
+ConditionPathExists=|!/var/run/ssh/ssh_host_dsa_key
+ConditionPathExists=|!/var/run/ssh/ssh_host_ecdsa_key
+ConditionPathExists=|!/var/run/ssh/ssh_host_ed25519_key
+ConditionPathExists=|!/etc/ssh/ssh_host_rsa_key
+ConditionPathExists=|!/etc/ssh/ssh_host_dsa_key
+ConditionPathExists=|!/etc/ssh/ssh_host_ecdsa_key
+ConditionPathExists=|!/etc/ssh/ssh_host_ed25519_key
 
 [Service]
 Environment="SYSCONFDIR=/etc/ssh"
 EnvironmentFile=-/etc/default/ssh
 ExecStart=@BASE_BINDIR@/mkdir -p $SYSCONFDIR
-ExecStart=@BINDIR@/ssh-keygen -q -f ${SYSCONFDIR}/ssh_host_rsa_key -N '' -t rsa
-ExecStart=@BINDIR@/ssh-keygen -q -f ${SYSCONFDIR}/ssh_host_dsa_key -N '' -t dsa
-ExecStart=@BINDIR@/ssh-keygen -q -f ${SYSCONFDIR}/ssh_host_ecdsa_key -N '' -t ecdsa
-ExecStart=@BINDIR@/ssh-keygen -q -f ${SYSCONFDIR}/ssh_host_ed25519_key -N '' -t ed25519
+ExecStart=@LIBEXECDIR@/sshd-check-key ${SYSCONFDIR}/ssh_host_rsa_key rsa
+ExecStart=@LIBEXECDIR@/sshd-check-key ${SYSCONFDIR}/ssh_host_dsa_key dsa
+ExecStart=@LIBEXECDIR@/sshd-check-key ${SYSCONFDIR}/ssh_host_ecdsa_key ecdsa
+ExecStart=@LIBEXECDIR@/sshd-check-key ${SYSCONFDIR}/ssh_host_ed25519_key ed25519
 Type=oneshot
 RemainAfterExit=yes
diff --git a/meta/recipes-connectivity/openssh/openssh_7.5p1.bb b/meta/recipes-connectivity/openssh/openssh_7.5p1.bb
index 5b96745..ede8823 100644
--- a/meta/recipes-connectivity/openssh/openssh_7.5p1.bb
+++ b/meta/recipes-connectivity/openssh/openssh_7.5p1.bb
@@ -25,6 +25,7 @@ SRC_URI = "http://ftp.openbsd.org/pub/OpenBSD/OpenSSH/portable/openssh-${PV}.tar
            file://openssh-7.1p1-conditional-compile-des-in-cipher.patch \
            file://openssh-7.1p1-conditional-compile-des-in-pkcs11.patch \
            file://fix-potential-signed-overflow-in-pointer-arithmatic.patch \
+           file://sshd-check-key \
            "
 
 PAM_SRC_URI = "file://sshd"
@@ -124,7 +125,14 @@ do_install_append () {
 	sed -i -e 's,@BASE_BINDIR@,${base_bindir},g' \
 		-e 's,@SBINDIR@,${sbindir},g' \
 		-e 's,@BINDIR@,${bindir},g' \
+		-e 's,@LIBEXECDIR@,${libexecdir}/${BPN},g' \
 		${D}${systemd_unitdir}/system/sshd.socket ${D}${systemd_unitdir}/system/*.service
+
+	sed -i -e 's,@LIBEXECDIR@,${libexecdir}/${BPN},g' \
+		${D}${sysconfdir}/init.d/sshd
+
+	install -d ${D}${libexecdir}/${BPN}
+	install -m 0755 ${WORKDIR}/sshd-check-key ${D}${libexecdir}/${BPN}
 }
 
 do_install_ptest () {
-- 
2.9.3