openssh: Atomically generate host keys

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.

[YOCTO #11671]

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>

1 files changed, 34 insertions, 8 deletions

diff --git a/meta/recipes-connectivity/openssh/openssh/sshd_check_keys b/meta/recipes-connectivity/openssh/openssh/sshd_check_keys
index f5bba53ca3..5463b1a4cb 100644
--- a/meta/recipes-connectivity/openssh/openssh/sshd_check_keys
+++ b/meta/recipes-connectivity/openssh/openssh/sshd_check_keys
@@ -1,5 +1,35 @@
 #! /bin/sh
 
+generate_key() {
+    local FILE=$1
+    local TYPE=$2
+    local DIR="$(dirname "$FILE")"
+
+    mkdir -p "$DIR"
+    ssh-keygen -q -f "${FILE}.tmp" -N '' -t $TYPE
+
+    # Atomically rename file public key
+    mv -f "${FILE}.tmp.pub" "${FILE}.pub"
+
+    # This sync does double duty: Ensuring that the data in the temporary
+    # private key file is on disk before the rename, and ensuring that the
+    # public key rename is completed before the private key rename, since we
+    # switch on the existence of the private key to trigger key generation.
+    # This does mean it is possible for the public key to exist, but be garbage
+    # but this is OK because in that case the private key won't exist and the
+    # keys will be regenerated.
+    #
+    # In the event that sync understands arguments that limit what it tries to
+    # fsync(), we provided them. If it does not, it will simply call sync()
+    # which is just as well
+    sync "${FILE}.pub" "$DIR" "${FILE}.tmp"
+
+    mv "${FILE}.tmp" "$FILE"
+
+    # sync to ensure the atomic rename is committed
+    sync "$DIR"
+}
+
 # /etc/default/ssh may set SYSCONFDIR and SSHD_OPTS
 if test -f /etc/default/ssh; then
     . /etc/default/ssh
@@ -43,22 +73,18 @@ HOST_KEY_ED25519=$(grep ^HostKey "${sshd_config}" | grep _ed25519_ | tail -1 | a
 # create keys if necessary
 if [ ! -f $HOST_KEY_RSA ]; then
     echo "  generating ssh RSA key..."
-    mkdir -p $(dirname $HOST_KEY_RSA)
-    ssh-keygen -q -f $HOST_KEY_RSA -N '' -t rsa
+    generate_key $HOST_KEY_RSA rsa
 fi
 if [ ! -f $HOST_KEY_ECDSA ]; then
     echo "  generating ssh ECDSA key..."
-    mkdir -p $(dirname $HOST_KEY_ECDSA)
-    ssh-keygen -q -f $HOST_KEY_ECDSA -N '' -t ecdsa
+    generate_key $HOST_KEY_ECDSA ecdsa
 fi
 if [ ! -f $HOST_KEY_DSA ]; then
     echo "  generating ssh DSA key..."
-    mkdir -p $(dirname $HOST_KEY_DSA)
-    ssh-keygen -q -f $HOST_KEY_DSA -N '' -t dsa
+    generate_key $HOST_KEY_DSA dsa
 fi
 if [ ! -f $HOST_KEY_ED25519 ]; then
     echo "  generating ssh ED25519 key..."
-    mkdir -p $(dirname $HOST_KEY_ED25519)
-    ssh-keygen -q -f $HOST_KEY_ED25519 -N '' -t ed25519
+    generate_key $HOST_KEY_ED25519 ed25519
 fi