[PATCH v10] 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.

[YOCTO #11671]

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
---
 .../openssh/openssh/sshd_check_keys                | 42 +++++++++++++++++-----
 1 file 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 f5bba53..5463b1a 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
 
-- 
2.9.4