Do GPG signature checking for the whole migration process

Before this commit, the Rocky system packages like rocky-repos,
rocky-release and similar, were not checked for their gpg signature,
while the rest of the packages were signature checked.
While the system packages were downloaded from https://dl.rockylinux.org,
this still created the risk that someone could attack the CDN infrastructure
behind dl.rockylinux.org and place malicous files there.

With this patch the GPG key is downloaded separately and then compared
against a checksum contained in the script. This adds an extra layer of
security.

migrate2rocky.sh

@@ -30,9 +30,15 @@ shopt -s nullglob
 SUPPORTED_MAJOR="8"
 SUPPORTED_PLATFORM="platform:el$SUPPORTED_MAJOR"
 ARCH=$(arch)
+
+gpg_key_url="https://dl.rockylinux.org/pub/rocky/RPM-GPG-KEY-rockyofficial"
+gpg_key_sha512="88fe66cf0a68648c2371120d56eb509835266d9efdf7c8b9ac8fc101bdf1f0e0197030d3ea65f4b5be89dc9d1ef08581adb068815c88d7b1dc40aa1c32990f6a"
+
+# all repos must be signed with the same key given in $gpg_key_url
+declare -A repo_urls
 repo_urls=(
-    "rockybaseos,https://dl.rockylinux.org/pub/rocky/${SUPPORTED_MAJOR}/BaseOS/$ARCH/os/"
-    "rockyappstream,https://dl.rockylinux.org/pub/rocky/${SUPPORTED_MAJOR}/AppStream/$ARCH/os/"
+    [rockybaseos]="https://dl.rockylinux.org/pub/rocky/${SUPPORTED_MAJOR}/BaseOS/$ARCH/os/"
+    [rockyappstream]="https://dl.rockylinux.org/pub/rocky/${SUPPORTED_MAJOR}/AppStream/$ARCH/os/"
 )
 
 unset CDPATH
@@ -99,6 +105,7 @@ bin_check() {
     bins=(
 	rpm dnf awk column tee tput mkdir
 	cat arch sort uniq rmdir rm head
+	curl sha512sum mktemp
     )
     if [[ $update_efi ]]; then
 	bins+=(findmnt grub2-mkconfig efibootmgr grep mokutil)
@@ -408,16 +415,28 @@ generate_rpm_info() {
 }
 
 package_swaps() {
+    # prepare repo parameters
+    local -a dnfparameters
+    for repo in "${!repo_urls[@]}"; do
+	dnfparameters+=( "--repofrompath=${repo},${repo_urls[${repo}]}" )
+	dnfparameters+=( "--setopt=${repo}.gpgcheck=1" )
+	dnfparameters+=( "--setopt=${repo}.gpgkey=file://${gpg_key_file}" )
+    done
+
     # Use dnf shell to swap the system packages out.
-    dnf -y shell --nogpg --disablerepo=\* --noautoremove \
+    dnf -y shell --disablerepo=\* --noautoremove \
 	--setopt=protected_packages= --setopt=keepcache=True \
-	"${repo_urls[@]/#/--repofrompath=}" <<EOF
+	"${dnfparameters[@]}" \
+	<<EOF
 	remove ${installed_pkg_map[@]} ${addl_pkg_removes[@]}
 	install ${!installed_pkg_map[@]}
 	run
 	exit
 EOF
 
+    # rocky-repos and rocky-gpg-keys are now installed, so we don't need the key file anymore
+    rm -rf "$gpg_tmp_dir"
+
     # We need to check to make sure that all of the original system packages
     # have been removed and all of the new ones have been added. If a package
     # was supposed to be removed and one with the same name added back then
@@ -549,6 +568,32 @@ fix_efi () (
 	exit_message "Error updating uEFI firmware."
 )
 
+# Download and verify the Rocky Linux package signing key
+establish_gpg_trust () {
+    # create temp dir and verify it is really created and empty, so we are sure deleting it afterwards won't cause any harm
+    declare -g gpg_tmp_dir
+    if ! gpg_tmp_dir=$(mktemp -d) || [[ ! -d "$gpg_tmp_dir" ]]; then
+	exit_message "Error creating temp dir"
+    fi
+    # failglob makes pathname expansion fail if empty, dotglob adds files starting with . to pathname expansion
+    if ( shopt -s failglob dotglob; : "$gpg_tmp_dir"/* ) 2>/dev/null ; then
+	exit_message "Temp dir not empty"
+    fi
+
+    # extract the filename from the url, use the temp dir just created
+    declare -g gpg_key_file="$gpg_tmp_dir/${gpg_key_url##*/}"
+
+    if ! curl -o "$gpg_key_file" --silent --show-error "$gpg_key_url"; then
+	rm -rf "$gpg_tmp_dir"
+	exit_message "Error downloading the Rocky Linux signing key."
+    fi
+
+    if ! sha512sum --quiet -c <<<"$gpg_key_sha512 $gpg_key_file"; then
+	rm -rf "$gpg_tmp_dir"
+	exit_message "Error validating the signing key."
+    fi
+}
+
 ## End actual work
 
 noopts=0
@@ -583,6 +628,7 @@ fi
 
 if [[ $convert_to_rocky ]]; then
     collect_system_info
+    establish_gpg_trust
     package_swaps
 fi