A script that can unpack initramfs from a citadel bzImage

scripts/extract-initramfs.sh

@@ -0,0 +1,190 @@
+#!/bin/bash
+
+#
+# extract-initramfs:
+#
+# Extracts and decompresses kernel binary (vmlinux) from a bzImage then searches
+# the extracted kernel for an initramfs cpio archive.
+#
+# Assumes kernel is compressed with LZ4 and that embedded cpio archive is not compressed.
+#
+
+usage() {
+cat <<-EOF
+USAGE: extract-initramfs [options] [kernel bzImage]
+
+OPTIONS
+
+    -d <directory-name>     Choose a non-default directory for extracted initramfs (default ${PWD}/initramfs)
+
+EOF
+exit 0
+}
+
+info() {
+    printf "[+] ${1}\n"
+}
+
+error() {
+    >&2 echo "[!] ${1}"
+    exit 1
+}
+
+REQUIRED_UTILS="cpio lz4 readelf"
+
+confirm_required_utils() {
+    local required=${1}
+    local missing=""
+
+    for util in ${required}; do
+        which ${util} > /dev/null || missing="${missing}${util} "
+    done
+
+    if [[ -n ${missing} ]]; then
+        error "Required utilities are not installed: ${missing}"
+    fi
+}
+
+confirm_required_utils "${REQUIRED_UTILS}"
+
+CPIO_HEADER='070701'
+LZ4_HEADER='\x02!L\x18'
+
+img=""
+workdir=$(mktemp -d /tmp/initramfs-extract.XXXX)
+vmlinux=${workdir}/vmlinux
+outdir=${PWD}/initramfs
+trap "rm -rf $workdir" 0
+
+#
+# Search for $pattern in $infile and output a list of offsets where the pattern is found
+#
+
+binary_grep() {
+    local pattern=$1 infile=$2
+    grep --only-matching --byte-offset --text --perl-regexp ${pattern} ${infile} | cut -d: -f1
+}
+
+#
+# Output $infile starting at byte position $offset
+#
+
+binary_tail() {
+    local offset=$1 infile=$2
+    tail -c +$(($offset + 1)) ${infile}
+}
+
+check_elf() {
+    readelf -h ${1} > /dev/null 2>&1
+}
+
+#
+# Search for $pattern in $img file and for each offset found, attempt to decompress 
+# the file starting at that position by piping through $cmdline
+#
+
+try_decompress() {
+    local pattern=$1 cmdline=$2
+    for pos in $(binary_grep ${pattern} ${img}); do
+        printf "Testing offset %08x\n" $pos
+        binary_tail $pos $img | ${cmdline} > ${vmlinux} 2> /dev/null
+        check_elf ${vmlinux} && return 0
+    done
+    return 1
+}
+
+
+#
+# Add more patterns and command lines from extract-vmlinux script in linux kernel source tree
+# to support more compression formats
+#
+
+decompress_kernel() {
+    try_decompress $LZ4_HEADER 'lz4 -d' && return 0
+    error "Failed to decompress kernel"
+}
+
+#
+#  CPIO format is a list of file entries where each entry begins with a header that contains only
+#  ASCII hexadecimal digits. To avoid false positives matching only the magic value, check to make
+#  sure that some of the the bytes following the magic field are ASCII hex digits.
+#
+#  From https://www.systutorials.com/docs/linux/man/5-cpio/
+#
+#  New ASCII Format
+#
+#  The "new" ASCII format uses 8-byte hexadecimal fields for all numbers and separates device numbers 
+#  into separate fields for major and minor numbers.
+#
+#    struct cpio_newc_header {
+#            char    c_magic[6];
+#            char    c_ino[8];
+#            char    c_mode[8];
+#            char    c_uid[8];
+#            char    c_gid[8];
+#            char    c_nlink[8];
+#            char    c_mtime[8];
+#            char    c_filesize[8];
+#            char    c_devmajor[8];
+#            char    c_devminor[8];
+#            char    c_rdevmajor[8];
+#            char    c_rdevminor[8];
+#            char    c_namesize[8];
+#            char    c_check[8];
+#    };
+#
+
+confirm_cpio() {
+    # Check that there are 100 ascii hex digit characters at offset
+    local offset=${1} checklen=100
+    dd if=${vmlinux} bs=1 count=${checklen} skip=${offset} 2> /dev/null | grep -q "[[:xdigit:]]\{${checklen}\}"
+}
+
+extract_cpio() {
+    cpio --make-directories --extract --no-absolute-filenames -D ${outdir} 2> /dev/null
+}
+
+extract_initramfs() {
+    for pos in $(binary_grep ${CPIO_HEADER} ${vmlinux}); do
+        printf "Found CPIO: %08x\n" $pos
+        if confirm_cpio ${pos}; then
+            mkdir ${outdir}
+            binary_tail $pos ${vmlinux} | extract_cpio
+            info "Initramfs files extracted to ${outdir}"
+            exit 0
+        fi
+    done
+}
+
+while [[ $# -gt 0 ]]; do
+    key=${1}
+    case $key in
+        -d)
+            outdir="$(realpath ${2})"
+            shift
+            shift
+            ;;
+        -h|--help)
+            usage
+            ;;
+        -*)
+            printf "Unknown option ${key}\n"
+            usage
+            ;;
+        *)
+            img=${1}
+            shift
+            ;;
+    esac
+done
+
+[[ -z ${img} ]] && usage
+[[ -f ${img} ]] || error "kernel image file ${img} does not exist"
+
+if [[ -d ${outdir} ]]; then
+    error "Output directory ${outdir} already exists. Remove it or choose a different directory with -d option."
+fi
+
+decompress_kernel
+extract_initramfs
+