initramfs: avoid filename buffer overrun

The initramfs filename field is defined in
Documentation/driver-api/early-userspace/buffer-format.rst as:

 37 cpio_file := ALGN(4) + cpio_header + filename + "\0" + ALGN(4) + data
...
 55 ============= ================== =========================
 56 Field name    Field size         Meaning
 57 ============= ================== =========================
...
 70 c_namesize    8 bytes            Length of filename, including final \0

When extracting an initramfs cpio archive, the kernel's do_name() path
handler assumes a zero-terminated path at @collected, passing it
directly to filp_open() / init_mkdir() / init_mknod().

If a specially crafted cpio entry carries a non-zero-terminated filename
and is followed by uninitialized memory, then a file may be created with
trailing characters that represent the uninitialized memory. The ability
to create an initramfs entry would imply already having full control of
the system, so the buffer overrun shouldn't be considered a security
vulnerability.

Append the output of the following bash script to an existing initramfs
and observe any created /initramfs_test_fname_overrunAA* path. E.g.
  ./reproducer.sh | gzip >> /myinitramfs

It's easiest to observe non-zero uninitialized memory when the output is
gzipped, as it'll overflow the heap allocated @out_buf in __gunzip(),
rather than the initrd_start+initrd_size block.

---- reproducer.sh ----
nilchar="A"	# change to "\0" to properly zero terminate / pad
magic="070701"
ino=1
mode=$(( 0100777 ))
uid=0
gid=0
nlink=1
mtime=1
filesize=0
devmajor=0
devminor=1
rdevmajor=0
rdevminor=0
csum=0
fname="initramfs_test_fname_overrun"
namelen=$(( ${#fname} + 1 ))	# plus one to account for terminator

printf "%s%08x%08x%08x%08x%08x%08x%08x%08x%08x%08x%08x%08x%08x%s" \
	$magic $ino $mode $uid $gid $nlink $mtime $filesize \
	$devmajor $devminor $rdevmajor $rdevminor $namelen $csum $fname

termpadlen=$(( 1 + ((4 - ((110 + $namelen) & 3)) % 4) ))
printf "%.s${nilchar}" $(seq 1 $termpadlen)
---- reproducer.sh ----

Symlink filename fields handled in do_symlink() won't overrun past the
data segment, due to the explicit zero-termination of the symlink
target.

Fix filename buffer overrun by aborting the initramfs FSM if any cpio
entry doesn't carry a zero-terminator at the expected (name_len - 1)
offset.

Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
Signed-off-by: David Disseldorp <ddiss@suse.de>
Link: https://lore.kernel.org/r/20241030035509.20194-2-ddiss@suse.de
Signed-off-by: Christian Brauner <brauner@kernel.org>

init/initramfs.c

@@ -360,6 +360,15 @@ static int __init do_name(void)
 {
 	state = SkipIt;
 	next_state = Reset;
+
+	/* name_len > 0 && name_len <= PATH_MAX checked in do_header */
+	if (collected[name_len - 1] != '\0') {
+		pr_err("initramfs name without nulterm: %.*s\n",
+		       (int)name_len, collected);
+		error("malformed archive");
+		return 1;
+	}
+
 	if (strcmp(collected, "TRAILER!!!") == 0) {
 		free_hash();
 		return 0;
@@ -424,6 +433,12 @@ static int __init do_copy(void)
 
 static int __init do_symlink(void)
 {
+	if (collected[name_len - 1] != '\0') {
+		pr_err("initramfs symlink without nulterm: %.*s\n",
+		       (int)name_len, collected);
+		error("malformed archive");
+		return 1;
+	}
 	collected[N_ALIGN(name_len) + body_len] = '\0';
 	clean_path(collected, 0);
 	init_symlink(collected + N_ALIGN(name_len), collected);