initramfs: avoid filename buffer overrun

In the Linux kernel, the following vulnerability has been resolved:

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.