CVE-2024-53142 Information

Description

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 + \�\ + 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\t change to \�\ to properly zero terminate / pad magic=�01\nino=1 mode=$(( 0100777 )) uid=0 gid=0 nlink=1 mtime=1 filesize=0 devmajor=0 devminor=1 rdevmajor=0 rdevminor=0 csum=0 fname=\initramfs_test_fname_overrun\nnamelen=$(( $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.

Reference

https://git.kernel.org/stable/c/49d01e736c3045319e030d1e75fb983011abaca7 https://git.kernel.org/stable/c/bb7ac96670ab1d8d681015f9d66e45dad579af4d https://git.kernel.org/stable/c/e017671f534dd3f568db9e47b0583e853d2da9b5 https://git.kernel.org/stable/c/fb83b093f75806333b6f4ae29b158d2e0e3ec971