CVE-2025-40049: Undefined Security Weakness

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

Squashfs: fix uninit-value in squashfs_get_parent

Syzkaller reports a "KMSAN: uninit-value in squashfs_get_parent" bug.

This is caused by open_by_handle_at() being called with a file handle containing an invalid parent inode number. In particular the inode number is that of a symbolic link, rather than a directory.

Squashfs_get_parent() gets called with that symbolic link inode, and accesses the parent member field.

unsigned int parent_ino = squashfs_i(inode)->parent;

Because non-directory inodes in Squashfs do not have a parent value, this is uninitialised, and this causes an uninitialised value access.

The fix is to initialise parent with the invalid inode 0, which will cause an EINVAL error to be returned.

Regular inodes used to share the parent field with the block_list_start field. This is removed in this commit to enable the parent field to contain the invalid inode number 0.