Advisory [CVE-2026-53320] reject zero bd_oblocknr in nilfs_ioctl_mark_blocks_dirty

In the Linux kernel, the following vulnerability has been resolved: nilfs2: reject zero bd_oblocknr in nilfs_ioctl_mark_blocks_dirty() nilfs_ioctl_mark_blocks_dirty() uses bd_oblocknr to detect dead blocks by comparing it with the current block number bd_blocknr. If they differ, the block is considered dead and skipped.

However, bd_oblocknr should never be 0 since block 0 typically stores the primary superblock and is never a valid GC target block.

A corrupted ioctl request with bd_oblocknr set to 0 causes the comparison to incorrectly match when the lookup returns -ENOENT and sets bd_blocknr to 0, bypassing the dead block check and calling nilfs_bmap_mark() on a non-existent block. This causes nilfs_btree_do_lookup() to return -ENOENT, triggering the WARN_ON(ret == -ENOENT).

Fix this by rejecting ioctl requests with bd_oblocknr set to 0 at the beginning of each iteration. [ryusuke: slightly modified the commit message and comments for accuracy] A local attacker could exploit this vulnerability by sending a specially crafted input/output control (ioctl) request to the `nilfs_ioctl_mark_blocks_dirty()` function.

By providing a zero block number, the attacker can bypass a critical dead block check, causing the system to attempt operations on a non-existent block. This ultimately leads to a system crash, resulting in a Denial of Service (DoS).

Red Hat severity: not rated.