CVE-2026-53320

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]