published July 4, 2025
In the Linux kernel, the following vulnerability has been resolved:
ext4: only dirty folios when data journaling regular files
fstest generic/388 occasionally reproduces a crash that looks as
follows:
BUG: kernel NULL pointer dereference, address: 0000000000000000
...
Call Trace:
<TASK>
ext4_block_zero_page_range+0x30c/0x380 [ext4]
ext4_truncate+0x436/0x440 [ext4]
ext4_process_orphan+0x5d/0x110 [ext4]
ext4_orphan_cleanup+0x124/0x4f0 [ext4]
ext4_fill_super+0x262d/0x3110 [ext4]
get_tree_bdev_flags+0x132/0x1d0
vfs_get_tree+0x26/0xd0
vfs_cmd_create+0x59/0xe0
__do_sys_fsconfig+0x4ed/0x6b0
do_syscall_64+0x82/0x170
...
This occurs when processing a symlink inode from the orphan list. The
partial block zeroing code in the truncate path calls
ext4_dirty_journalled_data() -> folio_mark_dirty(). The latter calls
mapping->a_ops->dirty_folio(), but symlink inodes are not assigned an
a_ops vector in ext4, hence the crash.
To avoid this problem, update the ext4_dirty_journalled_data() helper to
only mark the folio dirty on regular files (for which a_ops is
assigned). This also matches the journaling logic in the ext4_symlink()
creation path, where ext4_handle_dirty_metadata() is called directly.