CVE-2026-64359

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

nilfs2: reject CLEAN_SEGMENTS ioctl with out-of-range segment numbers

Syzbot reported a hung task in nilfs_transaction_begin() where multiple

tasks performing chmod() on a nilfs2 mount blocked for over 143 seconds

waiting to acquire ns_segctor_sem for read:

INFO: task syz.0.17:5918 blocked for more than 143 seconds.

Call Trace:

schedule+0x164/0x360

rwsem_down_read_slowpath+0x6d9/0x940

down_read+0x99/0x2e0

nilfs_transaction_begin+0x364/0x710 fs/nilfs2/segment.c:221

nilfs_setattr+0x124/0x2c0 fs/nilfs2/inode.c:921

notify_change+0xc1a/0xf40

chmod_common+0x273/0x4a0

do_fchmodat+0x12d/0x230

The writer holding ns_segctor_sem was a concurrent

NILFS_IOCTL_CLEAN_SEGMENTS caller, stuck inside printk while emitting

per-element warnings from nilfs_sufile_updatev():

__nilfs_msg+0x373/0x450 fs/nilfs2/super.c:78

nilfs_sufile_updatev+0x21c/0x6d0 fs/nilfs2/sufile.c:186

nilfs_sufile_freev fs/nilfs2/sufile.h:93 [inline]

nilfs_free_segments fs/nilfs2/segment.c:1140 [inline]

nilfs_segctor_collect_blocks fs/nilfs2/segment.c:1261 [inline]

nilfs_segctor_do_construct+0x1f55/0x76c0

nilfs_clean_segments+0x3bd/0xa50

nilfs_ioctl_clean_segments fs/nilfs2/ioctl.c:922 [inline]

nilfs_ioctl+0x261f/0x2780

The root cause is that user-supplied segment numbers are not validated

before nilfs_clean_segments() begins doing work; the range check on

each segnum is performed deep inside the call chain by

nilfs_sufile_updatev(), which emits a nilfs_warn() per invalid entry

while still holding the segctor lock and the sufile mi_sem. Under load

(repeated invocations across multiple mounts saturating the global

printk path), the cumulative printk latency keeps ns_segctor_sem held

long enough to trip the hung_task watchdog, blocking concurrent

operations such as chmod() that need ns_segctor_sem for read.

Fix by validating the contents of kbufs[4] in nilfs_clean_segments()

immediately after acquiring ns_segctor_sem via nilfs_transaction_lock().

Holding ns_segctor_sem serializes the check against

nilfs_ioctl_resize(), which can modify ns_nsegments, so the validation

uses a consistent value. Out-of-range segment numbers are rejected

with -EINVAL before any segment-cleaning work begins, so the bad

entries never reach the per-element diagnostic path inside

nilfs_sufile_updatev().