Description
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().
Published: 2026-07-25
Score: n/a
EPSS: < 1% Very Low
KEV: No
Impact: n/a
Action: n/a
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History

Sat, 25 Jul 2026 09:30:00 +0000

Type Values Removed Values Added
Description 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().
Title nilfs2: reject CLEAN_SEGMENTS ioctl with out-of-range segment numbers
First Time appeared Linux
Linux linux Kernel
CPEs cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Vendors & Products Linux
Linux linux Kernel
References

Subscriptions

Linux Linux Kernel
cve-icon MITRE

Status: PUBLISHED

Assigner: Linux

Published:

Updated: 2026-07-25T08:50:15.947Z

Reserved: 2026-07-19T15:36:31.782Z

Link: CVE-2026-64359

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