CVE-2026-64359

Summary

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().

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/3ed388ec3b8922383d1e2d4432d7bd4cbbf8364e
• https://git.kernel.org/stable/c/876c98e0fc65f071680c03c2e2ee3ef7ff9ca078
• https://git.kernel.org/stable/c/39607452b1400c7bf748f15122df4d058b768c5b
• https://git.kernel.org/stable/c/286f77d002a337735c0846d7480a82d9cda2aa31
• https://git.kernel.org/stable/c/0789f0a6710713254a08f3a7d2ecbb6d1cbcf0aa
• https://git.kernel.org/stable/c/223463c488b0554212a94de971ea538eb2805fc7
• https://git.kernel.org/stable/c/d26aef771b4f6923da9f89d6d5b70d8def5853de
• https://git.kernel.org/stable/c/0e7a690fe435f8d5ea3feb7c1d8d73ba7e8b8aa9

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