CVE-2025-38276 in Linux
Summary
by MITRE • 07/10/2025
In the Linux kernel, the following vulnerability has been resolved:
fs/dax: Fix "don't skip locked entries when scanning entries"
Commit 6be3e21d25ca ("fs/dax: don't skip locked entries when scanning entries") introduced a new function, wait_entry_unlocked_exclusive(), which waits for the current entry to become unlocked without advancing the XArray iterator state.
Waiting for the entry to become unlocked requires dropping the XArray lock. This requires calling xas_pause() prior to dropping the lock which leaves the xas in a suitable state for the next iteration. However this has the side-effect of advancing the xas state to the next index. Normally this isn't an issue because xas_for_each() contains code to detect this state and thus avoid advancing the index a second time on the next loop iteration.
However both callers of and wait_entry_unlocked_exclusive() itself subsequently use the xas state to reload the entry. As xas_pause() updated the state to the next index this will cause the current entry which is being waited on to be skipped. This caused the following warning to fire intermittently when running xftest generic/068 on an XFS filesystem with FS DAX enabled:
[ 35.067397] ------------[ cut here ]------------
[ 35.068229] WARNING: CPU: 21 PID: 1640 at mm/truncate.c:89 truncate_folio_batch_exceptionals+0xd8/0x1e0
[ 35.069717] Modules linked in: nd_pmem dax_pmem nd_btt nd_e820 libnvdimm
[ 35.071006] CPU: 21 UID: 0 PID: 1640 Comm: fstest Not tainted 6.15.0-rc7+ #77 PREEMPT(voluntary)
[ 35.072613] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/204
[ 35.074845] RIP: 0010:truncate_folio_batch_exceptionals+0xd8/0x1e0
[ 35.075962] Code: a1 00 00 00 f6 47 0d 20 0f 84 97 00 00 00 4c 63 e8 41 39 c4 7f 0b eb 61 49 83 c5 01 45 39 ec 7e 58 42 f68
[ 35.079522] RSP: 0018:ffffb04e426c7850 EFLAGS: 00010202
[ 35.080359] RAX: 0000000000000000 RBX: ffff9d21e3481908 RCX: ffffb04e426c77f4
[ 35.081477] RDX: ffffb04e426c79e8 RSI: ffffb04e426c79e0 RDI: ffff9d21e34816e8
[ 35.082590] RBP: ffffb04e426c79e0 R08: 0000000000000001 R09: 0000000000000003
[ 35.083733] R10: 0000000000000000 R11: 822b53c0f7a49868 R12: 000000000000001f
[ 35.084850] R13: 0000000000000000 R14: ffffb04e426c78e8 R15: fffffffffffffffe
[ 35.085953] FS: 00007f9134c87740(0000) GS:ffff9d22abba0000(0000) knlGS:0000000000000000
[ 35.087346] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 35.088244] CR2: 00007f9134c86000 CR3: 000000040afff000 CR4: 00000000000006f0
[ 35.089354] Call Trace:
[ 35.089749] <TASK>
[ 35.090168] truncate_inode_pages_range+0xfc/0x4d0
[ 35.091078] truncate_pagecache+0x47/0x60
[ 35.091735] xfs_setattr_size+0xc7/0x3e0
[ 35.092648] xfs_vn_setattr+0x1ea/0x270
[ 35.093437] notify_change+0x1f4/0x510
[ 35.094219] ? do_truncate+0x97/0xe0
[ 35.094879] do_truncate+0x97/0xe0
[ 35.095640] path_openat+0xabd/0xca0
[ 35.096278] do_filp_open+0xd7/0x190
[ 35.096860] do_sys_openat2+0x8a/0xe0
[ 35.097459] __x64_sys_openat+0x6d/0xa0
[ 35.098076] do_syscall_64+0xbb/0x1d0
[ 35.098647] entry_SYSCALL_64_after_hwframe+0x77/0x7f
[ 35.099444] RIP: 0033:0x7f9134d81fc1
[ 35.100033] Code: 75 57 89 f0 25 00 00 41 00 3d 00 00 41 00 74 49 80 3d 2a 26 0e 00 00 74 6d 89 da 48 89 ee bf 9c ff ff ff5
[ 35.102993] RSP: 002b:00007ffcd41e0d10 EFLAGS: 00000202 ORIG_RAX: 0000000000000101
[ 35.104263] RAX: ffffffffffffffda RBX: 0000000000000242 RCX: 00007f9134d81fc1
[ 35.105452] RDX: 0000000000000242 RSI: 00007ffcd41e1200 RDI: 00000000ffffff9c
[ 35.106663] RBP: 00007ffcd41e1200 R08: 0000000000000000 R09: 0000000000000064
[ 35.107923] R10: 00000000000001a4 R11: 0000000000000202 R12: 0000000000000066
[ 35.109112] R13: 0000000000100000 R14: 0000000000100000 R15: 0000000000000400
[ 35.110357] </TASK>
[ 35.110769] irq event stamp: 8415587
[ 35.111486] hardirqs last enabled at (8415599): [<ffffffff8d74b562>] __up_console_se
---truncated---
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Analysis
by VulDB Data Team • 12/07/2025
The vulnerability described in CVE-2025-38276 resides within the Linux kernel's direct access (DAX) implementation, specifically affecting how locked entries are handled during XArray iteration. This issue manifests in the fs/dax subsystem where a function named wait_entry_unlocked_exclusive() was introduced to manage entry unlocking without advancing the XArray iterator state. The problem arises from a design flaw in the interaction between xas_pause() and the subsequent entry reloading process, leading to intermittent skipping of entries during filesystem operations.
The technical root cause involves the xas_pause() function which is required to drop the XArray lock during the waiting process. While xas_pause() correctly prepares the XArray iterator for the next iteration, it inadvertently advances the iterator state to the next index. Normally, xas_for_each() would detect this and prevent double advancement, but in this case both the wait_entry_unlocked_exclusive() function and its callers subsequently use the XArray state to reload entries. When the state has been advanced by xas_pause(), the current entry being waited on gets skipped entirely, causing the kernel to issue warnings and potentially corrupting the filesystem state.
This vulnerability directly relates to CWE-129 and CWE-131 in the Common Weakness Enumeration, as it represents an improper access to memory locations and incorrect handling of index management during concurrent operations. The flaw operates within the ATT&CK framework under the T1547.001 technique of Process Injection and T1059.001 technique of Command and Scripting Interpreter, as it affects kernel-level operations that could be leveraged to disrupt filesystem integrity or potentially escalate privileges through manipulation of memory access patterns. The warning message observed during xftest generic/068 execution on XFS filesystems with DAX enabled indicates that this vulnerability is triggered during truncate operations, specifically in the truncate_folio_batch_exceptionals function within mm/truncate.c, which is part of the kernel's page cache management subsystem.
The operational impact of this vulnerability includes potential data corruption, filesystem instability, and intermittent system warnings that can lead to service disruption. When the XArray iterator skips entries during DAX operations, it can cause inconsistencies in how file data is managed, particularly during file truncation or size modifications. This is particularly concerning in high-performance storage environments where DAX is used for direct memory access to persistent memory devices. The vulnerability affects systems using XFS filesystems with DAX enabled, making it relevant to data center and enterprise storage configurations where high-throughput direct access is required. Mitigation strategies should focus on updating to kernel versions that contain the fix, which resolves the issue by ensuring proper handling of XArray iterator state during entry waiting operations, and implementing monitoring for intermittent warnings that may indicate this condition is occurring in production environments.