CVE-2025-68261 in Linux
Summary
by MITRE • 12/16/2025
In the Linux kernel, the following vulnerability has been resolved:
ext4: add i_data_sem protection in ext4_destroy_inline_data_nolock()
Fix a race between inline data destruction and block mapping.
The function ext4_destroy_inline_data_nolock() changes the inode data layout by clearing EXT4_INODE_INLINE_DATA and setting EXT4_INODE_EXTENTS. At the same time, another thread may execute ext4_map_blocks(), which tests EXT4_INODE_EXTENTS to decide whether to call ext4_ext_map_blocks() or ext4_ind_map_blocks().
Without i_data_sem protection, ext4_ind_map_blocks() may receive inode with EXT4_INODE_EXTENTS flag and triggering assert.
kernel BUG at fs/ext4/indirect.c:546! EXT4-fs (loop2): unmounting filesystem. invalid opcode: 0000 [#1] PREEMPT SMP KASAN NOPTI
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014 RIP: 0010:ext4_ind_map_blocks.cold+0x2b/0x5a fs/ext4/indirect.c:546
Call Trace: <TASK> ext4_map_blocks+0xb9b/0x16f0 fs/ext4/inode.c:681 _ext4_get_block+0x242/0x590 fs/ext4/inode.c:822 ext4_block_write_begin+0x48b/0x12c0 fs/ext4/inode.c:1124 ext4_write_begin+0x598/0xef0 fs/ext4/inode.c:1255 ext4_da_write_begin+0x21e/0x9c0 fs/ext4/inode.c:3000 generic_perform_write+0x259/0x5d0 mm/filemap.c:3846 ext4_buffered_write_iter+0x15b/0x470 fs/ext4/file.c:285 ext4_file_write_iter+0x8e0/0x17f0 fs/ext4/file.c:679 call_write_iter include/linux/fs.h:2271 [inline]
do_iter_readv_writev+0x212/0x3c0 fs/read_write.c:735 do_iter_write+0x186/0x710 fs/read_write.c:861 vfs_iter_write+0x70/0xa0 fs/read_write.c:902 iter_file_splice_write+0x73b/0xc90 fs/splice.c:685 do_splice_from fs/splice.c:763 [inline]
direct_splice_actor+0x10f/0x170 fs/splice.c:950 splice_direct_to_actor+0x33a/0xa10 fs/splice.c:896 do_splice_direct+0x1a9/0x280 fs/splice.c:1002 do_sendfile+0xb13/0x12c0 fs/read_write.c:1255 __do_sys_sendfile64 fs/read_write.c:1323 [inline]
__se_sys_sendfile64 fs/read_write.c:1309 [inline]
__x64_sys_sendfile64+0x1cf/0x210 fs/read_write.c:1309 do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x35/0x80 arch/x86/entry/common.c:81 entry_SYSCALL_64_after_hwframe+0x6e/0xd8
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Analysis
by VulDB Data Team • 02/22/2026
The vulnerability CVE-2025-68261 represents a race condition within the Linux kernel's ext4 filesystem implementation that occurs during the transition from inline data storage to extent-based storage. This issue manifests in the function ext4_destroy_inline_data_nolock() which modifies inode data layout by clearing the EXT4_INODE_INLINE_DATA flag while simultaneously setting the EXT4_INODE_EXTENTS flag. The race condition arises because multiple threads can concurrently access the same inode structure without proper synchronization, leading to inconsistent state management during data layout transitions. The vulnerability is particularly concerning as it can trigger kernel panics and system crashes, effectively causing denial of service conditions that impact system stability and availability.
The technical flaw stems from the absence of proper locking mechanisms around the inode data structure modification operations. When ext4_destroy_inline_data_nolock() executes, it alters the inode's internal flags that determine how subsequent block mapping operations should proceed. Concurrently, another thread executing ext4_map_blocks() may observe the inode in an inconsistent state where the EXT4_INODE_EXTENTS flag has been set but the underlying data structures are not yet properly initialized. This inconsistency causes ext4_ind_map_blocks() to encounter an assertion failure at fs/ext4/indirect.c:546, resulting in a kernel BUG trap with an invalid opcode exception. The call trace demonstrates a typical execution flow involving file write operations that eventually lead to the problematic code path, indicating that this vulnerability can be triggered through normal file I/O operations.
The operational impact of CVE-2025-68261 extends beyond simple system instability to potentially compromise system integrity and availability. When triggered, the vulnerability results in kernel panics that require system reboot to recover, effectively creating a denial of service condition for the affected system. The vulnerability can be exploited through normal file operations such as writing to files, particularly when the filesystem is under concurrent load with multiple processes attempting to modify the same inode structure. This makes the vulnerability particularly dangerous in production environments where system stability and uptime are critical requirements. The issue affects systems running Linux kernels with ext4 filesystem implementations and could potentially impact a wide range of applications and services that rely on file I/O operations.
The mitigation strategy for CVE-2025-68261 involves implementing proper synchronization through the addition of i_data_sem protection around the inode modification operations in ext4_destroy_inline_data_nolock(). This approach aligns with the established security principle of using appropriate locking mechanisms to prevent race conditions in concurrent environments. The fix should ensure that when inline data destruction occurs, no other threads can simultaneously access the inode structure for block mapping operations until the transition is complete. Organizations should prioritize applying the kernel patches that implement this protection mechanism, particularly in production environments where the vulnerability could be exploited to cause system disruptions. Additionally, system administrators should monitor for any signs of kernel panics or unexpected system reboots that might indicate exploitation of this vulnerability, and maintain regular kernel update schedules to ensure protection against similar race condition issues.
This vulnerability is classified under CWE-362, which specifically addresses race conditions in concurrent programming environments, and aligns with ATT&CK technique T1499.001 for operating system binary exploitation. The issue demonstrates how improper synchronization in kernel-level code can lead to critical system instability and provides insight into the importance of proper locking mechanisms in concurrent systems. The fix implemented in the kernel represents a standard approach to addressing race conditions through appropriate mutex or semaphore usage, ensuring that critical sections of code execute atomically and preventing inconsistent state transitions that could lead to kernel crashes or data corruption.