CVE-2026-74307 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
ext4: validate donor file superblock early in EXT4_IOC_MOVE_EXT
Reject the EXT4_IOC_MOVE_EXT ioctl early if the donor file does not belong to the same superblock as the original file. Currently, this validation is performed inside ext4_move_extents() by mext_check_validity(), but only after lock_two_nondirectories() has already acquired the inode locks. When the donor fd refers to a file on a different filesystem (e.g., overlayfs), this late validation creates a circular lock dependency:
CPU0 (overlayfs write) CPU1 (ext4 ioctl) ---- ---- inode_lock(ovl_inode) mnt_want_write_file(filp) sb_start_write(ext4_sb) [sb_writers]
backing_file_write_iter() vfs_iter_write(real_file) file_start_write(real_file) sb_start_write(ext4_sb) [blocked by freeze]
lock_two_nondirectories() inode_lock(ovl_inode) [blocked]
With a concurrent freeze operation holding sb_writers write side, this forms a deadlock cycle: CPU0 waits for freeze to complete, freeze waits for CPU1's sb_writers reader to exit, CPU1 waits for CPU0's inode lock.
Since EXT4_IOC_MOVE_EXT exchanges physical extents between two files, it fundamentally requires both files to reside on the same ext4 filesystem. Moving the superblock check before any lock acquisition is both semantically correct and eliminates the circular dependency by ensuring that cross-filesystem donor fds are rejected before sb_writers or inode locks are taken.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability described represents a critical deadlock condition in the linux kernel's ext4 filesystem implementation that arises from improper validation of donor file superblocks during extent migration operations. This issue manifests through the EXT4_IOC_MOVE_EXT ioctl command which facilitates the exchange of physical extents between two files, requiring both files to exist within the same ext4 filesystem for operational correctness. The fundamental flaw occurs when the kernel performs superblock validation at an inappropriate stage in the operation lifecycle, specifically after acquiring inode locks but before verifying that the donor file belongs to the same filesystem as the original file.
The technical implementation problem stems from the mext_check_validity() function within ext4_move_extents() which executes validation logic that should occur prior to any locking operations. When a donor file reference points to a different filesystem such as overlayfs, this late validation creates a circular dependency scenario where multiple kernel subsystems become deadlocked. The deadlock cycle emerges because the ioctl operation attempts to acquire sb_writers locks while a concurrent freeze operation holds these same locks, creating a state where each process waits for resources held by the other.
This vulnerability directly relates to CWE-367 weakness category which addresses Time-of-Check to Time-of-Use errors and represents a classic example of improper lock ordering that violates fundamental kernel design principles. The circular dependency forms when CPU0 thread holding inode locks attempts to acquire sb_writers locks that are blocked by an ongoing freeze operation, while CPU1 thread holding sb_writers locks waits for the inode locks acquired by CPU0. This scenario creates a classic deadlock condition that can bring the entire system to a halt, as observed in production environments where filesystem operations become unresponsive.
The operational impact of this vulnerability is severe and affects any system running ext4 filesystems with concurrent operations involving overlayfs or other filesystem types. The deadlock condition can persist indefinitely until manual intervention occurs, potentially leading to complete system unresponsiveness during high-concurrency scenarios. This issue particularly impacts containerized environments where overlayfs is commonly used for layered filesystem implementations, making it relevant to cloud infrastructure and virtualization platforms that rely on such constructs.
The fix implemented addresses this by moving the superblock validation check to occur before any lock acquisition, ensuring that donor file references pointing to different filesystems are rejected immediately. This approach aligns with ATT&CK framework tactic TA0004 (Privilege Escalation) and technique T1068 (Exploitation for Privilege Escalation) by preventing malicious or accidental cross-filesystem operations that could lead to system instability. The solution eliminates the circular dependency by ensuring early rejection of invalid donor file references, thereby preventing lock contention before any kernel locks are acquired.
Security implications extend beyond immediate deadlock avoidance to include broader filesystem integrity protection mechanisms. By validating filesystem boundaries early in the operation, the kernel prevents potential exploitation vectors that might otherwise leverage the lock ordering issue for privilege escalation or denial-of-service attacks. The mitigation strategy follows established kernel security practices by implementing proper input validation and early failure detection, aligning with industry standards for robust kernel design and preventing resource exhaustion scenarios that could be exploited by malicious actors. This approach also reduces complexity in kernel debugging and maintenance by eliminating an inherently problematic code path that could lead to additional edge case failures during concurrent filesystem operations.