CVE-2026-63818 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
f2fs: validate orphan inode entry count
f2fs_recover_orphan_inodes() trusts the orphan block entry_count when replaying orphan inodes from the checkpoint pack. A corrupted entry_count larger than F2FS_ORPHANS_PER_BLOCK makes the recovery loop read past the ino[] array and interpret footer or following data as inode numbers.
On a crafted image, mounting an unpatched kernel can drive orphan recovery into f2fs_bug_on() and panic the kernel. Validate entry_count before consuming entries so corrupted checkpoint data fails the mount with -EFSCORRUPTED and requests fsck instead.
Set ERROR_INCONSISTENT_ORPHAN as well, so the corruption reason can be recorded in the superblock s_errors[] field. This gives fsck a persistent
hint even though mount-time orphan recovery failure may leave no chance to persist SBI_NEED_FSCK through a checkpoint.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability described represents a critical buffer overflow condition within the f2fs filesystem implementation of the linux kernel, specifically affecting the orphan inode recovery mechanism during filesystem mounting operations. This flaw exists in the f2fs_recover_orphan_inodes() function which processes orphan block data from checkpoint information. The vulnerability stems from insufficient validation of the entry_count field within the orphan block structure, creating a scenario where maliciously crafted filesystem images can trigger undefined behavior through memory corruption.
The technical implementation flaw manifests when the kernel trusts unvalidated metadata from the checkpoint pack without proper bounds checking against the F2FS_ORPHANS_PER_BLOCK constant. This allows an attacker to manipulate the entry_count field to values exceeding the legitimate array boundaries, causing the recovery loop to read beyond the allocated ino[] array space. The overflow extends into subsequent data structures including footer information or following data sections, effectively interpreting arbitrary memory contents as inode numbers. This type of vulnerability falls under CWE-129 and CWE-787 categories, representing insufficient validation of input boundaries and out-of-bounds reads respectively.
The operational impact of this vulnerability is severe, as it can lead to complete kernel panics and system crashes when mounting affected filesystem images. The f2fs_bug_on() function call triggers during the recovery process, causing immediate system termination and potentially resulting in data loss or system downtime. This vulnerability directly maps to attack techniques outlined in the ATT&CK framework under T1490 (Inhibit System Recovery) and T1059 (Command and Scripting Interpreter), as it can be exploited to disrupt system availability through filesystem corruption attacks.
The mitigation strategy implemented in the patch involves comprehensive validation of entry_count values before any processing occurs, ensuring that the recovery loop operates within defined memory boundaries. This approach prevents the exploitation of corrupted checkpoint data by immediately failing the mount operation with -EFSCORRUPTED error code instead of allowing the kernel to proceed with potentially dangerous memory access patterns. Additionally, setting the ERROR_INCONSISTENT_ORPHAN flag enables persistent corruption tracking within the superblock's s_errors[] field, providing fsck utilities with reliable indicators of filesystem inconsistency even when runtime recovery failures prevent normal checkpoint persistence mechanisms from functioning.
The fix addresses fundamental security principles by implementing proper input validation and defensive programming practices that align with secure coding guidelines. The solution prevents information disclosure through controlled error handling rather than allowing corrupted data to propagate through the system, thereby maintaining system integrity and preventing potential escalation to privilege escalation or denial of service conditions. This vulnerability demonstrates the critical importance of validating filesystem metadata and implementing robust boundary checking mechanisms in kernel-level code that directly handles user-supplied data structures.