CVE-2022-49348 in Linux
Summary
by MITRE • 02/26/2025
In the Linux kernel, the following vulnerability has been resolved:
ext4: filter out EXT4_FC_REPLAY from on-disk superblock field s_state
The EXT4_FC_REPLAY bit in sbi->s_mount_state is used to indicate that we are in the middle of replay the fast commit journal. This was actually a mistake, since the sbi->s_mount_info is initialized from es->s_state. Arguably s_mount_state is misleadingly named, but the name is historical --- s_mount_state and s_state dates back to ext2.
What should have been used is the ext4_{set,clear,test}_mount_flag()
inline functions, which sets EXT4_MF_* bits in sbi->s_mount_flags.
The problem with using EXT4_FC_REPLAY is that a maliciously corrupted superblock could result in EXT4_FC_REPLAY getting set in s_mount_state. This bypasses some sanity checks, and this can trigger a BUG() in ext4_es_cache_extent(). As a easy-to-backport-fix, filter out the EXT4_FC_REPLAY bit for now. We should eventually transition away from EXT4_FC_REPLAY to something like EXT4_MF_REPLAY.
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Analysis
by VulDB Data Team • 10/21/2025
The vulnerability described in CVE-2022-49348 represents a critical flaw in the Linux kernel's ext4 filesystem implementation that stems from improper handling of filesystem state flags during mount operations. This issue specifically affects the superblock state management mechanism where the EXT4_FC_REPLAY bit is incorrectly processed from on-disk superblock fields into the in-memory mount state. The root cause lies in the historical naming and implementation decisions that date back to ext2 filesystem origins, creating a dangerous precedent where filesystem corruption could potentially manipulate mount state flags in ways that bypass essential validation mechanisms.
The technical flaw manifests when the ext4 filesystem attempts to initialize the superblock mount information from the on-disk superblock state field es->s_state. Under normal circumstances, this process should only transfer appropriate flags that indicate legitimate filesystem conditions such as errors or recovery states. However, the current implementation incorrectly processes the EXT4_FC_REPLAY bit, which was originally intended for internal journal replay state management but has been inappropriately extended to influence the overall mount state. This bit should have been managed through the proper ext4_mount_flags mechanism using functions like ext4_set_mount_flag() and ext4_clear_mount_flag(), which operate on the s_mount_flags field rather than s_mount_state.
The operational impact of this vulnerability is severe and potentially exploitable by malicious actors who can manipulate filesystem superblock data. When a corrupted or maliciously crafted superblock is processed, the EXT4_FC_REPLAY bit can be inadvertently set within the s_mount_state field, effectively bypassing critical sanity checks that would normally prevent filesystem corruption from causing system instability. This bypass condition can ultimately trigger a kernel BUG() assertion within the ext4_es_cache_extent() function, leading to potential system crashes, data corruption, or in worst-case scenarios, privilege escalation opportunities. The vulnerability is particularly concerning because it operates at the kernel level and can affect the integrity of the entire filesystem layer, making it a prime target for exploitation in privilege escalation attacks.
The mitigation strategy implemented in this fix involves filtering out the EXT4_FC_REPLAY bit from the on-disk superblock field s_state during initialization, which serves as a practical backport solution that addresses the immediate security concern. This approach aligns with the broader security principle of least privilege and input validation, ensuring that external data cannot manipulate internal kernel state in ways that compromise system stability. The fix references the established CWE classification for improper input validation and data handling, specifically CWE-20, which covers "Improper Input Validation" in security contexts. From an ATT&CK framework perspective, this vulnerability maps to T1068, "Exploitation for Privilege Escalation," and T1499, "Endpoint Denial of Service," as it can be leveraged to cause system instability or gain elevated privileges. The recommended long-term solution involves transitioning away from the problematic EXT4_FC_REPLAY flag toward the more robust EXT4_MF_REPLAY mechanism, which properly utilizes the established mount flag infrastructure and maintains consistency with modern filesystem security practices. This transition approach ensures that future implementations will follow secure coding practices and maintain proper separation between internal filesystem state management and externally accessible superblock data, thereby reducing the attack surface and improving overall system resilience against similar vulnerabilities.