CVE-2023-54021 in Linux
Summary
by MITRE • 12/24/2025
In the Linux kernel, the following vulnerability has been resolved:
ext4: set goal start correctly in ext4_mb_normalize_request
We need to set ac_g_ex to notify the goal start used in ext4_mb_find_by_goal. Set ac_g_ex instead of ac_f_ex in ext4_mb_normalize_request. Besides we should assure goal start is in range [first_data_block,
blocks_count) as ext4_mb_initialize_context does.
[ Added a check to make sure size is less than ar->pright; otherwise
we could end up passing an underflowed value of ar->pright - size to ext4_get_group_no_and_offset(), which will trigger a BUG_ON later on. - TYT ]
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Analysis
by VulDB Data Team • 12/26/2025
The vulnerability CVE-2023-54021 represents a critical flaw in the Linux kernel's ext4 filesystem implementation that affects memory allocation and block management during filesystem operations. This issue specifically resides in the ext4_mb_normalize_request function where improper handling of goal start parameters can lead to significant system instability and potential security implications. The vulnerability stems from a fundamental misconfiguration in how the kernel manages ext4 filesystem allocation contexts, particularly in the relationship between goal start values and the underlying block allocation mechanisms. The flaw manifests when the kernel attempts to normalize memory allocation requests for ext4 filesystems, creating conditions where allocation parameters become inconsistent with expected filesystem boundaries.
The technical root cause of this vulnerability involves the incorrect assignment of goal start parameters within the ext4 memory allocation subsystem. Specifically, the code mistakenly assigns the goal start value to ac_f_ex instead of the correct ac_g_ex parameter that should be used for notifying goal start information to ext4_mb_find_by_goal function. This misassignment creates a cascade of issues where the filesystem's block allocation logic operates with incorrect contextual information, potentially leading to allocation failures or memory corruption. The flaw also includes inadequate validation of parameter ranges, particularly concerning the relationship between requested allocation sizes and available block ranges within the filesystem. The absence of proper bounds checking means that when size parameters exceed ar->pright boundaries, underflow conditions can occur that propagate to critical kernel functions like ext4_get_group_no_and_offset(), triggering BUG_ON assertions that can cause system crashes or unpredictable behavior.
The operational impact of CVE-2023-54021 extends beyond simple filesystem corruption to potentially enable privilege escalation scenarios and denial-of-service conditions within Linux systems. When exploited, this vulnerability can cause the kernel to enter unstable states during ext4 filesystem operations, particularly affecting applications that rely heavily on dynamic block allocation. The flaw is particularly concerning in environments where ext4 filesystems are heavily utilized for storage operations, database workloads, or virtualization platforms that depend on robust filesystem allocation mechanisms. Systems running affected kernel versions may experience spontaneous crashes, data corruption, or complete system hangs during normal filesystem operations. From a cybersecurity perspective, this vulnerability aligns with CWE-129 Input Validation and CWE-125 Out-of-bounds Read categories, as it involves improper parameter validation and boundary checking within kernel memory management subsystems. The vulnerability can be leveraged by attackers to create persistent denial-of-service conditions or potentially gain elevated privileges through carefully crafted filesystem operations that exploit the memory allocation inconsistencies.
Mitigation strategies for CVE-2023-54021 focus primarily on kernel updates and patches that correct the parameter assignment logic and implement proper range validation for goal start values. System administrators should prioritize applying the official kernel patches released by Linux kernel maintainers, as these updates specifically address the incorrect assignment of ac_g_ex versus ac_f_ex parameters and introduce the necessary bounds checking for allocation size validation. Additional protective measures include monitoring filesystem operations for unusual allocation patterns that might indicate exploitation attempts, implementing proper filesystem quotas to limit allocation scope, and maintaining regular system updates to prevent exploitation of similar vulnerabilities. The fix addresses the issue through direct code corrections that ensure proper parameter handling within the ext4 memory allocation subsystem, aligning with ATT&CK techniques related to privilege escalation and system stability compromise. Organizations should also consider implementing filesystem-level monitoring tools that can detect anomalous allocation patterns and provide early warning of potential exploitation attempts, as the vulnerability's impact is most pronounced during active filesystem operations and memory allocation activities.