CVE-2025-38579 in Linux
Summary
by MITRE • 08/19/2025
In the Linux kernel, the following vulnerability has been resolved:
f2fs: fix KMSAN uninit-value in extent_info usage
KMSAN reported a use of uninitialized value in `__is_extent_mergeable()` and `__is_back_mergeable()` via the read extent tree path.
The root cause is that `get_read_extent_info()` only initializes three fields (`fofs`, `blk`, `len`) of `struct extent_info`, leaving the remaining fields uninitialized. This leads to undefined behavior when those fields are accessed later, especially during extent merging.
Fix it by zero-initializing the `extent_info` struct before population.
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Analysis
by VulDB Data Team • 01/09/2026
The vulnerability CVE-2025-38579 represents a critical uninitialized memory access issue within the Linux kernel's f2fs filesystem implementation that was identified through KMSAN static analysis. This flaw exists in the extent management subsystem where the `get_read_extent_info()` function fails to properly initialize all fields of the `struct extent_info` structure before populating it with data. The incomplete initialization affects the `__is_extent_mergeable()` and `__is_back_mergeable()` functions which operate on the read extent tree path, creating a scenario where undefined behavior occurs when accessing uninitialized memory regions.
The technical root cause stems from the incomplete initialization of the `extent_info` structure in the `get_read_extent_info()` function which only sets three out of potentially multiple fields including `fofs`, `blk`, and `len` while leaving other fields uninitialized. This partial initialization creates a dangerous state where subsequent operations in the extent merging logic may inadvertently access garbage values stored in the uninitialized memory locations. The vulnerability manifests during extent merging operations when the kernel attempts to evaluate mergeability conditions based on potentially corrupted or undefined data from the uninitialized fields, leading to unpredictable behavior that could compromise system stability or security.
The operational impact of this vulnerability extends beyond simple system instability into potential security implications within the kernel's memory management subsystem. When the f2fs filesystem processes read operations involving extent trees, the uninitialized memory values can cause incorrect merge decisions, potentially leading to data corruption, memory access violations, or even privilege escalation opportunities. The undefined behavior introduced by accessing uninitialized memory fields creates a vector for exploitation that could be leveraged by malicious actors to disrupt normal system operations or potentially gain elevated privileges within the kernel space.
This vulnerability aligns with CWE-457: Use of Uninitialized Variable, which specifically addresses the risks associated with accessing variables that have not been properly initialized. The flaw also demonstrates characteristics consistent with ATT&CK technique T1068: Exploitation for Privilege Escalation, as uninitialized memory access can create conditions that enable attackers to manipulate kernel data structures. The fix implemented addresses the core issue by ensuring complete zero-initialization of the `extent_info` structure before population, which prevents access to undefined memory values and maintains the integrity of the extent merging logic.
The resolution for CVE-2025-38579 involves a straightforward but critical code modification that ensures the `extent_info` structure is properly initialized before being populated with data from `get_read_extent_info()`. This fix prevents the use of uninitialized memory values during extent merging operations and eliminates the potential for undefined behavior that could lead to system crashes or security vulnerabilities. The solution follows best practices for kernel memory management and aligns with security guidelines that emphasize proper initialization of data structures to prevent memory safety issues. The implementation requires minimal code changes while providing comprehensive protection against the uninitialized memory access pattern that was previously exploited in the f2fs filesystem implementation.