CVE-2025-38691 in Linux
Summary
by MITRE • 09/04/2025
In the Linux kernel, the following vulnerability has been resolved:
pNFS: Fix uninited ptr deref in block/scsi layout
The error occurs on the third attempt to encode extents. When function ext_tree_prepare_commit() reallocates a larger buffer to retry encoding extents, the "layoutupdate_pages" page array is initialized only after the retry loop. But ext_tree_free_commitdata() is called on every iteration and tries to put pages in the array, thus dereferencing uninitialized pointers.
An additional problem is that there is no limit on the maximum possible buffer_size. When there are too many extents, the client may create a layoutcommit that is larger than the maximum possible RPC size accepted by the server.
During testing, we observed two typical scenarios. First, one memory page for extents is enough when we work with small files, append data to the end of the file, or preallocate extents before writing. But when we fill a new large file without preallocating, the number of extents can be huge, and counting the number of written extents in ext_tree_encode_commit() does not help much. Since this number increases even more between unlocking and locking of ext_tree, the reallocated buffer may not be large enough again and again.
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Analysis
by VulDB Data Team • 02/10/2026
The vulnerability CVE-2025-38691 represents a critical uninitialized pointer dereference in the Linux kernel's pNFS implementation, specifically within the block/scsi layout handling subsystem. This flaw exists in the ext_tree_prepare_commit() function where a buffer reallocation occurs during the third attempt to encode extents. The root cause stems from the improper initialization sequence of the layoutupdate_pages page array, which remains uninitialized when ext_tree_free_commitdata() is invoked during each iteration of the retry loop. This creates a scenario where the kernel attempts to dereference uninitialized memory pointers, potentially leading to system instability or arbitrary code execution.
The technical implementation of this vulnerability demonstrates a classic race condition and memory management error within the pNFS subsystem. When the system encounters large files without preallocation, the number of extents required for encoding can grow exponentially, causing the buffer reallocation process to fail repeatedly. The ext_tree_free_commitdata() function attempts to manage page references in the layoutupdate_pages array before it has been properly initialized, creating a dangerous state where kernel memory is accessed without proper validation. This issue is particularly severe because pNFS (parallel Network File System) is designed for high-performance storage environments where such memory corruption can lead to complete system compromise.
The operational impact of this vulnerability extends beyond simple memory corruption to potentially enable privilege escalation and system denial of service attacks. According to CWE-476, this represents an uninitialized pointer dereference that can be exploited by malicious actors to gain elevated privileges or crash the kernel. The attack surface is particularly concerning in enterprise storage environments where pNFS is commonly deployed, as it can be triggered through normal file operations on large files that require extensive extent tracking. The lack of buffer size limiting mechanisms means that attackers could potentially craft malicious file operations that generate layoutcommit messages exceeding RPC size limits, causing server-side rejection or resource exhaustion.
Security mitigations for this vulnerability should focus on implementing proper initialization sequences in the pNFS commit processing code, establishing reasonable limits on buffer growth, and adding comprehensive validation checks before pointer dereference operations. The fix must ensure that layoutupdate_pages array is properly initialized before any operations attempt to access it, and that buffer reallocation logic accounts for potential growth factors during the retry loop. Additionally, implementing maximum buffer size constraints prevents the generation of excessively large layoutcommit messages that could overwhelm server resources. Organizations should prioritize patching this vulnerability as it represents a direct threat to kernel stability and system security, particularly in environments where pNFS is actively used for storage operations. The ATT&CK framework categorizes this as a kernel-level privilege escalation technique, making it a critical target for immediate remediation.