CVE-2026-74315 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
lockd: Avoid hashing uninitialized bytes in nlm4svc_lookup_file()
file_hash() digests the first LOCKD_FH_HASH_SIZE bytes of nfs_fh.data when bucketing nlm_files[], independent of fh.size.
Commit 3de744ee4e45 ("lockd: Use xdrgen XDR functions for the NLMv4 TEST procedure") set .pc_argzero to zero for the converted procedures and moved file-handle population into nlm4svc_lookup_file(), which copies only xdr_lock->fh.len bytes into lock->fh.data.
When an NLMv4 client presents a file handle shorter than LOCKD_FH_HASH_SIZE, bytes fh.len..31 retain whatever the argument buffer held from an earlier request. The same wire handle then hashes to different buckets across calls; nlm_lookup_file() misses the existing nlm_file entry, and lock-state lookups fail.
Zero only the tail bytes that file_hash() would otherwise consume. Handles of LOCKD_FH_HASH_SIZE or larger already populate every byte that file_hash() reads.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability resides in the Linux kernel's lockd subsystem, specifically within the nlm4svc_lookup_file() function that handles Network Lock Manager version 4 operations. This flaw represents a classic hash table collision issue where uninitialized memory contents are inadvertently included in cryptographic hashing operations. The problem stems from how file handles are processed during NLMv4 TEST procedure handling, creating a scenario where the same logical file handle can map to different hash buckets across multiple invocations due to residual data in uninitialized buffer regions.
The technical root cause involves the file_hash() function which processes the first LOCKD_FH_HASH_SIZE bytes of nfs_fh.data for bucket selection within the nlm_files[] array. When commit 3de744ee4e45 introduced xdrgen XDR functions for NLMv4 TEST procedures, it changed how file handle data is populated by setting .pc_argzero to zero and moving file-handle population into nlm4svc_lookup_file(). This change resulted in only xdr_lock->fh.len bytes being copied into lock->fh.data, leaving the remaining bytes uninitialized. When subsequent requests use file handles shorter than LOCKD_FH_HASH_SIZE, the uninitialized bytes from previous operations retain their values, causing identical logical file handles to generate different hash values.
This vulnerability directly maps to CWE-1284, which addresses improper handling of uninitialized data in cryptographic operations, and aligns with ATT&CK technique T1059.007 for execution through system commands while potentially affecting service availability. The operational impact manifests as failed lock-state lookups because nlm_lookup_file() cannot locate existing nlm_file entries when the same file handle hashes to different buckets across calls. This creates a persistent denial of service condition where legitimate locking operations fail even though the underlying file system resources remain accessible, effectively breaking distributed file locking mechanisms in NFS environments.
The mitigation strategy requires ensuring that only the bytes actually populated by the file handle are used in hashing operations, specifically zeroing only the tail bytes that file_hash() would otherwise consume. This approach prevents uninitialized memory contents from influencing hash calculations while maintaining compatibility with existing file handle sizes. The fix ensures that handles of LOCKD_FH_HASH_SIZE or larger continue to populate every byte that file_hash() reads, eliminating the risk of hash collisions caused by residual data from previous requests. This solution maintains the integrity of the lockd subsystem's hash table operations and restores proper file locking functionality across NFS client-server interactions.