CVE-2025-71090 in Linux
Summary
by MITRE • 01/13/2026
In the Linux kernel, the following vulnerability has been resolved:
nfsd: fix nfsd_file reference leak in nfsd4_add_rdaccess_to_wrdeleg()
nfsd4_add_rdaccess_to_wrdeleg() unconditionally overwrites fp->fi_fds[O_RDONLY] with a newly acquired nfsd_file. However, if
the client already has a SHARE_ACCESS_READ open from a previous OPEN operation, this action overwrites the existing pointer without releasing its reference, orphaning the previous reference.
Additionally, the function originally stored the same nfsd_file pointer in both fp->fi_fds[O_RDONLY] and fp->fi_rdeleg_file with
only a single reference. When put_deleg_file() runs, it clears fi_rdeleg_file and calls nfs4_file_put_access() to release the file.
However, nfs4_file_put_access() only releases fi_fds[O_RDONLY] when
the fi_access[O_RDONLY] counter drops to zero. If another READ open
exists on the file, the counter remains elevated and the nfsd_file reference from the delegation is never released. This potentially causes open conflicts on that file.
Then, on server shutdown, these leaks cause __nfsd_file_cache_purge() to encounter files with an elevated reference count that cannot be cleaned up, ultimately triggering a BUG() in kmem_cache_destroy() because there are still nfsd_file objects allocated in that cache.
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Analysis
by VulDB Data Team • 05/21/2026
The vulnerability described in CVE-2025-71090 represents a critical reference counting flaw within the Linux kernel's Network File System daemon implementation. This issue specifically affects the nfsd4_add_rdaccess_to_wrdeleg() function which handles the management of file access delegations in NFSv4. The flaw manifests as a memory leak through improper reference handling, creating persistent resource leaks that can escalate to system instability. The vulnerability operates at the kernel level where the nfsd subsystem manages file access for network clients, making it particularly dangerous as it affects the core file system operations of the NFS service.
The technical implementation flaw stems from the function's unconditional overwriting of file descriptor pointers without proper reference release mechanisms. When a client already holds an existing read share access from a previous OPEN operation, the function simply overwrites the fp->fi_fds[O_RDONLY] pointer with a new nfsd_file reference while leaving the previous reference unreleased. This creates an orphaned reference that persists in memory, effectively leaking the associated file resource. The issue is compounded by the fact that the same nfsd_file pointer is stored in both fp->fi_fds[O_RDONLY] and fp->fi_rdeleg_file, but only a single reference is maintained for both locations. This dual storage pattern with single reference management creates a scenario where the reference counting logic fails to properly account for all access paths to the same underlying file resource.
The operational impact of this vulnerability extends beyond simple memory leaks to potentially cause file access conflicts and system crashes. During normal operation, the elevated reference counts prevent proper cleanup of file resources when clients close their access, leading to persistent file locks that can cause open conflicts. The most severe consequence occurs during server shutdown when the __nfsd_file_cache_purge() function attempts to clean up all allocated nfsd_file objects. Due to the elevated reference counts from leaked references, the cleanup process cannot properly release all objects, ultimately triggering a kernel BUG() in kmem_cache_destroy() which results in system panic and potential denial of service. This vulnerability directly relates to CWE-401 which describes improper resource management and can be categorized under ATT&CK technique T1489 which involves data destruction through system resource exhaustion or corruption.
Mitigation strategies should focus on implementing proper reference counting mechanisms within the nfsd subsystem to ensure that all file references are properly released when overwritten or when delegations are removed. The fix requires modifying the nfsd4_add_rdaccess_to_wrdeleg() function to properly decrement reference counts before overwriting pointers and ensuring that nfs4_file_put_access() correctly handles all access paths to file resources. System administrators should monitor NFS service stability and implement proper resource monitoring to detect potential reference leak accumulation. Additionally, kernel updates should be applied immediately to address this vulnerability, as the memory leak can accumulate over time and eventually lead to system crashes. Organizations should also consider implementing automated monitoring for NFS service resource usage and file descriptor counts to detect anomalous behavior that might indicate reference leak accumulation before it reaches critical levels.