CVE-2026-89666 in Linux
Summary
by MITRE • 09/12/2026
In the Linux kernel, the following vulnerability has been resolved:
nfsd: reject out-of-range nseconds in NFSv3 SETATTR and create ops
A client can send an NFSv3 SETATTR, CREATE, MKDIR, SYMLINK or MKNOD carrying an atime or mtime whose nseconds field is out of range. The value is well-formed on the wire and decodes cleanly into a valid uint32, but it is not a valid timespec64: tv_nsec must be less than NSEC_PER_SEC.
Nothing in the setattr path clamps it. notify_change() runs the time through timestamp_truncate(), which does not reduce tv_nsec below NSEC_PER_SEC when the filesystem supports nanosecond granularity (s_time_gran == 1), and the inode atime/mtime setters store it verbatim (only ctime is normalized, via inode_set_ctime_to_ts()). The un-normalized value then corrupts on-disk metadata: ext4's ext4_encode_extra_time() shifts tv_nsec left by EXT4_EPOCH_BITS, which overflows the 32-bit extra field and clobbers the seconds-epoch bits, so the stored seconds (and thus the year) are wrong on read-back. XFS with bigtime mis-stores the timestamp for the same reason.
Validate the client-supplied atime/mtime in the proc handlers and return NFS3ERR_INVAL before anything is changed. RFC 1813 lists NFS3ERR_INVAL for SETATTR and describes it as the error for a value the server 'can not store ... in its own representation'; the client maps it to EINVAL.
Checking in the proc handlers, rather than in nfsd_setattr(), keeps the rejection in front of object creation. The create operations create the object before nfsd_create_setattr() runs, so a late failure would leave the new object behind and turn a non-idempotent request into a namespace change that reports failure. The check is therefore done up front, for the create operations before the object is created.
tv_nsec is a long, so the comparison casts it to unsigned long (the same width) rather than to u32, matching timespec64_valid(). A u32 cast would truncate on 64-bit; the unsigned long cast also rejects a value that became negative when an out-of-range u32 wire nseconds was assigned to a 32-bit long.
Only client-supplied times are checked: SET_TO_SERVER_TIME requests carry no client value. The sattrguard3 ctime is deliberately left alone: an out-of-range guard simply never matches the object's ctime and yields NFS3ERR_NOT_SYNC via the existing guardtime comparison, which is the protocol-correct outcome rather than rejecting the request.
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Analysis
by VulDB Data Team • 09/12/2026
The Linux kernel has addressed a critical input validation vulnerability within the Network File System daemon (nfsd) component that affects NFSv3 operations including SETATTR, CREATE, MKDIR, SYMLINK, and MKNOD. This flaw allows remote clients to supply timestamp values where the nanosecond field exceeds the maximum valid value for a timespec64 structure. Specifically, while the incoming nseconds field is well-formed as an unsigned 32-bit integer on the network wire, it may exceed NSEC_PER_SEC, which represents one billion nanoseconds and constitutes the upper bound for valid time values in POSIX systems. The vulnerability arises because the NFS server implementation fails to clamp or validate this specific range before processing the request, allowing malformed timestamps to propagate through the kernel's timestamp handling subsystems without proper normalization.
The technical root cause lies in how the setattr path processes incoming time attributes. When a client submits an out-of-range nanosecond value, it decodes cleanly into a valid uint32 but fails validation against timespec64 standards where tv_nsec must be strictly less than NSEC_PER_SEC. The notify_change function invokes timestamp_truncate to normalize timestamps; however, this routine does not reduce tv_nsec below the maximum limit when the underlying filesystem supports nanosecond granularity via s_time_gran set to one. Consequently, the inode atime and mtime setters store these un-normalized values verbatim into the file system metadata structures. Although ctime is normalized through inode_set_ctime_to_ts(), the lack of normalization for access and modification times leads directly to data corruption on disk.
The operational impact of this vulnerability involves severe integrity issues with stored file metadata, particularly in filesystems like ext4 and XFS that utilize extended time fields. In ext4, the function ext4_encode_extra_time shifts tv_nsec left by EXT4_EPOCH_BITS bits. When tv_nsec exceeds valid bounds due to the missing validation, this shift operation causes an overflow of the 32-bit extra field. This overflow clobbers the seconds-epoch bits embedded within that same field, resulting in incorrect storage of the second component and consequently corrupting the year value upon read-back operations. Similarly, XFS with bigtime support mis-stores timestamps for identical reasons due to the lack of boundary checking prior to metadata encoding.
To mitigate this risk, the fix implements strict validation at the protocol handler level rather than relying solely on lower-level inode setters. The server now validates client-supplied atime and mtime values in the proc handlers before any state changes occur. If an out-of-range value is detected, the server returns NFS3ERR_INVAL, which aligns with RFC 1813 guidelines stating that this error code should be used for values a server cannot store in its own representation. Clients typically map this protocol-level error to EINVAL on their side. This approach ensures idempotency by rejecting invalid requests before object creation takes place during CREATE operations, preventing the scenario where a failed setattr leaves behind orphaned or corrupted directory entries.
The implementation details of the fix demonstrate careful attention to data type widths and edge cases. Since tv_nsec is defined as a long integer in the kernel structures, the validation logic casts it to unsigned long for comparison rather than u32. This prevents truncation issues on 64-bit architectures where casting directly from wire-format uint32 might behave unexpectedly if negative values were introduced via overflow during assignment. The check specifically targets client-supplied times and intentionally excludes SET_TO_SERVER_TIME requests which do not carry external input, as well as sattrguard3 ctime checks which are handled by existing guard time comparison logic that naturally rejects non-matching timestamps with NFS3ERR_NOT_SYNC.
From a security architecture perspective, this vulnerability maps to CWE-20 Improper Input Validation and CWE-190 Integer Overflow or Wraparound within the context of timestamp encoding. The exploitation vector aligns with MITRE ATT&CK technique T1496 Resource Hijacking if an attacker were to attempt denial-of-service through metadata corruption, though primarily it represents a data integrity risk rather than immediate code execution. System administrators should ensure that NFS servers are updated to include this patch to prevent remote clients from corrupting file system metadata on ext4 and XFS volumes. The fix ensures robust handling of edge-case timestamp values while maintaining protocol compliance with RFC 1813 standards for error reporting and state management during attribute modification operations.