CVE-2026-90103 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
NFSv4.2: fix LAYOUTSTATS send buffer exhaustion
encode_layoutstats_maxsz budgets XDR_QUADLEN(PNFS_LAYOUTSTATS_MAXSIZE), i.e. 256 bytes, for the layoutupdate4 body written by the layout driver. The flexfiles record can exceed that.
ff_layout_encode_ff_layoutupdate() emits, per data server, a netaddr4, an nfs_fh4, two ff_io_latency4, an nfstime4 and a bool. A data server whose filehandle is NFS_MAXFHSIZE bytes long already accounts for 132 of those bytes, and the two ff_io_latency4 at 64 bytes each, the nfstime4 and the bool add a further 144, so the body passes 256 bytes before the netaddr4 is encoded at all. encode_layoutstats() additionally writes the deviceid4 and the layoutupdate4 lou_type word, neither of which the macro accounts for.
The filehandle and the address are both chosen by the server, through LAYOUTGET and GETDEVICEINFO, so it can drive the encoder past the end of the send buffer. xdr_reserve_space() returns NULL once that happens, and the two ff_layout_encode_io_latency() calls run with dss_info->mirror->lock held, so a NULL return there leaves the lock permanently held.
Raise PNFS_LAYOUTSTATS_MAXSIZE to 384 so that the record fits inside the reservation.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel contains a buffer exhaustion vulnerability within the NFSv4.2 client implementation specifically affecting the LAYOUTSTATS operation in the flexfiles layout driver. This issue stems from an incorrect budgeting of space for the XDR encoded data structure used to transmit layout statistics back to the server. The function encode_layoutstats_maxsz incorrectly allocates only 256 bytes, calculated as XDR_QUADLEN(PNFS_LAYOUTSTATS_MAXSIZE), for the body of the layoutupdate4 message generated by the layout driver. This allocation is insufficient because certain data servers can produce records that exceed this limit due to variable-length fields such as file handles and network addresses which are determined dynamically during operation rather than being fixed size constants known at compile time.
The technical flaw arises from a miscalculation of the maximum possible size of the ff_layout_encode_ff_layoutupdate structure. For each data server involved, the encoder must transmit several components including a netaddr4, an nfs_fh4, two ff_io_latency4 structures, one nfstime4, and a boolean flag. The file handle field alone can consume up to 132 bytes when using NFS_MAXFHSIZE, while the latency structures contribute another 144 bytes combined with the time stamp and boolean flags. This totals 276 bytes before even accounting for the network address encoding or other overhead fields like deviceid4 and layoutupdate type identifiers that are appended by encode_layoutstats but not included in the initial size reservation macro. Consequently, when a server provides large file handles or addresses, the encoder attempts to write beyond the reserved buffer space.
The operational impact of this vulnerability is severe as it leads to send buffer exhaustion within the kernel networking stack. When xdr_reserve_space detects that there is insufficient room for the required data, it returns NULL instead of providing a valid pointer. Because the encoding process occurs while holding dss_info->mirror->lock, and specifically during calls to ff_layout_encode_io_latency which are protected by this mutex, returning NULL leaves the lock permanently held without being released. This results in a local denial of service condition where subsequent operations attempting to acquire the same lock will hang indefinitely, effectively freezing the NFS client's interaction with that specific data server or potentially impacting broader system stability depending on how the kernel handles such hangs in I/O paths.
To mitigate this vulnerability and restore normal operation, the PNFS_LAYOUTSTATS_MAXSIZE constant must be increased from its current value to 384 bytes. This adjustment ensures that the reserved buffer space is sufficient to accommodate the maximum possible size of the layout update record including all variable-length fields such as large file handles and network addresses. By aligning the allocated memory with the actual worst-case scenario for data transmission, the encoder will no longer exceed the send buffer limits, thereby preventing xdr_reserve_space from failing and avoiding the subsequent lock retention issue that causes system hangs. This fix ensures robust handling of diverse server configurations without compromising security or stability through resource exhaustion attacks facilitated by legitimate but large metadata responses.