FreeBSD VuXML: Documenting security issues in FreeBSD and the FreeBSD Ports Collection

FreeBSD -- NFS client data corruption and kernel memory disclosure

Affected packages
14.0 <= FreeBSD-kernel < 14.0_3
13.2 <= FreeBSD-kernel < 13.2_8


VuXML ID 8eefff69-997f-11ee-8e38-002590c1f29c
Discovery 2023-12-12
Entry 2023-12-13

Problem Description:

In FreeBSD 13.2 and 14.0, the NFS client was optimized to improve the performance of IO_APPEND writes, that is, writes which add data to the end of a file and so extend its size. This uncovered an old bug in some routines which copy userspace data into the kernel. The bug also affects the NFS client's implementation of direct I/O; however, this implementation is disabled by default by the vfs.nfs.nfs_directio_enable sysctl and is only used to handle synchronous writes.


When a program running on an affected system appends data to a file via an NFS client mount, the bug can cause the NFS client to fail to copy in the data to be written but proceed as though the copy operation had succeeded. This means that the data to be written is instead replaced with whatever data had been in the packet buffer previously. Thus, an unprivileged user with access to an affected system may abuse the bug to trigger disclosure of sensitive information. In particular, the leak is limited to data previously stored in mbufs, which are used for network transmission and reception, and for certain types of inter-process communication.

The bug can also be triggered unintentionally by system applications, in which case the data written by the application to an NFS mount may be corrupted. Corrupted data is written over the network to the NFS server, and thus also susceptible to being snooped by other hosts on the network.

Note that the bug exists only in the NFS client; the version and implementation of the server has no effect on whether a given system is affected by the problem.


CVE Name CVE-2023-6660
FreeBSD Advisory SA-23:18.nfsclient