The FreeBSD Security Team have identified a little bug in FreeBSD where a lost mbuf flag can result in data loss.
An mbuf is a basic unit of memory management in the FreeBSD kernel inter-process communication and networking subsystem. Network packets and socket buffers are dependent on mbufs for their storage.
Data can be embedded directly in mbufs, or mbufs can instead reference external buffers. The sendfile(2) system call uses external mbuf storage to directly map the contents of a file into a chain of mbufs for
transmission purposes. The mbuf object supports a read-only flag that must be honored to prevent modification or writes to buffer data in cases like these.
II. Problem Description
The read-only flag is not correctly copied when a mbuf buffer reference is duplicated. When the sendfile(2) system call is used to transmit data over the loopback interface, this can result in the backing pages
for the transmitted file being modified, causing data corruption.
This data corruption can be exploited by an local attacker to escalate their privilege by carefully controlling the corruption of system files. It should be noted that the attacker can corrupt any file they have read
For a workaround and steps to fix this, have a look at the announcement