FreeBSD Security Advisory - The FreeBSD operating system implements a rings model of security, where privileged operations are done in the kernel, and most applications request access to these operations by making a system call, which puts the CPU into the required privilege level and passes control to the kernel. FreeBSD/amd64 runs on CPUs from different vendors. Due to varying behaviour of CPUs in 64 bit mode a sanity check of the kernel may be insufficient when returning from a system call. Successful exploitation of the problem can lead to local kernel privilege escalation, kernel data corruption and/or crash.
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