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Vigil@nce: FreeBSD, privilege elevation on amd64

September 2008 by Vigil@nce

A local attacker can obtain root privileges on a FreeBSD/amd64 system.

- Gravity: 2/4
- Consequences: administrator access/rights
- Provenance: user shell
- Means of attack: no proof of concept, no attack
- Ability of attacker: expert (4/4)
- Confidence: confirmed by the editor (5/5)
- Diffusion of the vulnerable configuration: high (3/3)
- Creation date: 04/09/2008
- Identifier: VIGILANCE-VUL-8087


- FreeBSD [confidential versions]


The FreeBSD/amd64 system uses the GS segment to store:
- in user mode : information on threads
- in kernel mode : information on processes

The SwapGS assembler instruction swaps GS and the "KernelGSbase" MSR (Model Specific Register). This instruction is called for each mode change.

However, if a GPF (General Protection Fault) occurs when returning from an interrupt, trap or system call, SwapGS is called twice. Kernel GS data can thus be altered by the user.

A local attacker can therefore execute code with kernel privileges.


- Identifiers: BID-31003, CVE-2008-3890, FreeBSD-SA-08:07.amd64, VIGILANCE-VUL-8087
- Url:

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