Hi,
Letting processes set their own GS.base would expose the classic SWAPGS speculative side-channel class of bugs (CVE-2019-1125 and friends). Keeping GS under exclusive kernel control removes that attack surface and keeps the entry/exit paths simple. Here is a Wikipedia article. <https://en.wikipedia.org/wiki/Spectre_(security_vulnerability)>
I'm not an expert, I'm still reading the man pages, but I do know there is no plan to add a syscall for it.
Thanks,
Story


On September 29, 2026 5:32:51 AM EDT, Neth Iafin <NethIafin-obsd@proton.me> wrote:
>Hi OpenBSD,
>
>I was wondering why OpenBSD has no ability to set %gs in userland?
>
>Is this to avoid swapgs vulnerabilities? Something else?
>I understand that supporting wine is probably below 0 on the priority list for OpenBSD, but I wonder about the reasoning behind %gs being locked to kernel.
>I assume there is no chance that setting %gs will ever come back as a syscall?
>
>Thanks,
>Neth