On Mon, Feb 20, 2023 at 10:22 AM Borislav Petkov <bp@xxxxxxxxx> wrote: > > On Mon, Feb 20, 2023 at 10:01:57AM -0800, KP Singh wrote: > > Well, we disable IBRS userspace (this is KENREL_IBRS), because it is > > slow. Now if a user space process wants to protect itself from cross > > thread training, it should be able to do it, either by turning STIBP > > always on or using a prctl to enable. With the current logic, it's > > unable to do so. > > Ofcourse it can: > > [SPECTRE_V2_USER_PRCTL] = "User space: Mitigation: STIBP via prctl", > > we did this at the time so that a userspace process can control it via > prctl(). No it cannot with IBRS which is really just KERNEL_IBRS enabled, we bail out if spectre_v2_in_inbrs_mode and ignore any selections: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/cpu/bugs.c#n1202 The whole confusion spews from the fact that while the user thinks they are enabling spectre_v2=ibrs, they only really get KERNEL_IBRS and IBRS is dropped in userspace. This in itself seems like a decision the kernel implicitly took on behalf of the user. Now it also took their ability to enable spectre_v2_user in this case, which is what this patch is fixing. > > So, maybe you should explain what you're trying to accomplish in detail > and where it fails... > > -- > Regards/Gruss, > Boris. > > https://people.kernel.org/tglx/notes-about-netiquette
