Re: [PATCH RESEND] x86/speculation: Fix user-mode spectre-v2 protection with KERNEL_IBRS

KP Singh
2023-02-20T18:44:41Z

On Mon, Feb 20, 2023 at 10:22 AM Borislav Petkov <bp@alien8.de> 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