[PATCH] arm64: Avoid enabling KPTI unnecessarily

[Ard Biesheuvel]

On Mon, 27 Nov 2023 at 11:34, Marc Zyngier <maz at kernel.org> wrote:
>
> On Mon, 27 Nov 2023 10:21:42 +0000,
> Ard Biesheuvel <ardb at google.com> wrote:
> >
> > From: Ard Biesheuvel <ardb at kernel.org>
> >
> > Commit 42c5a3b04bf6 refactored the KPTI init code in a way that results
> > in the use of non-global kernel mappings even on systems that have no
> > need for it, and even when KPTI has been disabled explicitly via the
> > command line.
> >
> > Ensure that this only happens when we have decided (based on the
> > detected CPU features) that KPTI should be enabled.
> >
> > Fixes: 42c5a3b04bf6 ("arm64: Split kpti_install_ng_mappings()")
> > Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> > ---
> >  arch/arm64/kernel/cpufeature.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 646591c67e7a..4b1762b9c37c 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -3352,7 +3352,8 @@ void __init setup_system_features(void)
> >        */
> >       enable_cpu_capabilities(SCOPE_ALL & ~SCOPE_BOOT_CPU);
> >
> > -     kpti_install_ng_mappings();
> > +     if (arm64_kernel_unmapped_at_el0())
> > +             kpti_install_ng_mappings();
> >
> >       sve_setup();
> >       sme_setup();
>
> Reviewed-by: Marc Zyngier <maz at kernel.org>
> Tested-by: Marc Zyngier <maz at kernel.org>
>

Bah now kpti=on is broken ...

Will respin,