[PATCH v2 1/3] arm64: Avoid enabling KPTI unnecessarily

[Ard Biesheuvel]

On Mon, 27 Nov 2023 at 16:48, Will Deacon <will at kernel.org> wrote:
>
> On Mon, Nov 27, 2023 at 01:00:51PM +0100, Ard Biesheuvel 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 system-wide 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 | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 646591c67e7a..91d2d6714969 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1839,6 +1839,10 @@ static int __init __kpti_install_ng_mappings(void *__unused)
> >
> >  static void __init kpti_install_ng_mappings(void)
> >  {
> > +     /* Check whether KPTI is going to be used */
> > +     if (!cpus_have_cap(ARM64_UNMAP_KERNEL_AT_EL0))
> > +             return;
>
> Why can't you use arm64_kernel_unmapped_at_el0() here?
>

Because it relies on alternatives patching, which hasn't occurred yet
at this point.