Subject: Re: [PATCH 1/9] arm64: Add logic to fully remove features from sanitised id registers

From: Marc Zyngier <[email-masked]>
Date: Mon, 23 Feb 2026 09:48:24 +0000

Hi Fuad,

On Fri, 20 Feb 2026 15:36:37 +0000,
Fuad Tabba <[email-masked]> wrote:
> 
> > I think we must prevent this downgrade the same way, meaning that
> > ALL_HIDDEN and FTR_HIGHER are mutually exclusive.
> >
> > How about that:
> >
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index d58931e63a0b6..2cae00b4b0c5f 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -1067,7 +1067,14 @@ static void init_cpu_ftr_reg(u32 sys_reg, u64 new)
> >                         user_mask |= ftr_mask;
> >                         break;
> >                 case FTR_ALL_HIDDEN:
> > -                       val = arm64_ftr_set_value(ftrp, val, ftrp->safe_val);
> > +                       /*
> > +                        * ALL_HIDDEN and HIGHER_SAFE are incompatible.
> > +                        * Only hide from userspace, and log the oddity.
> > +                        */
> > +                       if (WARN_ON(ftrp->type == FTR_HIGHER_SAFE))
> > +                               val = arm64_ftr_set_value(ftrp, val, ftr_new);
> > +                       else
> > +                               val = arm64_ftr_set_value(ftrp, val, ftrp->safe_val);
> >                         reg->user_val = arm64_ftr_set_value(ftrp,
> >                                                             reg->user_val,
> >                                                             ftrp->safe_val);
> >
> 
> Yes, I think WARN_ON() here is the right call.
> 
> That said, I still think you should explicitly short-circuit
> update_cpu_ftr_reg() for FTR_ALL_HIDDEN features, in addition to the
> WARN_ON(). Relying on arm64_ftr_safe_value() to naturally preserve the
> safe_val during secondary CPU boot seems mathematically fragile.
> 
> Take MTE_frac as an example. It uses S_ARM64_FTR_BITS and
> FTR_LOWER_SAFE with a safe_val of 0. If it were marked FTR_ALL_HIDDEN,
> init_cpu_ftr_reg() would prime sys_val with 0. But if a secondary CPU
> boots and reports -1 (NI), arm64_ftr_safe_value() will execute min(-1,
> 0) and return -1. update_cpu_ftr_reg() will then overwrite the primed
> safe_val (0) with -1. The "hidden" state established by the boot CPU
> is gone, and the feature's hardware state is now exposed globally.
> 
> Note that MTE is currently ALL_HIDDEN when configured out, so it's not
> totally inconceivable that someone decides to make MTE_frac ALL_HIDDEN
> as well. Explicitly short-circuiting for FTR_ALL_HIDDEN features in
> update_cpu_ftr_reg() seems to be the safer bet here.

Right, the signed feature is a pretty compelling argument. And we
should do the same thing for overrides, probably as a preliminary
patch.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.