Subject: Re: [PATCH v6 04/22] powerpc/book3s64/kuap/kuep: Move uamor setup to pkey init

From: Michael Ellerman <[email-masked]>
Date: Thu, 26 Nov 2020 14:28:15 +1100

"Aneesh Kumar K.V" <[email-masked]> writes:
> This patch consolidates UAMOR update across pkey, kuap and kuep features.
> The boot cpu initialize UAMOR via pkey init and both radix/hash do the
> secondary cpu UAMOR init in early_init_mmu_secondary.
>
> We don't check for mmu_feature in radix secondary init because UAMOR
> is a supported SPRN with all CPUs supporting radix translation.
> The old code was not updating UAMOR if we had smap disabled and smep enabled.
> This change handles that case.
>
> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
> ---
>  arch/powerpc/mm/book3s64/radix_pgtable.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/mm/book3s64/radix_pgtable.c b/arch/powerpc/mm/book3s64/radix_pgtable.c
> index 3adcf730f478..bfe441af916a 100644
> --- a/arch/powerpc/mm/book3s64/radix_pgtable.c
> +++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
> @@ -620,9 +620,6 @@ void setup_kuap(bool disabled)
>  		cur_cpu_spec->mmu_features |= MMU_FTR_RADIX_KUAP;
>  	}
>  
> -	/* Make sure userspace can't change the AMR */
> -	mtspr(SPRN_UAMOR, 0);
> -
>  	/*
>  	 * Set the default kernel AMR values on all cpus.
>  	 */
> @@ -721,6 +718,11 @@ void radix__early_init_mmu_secondary(void)
>  
>  	radix__switch_mmu_context(NULL, &init_mm);
>  	tlbiel_all();
> +
> +#ifdef CONFIG_PPC_PKEY
> +	/* Make sure userspace can't change the AMR */
> +	mtspr(SPRN_UAMOR, 0);
> +#endif

If PPC_PKEY is disabled I think this leaves UAMOR unset, which means it
could potentially allow AMR to be used as a covert channel between
processes.

cheers