Re: [PATCH v2 2/5] arm64: Clear HCR_EL2.ATA when MTE is not supported or disabled

[Marc Zyngier <maz@kernel.org>]

On Thu, 11 Dec 2025 11:38:25 +0000,
Fuad Tabba <tabba@google.com> wrote:
> 
> If MTE is not supported by the hardware, or is disabled in the kernel
> configuration (CONFIG_ARM64_MTE=n) or command line (arm64.nomte), the
> kernel stops advertising MTE to userspace and avoids using MTE
> instructions. However, this is a software-level disable only.
> 
> When MTE hardware is present and enabled by EL3 firmware, leaving
> HCR_EL2.ATA set allows the host to execute MTE instructions (STG, LDG,
> etc.) and access allocation tags in physical memory. This creates a
> security risk where a malicious or buggy host could lead to system
> crashes, undefined behavior, or compromise guests.
> 
> Prevent this by clearing HCR_EL2.ATA when MTE is disabled. Remove it
> from the HCR_HOST_NVHE_FLAGS default, and conditionally set it in
> cpu_prepare_hyp_mode() only when system_supports_mte() returns true.
> This causes MTE instructions to trap to EL2 when HCR_EL2.ATA is cleared.
> 
> Early boot code in head.S temporarily keeps HCR_ATA set to avoid
> special-casing initialization paths. This is safe because this code
> executes before untrusted code runs and will clear HCR_ATA if MTE is
> disabled.
> 
> Signed-off-by: Fuad Tabba <tabba@google.com>
> ---
>  arch/arm64/include/asm/kvm_arm.h | 2 +-
>  arch/arm64/kernel/head.S         | 2 +-
>  arch/arm64/kvm/arm.c             | 4 ++++
>  3 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> index 1da290aeedce..a41e3087e00a 100644
> --- a/arch/arm64/include/asm/kvm_arm.h
> +++ b/arch/arm64/include/asm/kvm_arm.h
> @@ -101,7 +101,7 @@
>  			 HCR_BSU_IS | HCR_FB | HCR_TACR | \
>  			 HCR_AMO | HCR_SWIO | HCR_TIDCP | HCR_RW | HCR_TLOR | \
>  			 HCR_FMO | HCR_IMO | HCR_PTW | HCR_TID3 | HCR_TID1)
> -#define HCR_HOST_NVHE_FLAGS (HCR_RW | HCR_API | HCR_APK | HCR_ATA)
> +#define HCR_HOST_NVHE_FLAGS (HCR_RW | HCR_API | HCR_APK)
>  #define HCR_HOST_NVHE_PROTECTED_FLAGS (HCR_HOST_NVHE_FLAGS | HCR_TSC)
>  #define HCR_HOST_VHE_FLAGS (HCR_RW | HCR_TGE | HCR_E2H | HCR_AMO | HCR_IMO | HCR_FMO)
>  
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index ca04b338cb0d..87a822e5c4ca 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -299,7 +299,7 @@ SYM_INNER_LABEL(init_el2, SYM_L_LOCAL)
>  	isb
>  0:
>  
> -	init_el2_hcr	HCR_HOST_NVHE_FLAGS
> +	init_el2_hcr	HCR_HOST_NVHE_FLAGS | HCR_ATA
>  	init_el2_state
>  
>  	/* Hypervisor stub */
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 052bf0d4d0b0..c03006b1c5bc 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -2030,6 +2030,10 @@ static void __init cpu_prepare_hyp_mode(int cpu, u32 hyp_va_bits)
>  		params->hcr_el2 = HCR_HOST_NVHE_PROTECTED_FLAGS;
>  	else
>  		params->hcr_el2 = HCR_HOST_NVHE_FLAGS;
> +
> +	if (system_supports_mte())
> +		params->hcr_el2 |= HCR_ATA;
> +

How about TID5? If you really want to hide MTE, you also need to catch
accesses to GMID_EL1.

Thanks,

	M.

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