linux-kernel - Re: [PATCH 1/2] arm64: ARM_SMCCC_ARCH_WORKAROUND_1 doesn't return SMCCC_RET_NOT

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Date:   Wed, 21 Oct 2020 08:57:23 +0100
From:   Will Deacon <will@...nel.org>
To:     Stephen Boyd <swboyd@...omium.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Andre Przywara <andre.przywara@....com>,
        Steven Price <steven.price@....com>,
        Marc Zyngier <maz@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH 1/2] arm64: ARM_SMCCC_ARCH_WORKAROUND_1 doesn't return
 SMCCC_RET_NOT_REQUIRED

On Tue, Oct 20, 2020 at 02:45:43PM -0700, Stephen Boyd wrote:
> According to the SMCCC spec (7.5.2 Discovery) the
> ARM_SMCCC_ARCH_WORKAROUND_1 function id only returns 0, 1, and
> SMCCC_RET_NOT_SUPPORTED corresponding to "workaround required",
> "workaround not required but implemented", and "who knows, you're on
> your own" respectively. For kvm hypercalls (hvc), we've implemented this
> function id to return SMCCC_RET_NOT_SUPPORTED, 1, and
> SMCCC_RET_NOT_REQUIRED. The SMCCC_RET_NOT_REQUIRED return value is not a
> thing for this function id, and is probably copy/pasted from the
> SMCCC_ARCH_WORKAROUND_2 function id that does support it.
> 
> Clean this up by returning 0, 1, and SMCCC_RET_NOT_SUPPORTED
> appropriately. Changing this exposes the problem that
> spectre_v2_get_cpu_fw_mitigation_state() assumes a
> SMCCC_RET_NOT_SUPPORTED return value means we are vulnerable, but really
> it means we have no idea and should assume we can't do anything about
> mitigation. Put another way, it better be unaffected because it can't be
> mitigated in the firmware (in this case kvm) as the call isn't
> implemented!
> 
> Cc: Andre Przywara <andre.przywara@....com>
> Cc: Steven Price <steven.price@....com>
> Cc: Marc Zyngier <maz@...nel.org>
> Cc: stable@...r.kernel.org
> Fixes: c118bbb52743 ("arm64: KVM: Propagate full Spectre v2 workaround state to KVM guests")
> Fixes: 73f381660959 ("arm64: Advertise mitigation of Spectre-v2, or lack thereof")
> Signed-off-by: Stephen Boyd <swboyd@...omium.org>
> ---
> 
> This will require a slightly different backport to stable kernels, but
> at least it looks like this is a problem given that this return value
> isn't valid per the spec and we've been going around it by returning
> something invalid for some time.
> 
>  arch/arm64/kernel/proton-pack.c | 3 +--
>  arch/arm64/kvm/hypercalls.c     | 2 +-
>  2 files changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/proton-pack.c b/arch/arm64/kernel/proton-pack.c
> index 68b710f1b43f..00bd54f63f4f 100644
> --- a/arch/arm64/kernel/proton-pack.c
> +++ b/arch/arm64/kernel/proton-pack.c
> @@ -149,10 +149,9 @@ static enum mitigation_state spectre_v2_get_cpu_fw_mitigation_state(void)
>  	case SMCCC_RET_SUCCESS:
>  		return SPECTRE_MITIGATED;
>  	case SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED:
> +	case SMCCC_RET_NOT_SUPPORTED: /* Good luck w/ the Gatekeeper of Gozer */
>  		return SPECTRE_UNAFFECTED;

Hmm, I'm not sure this is correct. The SMCCC spec is terrifically
unhelpful:

  NOT_SUPPORTED:
  Either:
  * None of the PEs in the system require firmware mitigation for CVE-2017-5715.
  * The system contains at least 1 PE affected by CVE-2017-5715 that has no firmware
    mitigation available.
  * The firmware does not provide any information about whether firmware mitigation is
    required.

so we can't tell whether the thing is vulnerable or not in this case, and
have to assume that it is.

>  	default:
> -		fallthrough;
> -	case SMCCC_RET_NOT_SUPPORTED:
>  		return SPECTRE_VULNERABLE;
>  	}
>  }
> diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
> index 9824025ccc5c..868486957808 100644
> --- a/arch/arm64/kvm/hypercalls.c
> +++ b/arch/arm64/kvm/hypercalls.c
> @@ -31,7 +31,7 @@ int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)
>  				val = SMCCC_RET_SUCCESS;
>  				break;
>  			case SPECTRE_UNAFFECTED:
> -				val = SMCCC_RET_NOT_REQUIRED;
> +				val = SMCCC_RET_NOT_SUPPORTED;

Which means we need to return SMCCC_ARCH_WORKAROUND_RET_UNAFFECTED here, I
suppose?

Will