Re: [PATCH 1/3] KVM: arm64: Don't hide ID registers from userspace

From: Marc Zyngier <maz@kernel.org>
To: Andrew Jones <drjones@redhat.com>
Cc: xu910121@sina.com, kvmarm@lists.cs.columbia.edu, Dave.Martin@arm.com
Subject: Re: [PATCH 1/3] KVM: arm64: Don't hide ID registers from userspace
Date: Sat, 31 Oct 2020 18:09:50 +0000	[thread overview]
Message-ID: <87a6w2e07l.wl-maz@kernel.org> (raw)
In-Reply-To: <20201029201105.101910-2-drjones@redhat.com>

On Thu, 29 Oct 2020 20:11:03 +0000,
Andrew Jones <drjones@redhat.com> wrote:
> 
> ID registers are RAZ until they've been allocated a purpose, but
> that doesn't mean they should be removed from the KVM_GET_REG_LIST
> list. So far we only have one register, SYS_ID_AA64ZFR0_EL1, that
> is hidden from userspace when its function is not present. Removing
> the userspace visibility checks is enough to reexpose it, as it
> already behaves as RAZ when the function is not present.
> 
> Fixes: 73433762fcae ("KVM: arm64/sve: System register context switch and access support")
> Reported-by: 张东旭 <xu910121@sina.com>
> Signed-off-by: Andrew Jones <drjones@redhat.com>

Since this was broken in 5.2, it probably deserves a Cc stable (which
I can add when applying this).

> ---
>  arch/arm64/kvm/sys_regs.c | 18 +-----------------
>  1 file changed, 1 insertion(+), 17 deletions(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index d9117bc56237..d24e66ee59b3 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1195,16 +1195,6 @@ static unsigned int sve_visibility(const struct kvm_vcpu *vcpu,
>  	return REG_HIDDEN_USER | REG_HIDDEN_GUEST;
>  }
>  
> -/* Visibility overrides for SVE-specific ID registers */
> -static unsigned int sve_id_visibility(const struct kvm_vcpu *vcpu,
> -				      const struct sys_reg_desc *rd)
> -{
> -	if (vcpu_has_sve(vcpu))
> -		return 0;
> -
> -	return REG_HIDDEN_USER;
> -}
> -
>  /* Generate the emulated ID_AA64ZFR0_EL1 value exposed to the guest */
>  static u64 guest_id_aa64zfr0_el1(const struct kvm_vcpu *vcpu)
>  {
> @@ -1231,9 +1221,6 @@ static int get_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
>  {
>  	u64 val;
>  
> -	if (WARN_ON(!vcpu_has_sve(vcpu)))
> -		return -ENOENT;
> -
>  	val = guest_id_aa64zfr0_el1(vcpu);
>  	return reg_to_user(uaddr, &val, reg->id);
>  }
> @@ -1246,9 +1233,6 @@ static int set_id_aa64zfr0_el1(struct kvm_vcpu *vcpu,
>  	int err;
>  	u64 val;
>  
> -	if (WARN_ON(!vcpu_has_sve(vcpu)))
> -		return -ENOENT;
> -
>  	err = reg_from_user(&val, uaddr, id);
>  	if (err)
>  		return err;
> @@ -1518,7 +1502,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	ID_SANITISED(ID_AA64PFR1_EL1),
>  	ID_UNALLOCATED(4,2),
>  	ID_UNALLOCATED(4,3),
> -	{ SYS_DESC(SYS_ID_AA64ZFR0_EL1), access_id_aa64zfr0_el1, .get_user = get_id_aa64zfr0_el1, .set_user = set_id_aa64zfr0_el1, .visibility = sve_id_visibility },
> +	{ SYS_DESC(SYS_ID_AA64ZFR0_EL1), access_id_aa64zfr0_el1, .get_user = get_id_aa64zfr0_el1, .set_user = set_id_aa64zfr0_el1, },
>  	ID_UNALLOCATED(4,5),
>  	ID_UNALLOCATED(4,6),
>  	ID_UNALLOCATED(4,7),
> -- 
> 2.27.0
> 
> 

Looks like the right thing to do, thanks for following up on this.
Dave, are you happy with this?

	M.

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