Re: [PATCH v2 4/6] KVM: arm64: Limit length in kvm_vm_ioctl_mte_copy_tags() to INT_MAX

[Steven Price]

On 08/02/2023 14:01, Thomas Huth wrote:
> In case of success, this function returns the amount of handled bytes.
> However, this does not work for large values: The function is called
> from kvm_arch_vm_ioctl() (which still returns a long), which in turn
> is called from kvm_vm_ioctl() in virt/kvm/kvm_main.c. And that function
> stores the return value in an "int r" variable. So the upper 32-bits
> of the "long" return value are lost there.
> 
> KVM ioctl functions should only return "int" values, so let's limit
> the amount of bytes that can be requested here to INT_MAX to avoid
> the problem with the truncated return value. We can then also change
> the return type of the function to "int" to make it clearer that it
> is not possible to return a "long" here.
> 
> Fixes: f0376edb1ddc ("KVM: arm64: Add ioctl to fetch/store tags in a guest")
> Signed-off-by: Thomas Huth <thuth@xxxxxxxxxx>

Thanks for fixing this.

Reviewed-by: Steven Price <steven.price@xxxxxxx>

> ---
>  Documentation/virt/kvm/api.rst    | 3 ++-
>  arch/arm64/include/asm/kvm_host.h | 4 ++--
>  arch/arm64/kvm/guest.c            | 8 ++++++--
>  3 files changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 0a67cb738013..f184427931fa 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -5553,7 +5553,8 @@ with the KVM_XEN_VCPU_GET_ATTR ioctl.
>    };
>  
>  Copies Memory Tagging Extension (MTE) tags to/from guest tag memory. The
> -``guest_ipa`` and ``length`` fields must be ``PAGE_SIZE`` aligned. The ``addr``
> +``guest_ipa`` and ``length`` fields must be ``PAGE_SIZE`` aligned.
> +``length`` must not be bigger than 2^31 - PAGE_SIZE bytes. The ``addr``
>  field must point to a buffer which the tags will be copied to or from.
>  
>  ``flags`` specifies the direction of copy, either ``KVM_ARM_TAGS_TO_GUEST`` or
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 35a159d131b5..b1a16343767f 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -963,8 +963,8 @@ int kvm_arm_vcpu_arch_get_attr(struct kvm_vcpu *vcpu,
>  int kvm_arm_vcpu_arch_has_attr(struct kvm_vcpu *vcpu,
>  			       struct kvm_device_attr *attr);
>  
> -long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> -				struct kvm_arm_copy_mte_tags *copy_tags);
> +int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> +			       struct kvm_arm_copy_mte_tags *copy_tags);
>  
>  /* Guest/host FPSIMD coordination helpers */
>  int kvm_arch_vcpu_run_map_fp(struct kvm_vcpu *vcpu);
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index cf4c495a4321..cadef953046f 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -1013,8 +1013,8 @@ int kvm_arm_vcpu_arch_has_attr(struct kvm_vcpu *vcpu,
>  	return ret;
>  }
>  
> -long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> -				struct kvm_arm_copy_mte_tags *copy_tags)
> +int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> +			       struct kvm_arm_copy_mte_tags *copy_tags)
>  {
>  	gpa_t guest_ipa = copy_tags->guest_ipa;
>  	size_t length = copy_tags->length;
> @@ -1035,6 +1035,10 @@ long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
>  	if (length & ~PAGE_MASK || guest_ipa & ~PAGE_MASK)
>  		return -EINVAL;
>  
> +	/* Lengths above INT_MAX cannot be represented in the return value */
> +	if (length > INT_MAX)
> +		return -EINVAL;
> +
>  	gfn = gpa_to_gfn(guest_ipa);
>  
>  	mutex_lock(&kvm->slots_lock);