Re: [PATCH v5 06/13] KVM: Use kvm_userspace_memory_region_ext

[Sean Christopherson <seanjc@google.com>]

On Thu, Mar 10, 2022, Chao Peng wrote:
> @@ -4476,14 +4477,23 @@ static long kvm_vm_ioctl(struct file *filp,
>  		break;
>  	}
>  	case KVM_SET_USER_MEMORY_REGION: {
> -		struct kvm_userspace_memory_region kvm_userspace_mem;
> +		struct kvm_userspace_memory_region_ext region_ext;

It's probably a good idea to zero initialize the full region to avoid consuming
garbage stack data if there's a bug and an _ext field is accessed without first
checking KVM_MEM_PRIVATE.  I'm usually opposed to unnecessary initialization, but
this seems like something we could screw up quite easily.

>  		r = -EFAULT;
> -		if (copy_from_user(&kvm_userspace_mem, argp,
> -						sizeof(kvm_userspace_mem)))
> +		if (copy_from_user(&region_ext, argp,
> +				sizeof(struct kvm_userspace_memory_region)))
>  			goto out;
> +		if (region_ext.region.flags & KVM_MEM_PRIVATE) {
> +			int offset = offsetof(
> +				struct kvm_userspace_memory_region_ext,
> +				private_offset);
> +			if (copy_from_user(&region_ext.private_offset,
> +					   argp + offset,
> +					   sizeof(region_ext) - offset))

In this patch, KVM_MEM_PRIVATE should result in an -EINVAL as it's not yet
supported.  Copying the _ext on KVM_MEM_PRIVATE belongs in the "Expose KVM_MEM_PRIVATE"
patch.

Mechnically, what about first reading flags via get_user(), and then doing a single
copy_from_user()?  It's technically more work in the common case, and requires an
extra check to guard against TOCTOU attacks, but this isn't a fast path by any means
and IMO the end result makes it easier to understand the relationship between
KVM_MEM_PRIVATE and the two different structs.

E.g.

	case KVM_SET_USER_MEMORY_REGION: {
		struct kvm_user_mem_region region;
		unsigned long size;
		u32 flags;

		memset(&region, 0, sizeof(region));

		r = -EFAULT;
		if (get_user(flags, (u32 __user *)(argp + offsetof(typeof(region), flags))))
			goto out;

		if (flags & KVM_MEM_PRIVATE)
			size = sizeof(struct kvm_userspace_memory_region_ext);
		else
			size = sizeof(struct kvm_userspace_memory_region);
		if (copy_from_user(&region, argp, size))
			goto out;

		r = -EINVAL;
		if ((flags ^ region.flags) & KVM_MEM_PRIVATE)
			goto out;

		r = kvm_vm_ioctl_set_memory_region(kvm, &region);
		break;
	}

> +				goto out;
> +		}
>  
> -		r = kvm_vm_ioctl_set_memory_region(kvm, &kvm_userspace_mem);
> +		r = kvm_vm_ioctl_set_memory_region(kvm, &region_ext);
>  		break;
>  	}
>  	case KVM_GET_DIRTY_LOG: {
> -- 
> 2.17.1
>