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

[Sean Christopherson]

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
>