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

[Chao Peng]

On Mon, Mar 28, 2022 at 10:26:55PM +0000, Sean Christopherson wrote:
> 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.

Agreed.

> 
> 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.

Will use this code, thanks for typing.

Chao
> 
> 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
> >