Re: [PATCH v9 5/8] KVM: Register/unregister the guest private memory regions

[Chao Peng]

On Wed, Nov 16, 2022 at 10:24:11PM +0000, Sean Christopherson wrote:
> On Tue, Oct 25, 2022, Chao Peng wrote:
> > +static int kvm_vm_ioctl_set_mem_attr(struct kvm *kvm, gpa_t gpa, gpa_t size,
> > +				     bool is_private)
> > +{
> > +	gfn_t start, end;
> > +	unsigned long i;
> > +	void *entry;
> > +	int idx;
> > +	int r = 0;
> > +
> > +	if (size == 0 || gpa + size < gpa)
> > +		return -EINVAL;
> > +	if (gpa & (PAGE_SIZE - 1) || size & (PAGE_SIZE - 1))
> > +		return -EINVAL;
> > +
> > +	start = gpa >> PAGE_SHIFT;
> > +	end = (gpa + size - 1 + PAGE_SIZE) >> PAGE_SHIFT;
> > +
> > +	/*
> > +	 * Guest memory defaults to private, kvm->mem_attr_array only stores
> > +	 * shared memory.
> > +	 */
> > +	entry = is_private ? NULL : xa_mk_value(KVM_MEM_ATTR_SHARED);
> > +
> > +	idx = srcu_read_lock(&kvm->srcu);
> > +	KVM_MMU_LOCK(kvm);
> > +	kvm_mmu_invalidate_begin(kvm, start, end);
> > +
> > +	for (i = start; i < end; i++) {
> > +		r = xa_err(xa_store(&kvm->mem_attr_array, i, entry,
> > +				    GFP_KERNEL_ACCOUNT));
> > +		if (r)
> > +			goto err;
> > +	}
> > +
> > +	kvm_unmap_mem_range(kvm, start, end);
> > +
> > +	goto ret;
> > +err:
> > +	for (; i > start; i--)
> > +		xa_erase(&kvm->mem_attr_array, i);
> 
> I don't think deleting previous entries is correct.  To unwind, the correct thing
> to do is restore the original values.  E.g. if userspace space is mapping a large
> range as shared, and some of the previous entries were shared, deleting them would
> incorrectly "convert" those entries to private.

Ah, right!

> 
> Tracking the previous state likely isn't the best approach, e.g. it would require
> speculatively allocating extra memory for a rare condition that is likely going to
> lead to OOM anyways.

Agree.

> 
> Instead of trying to unwind, what about updating the ioctl() params such that
> retrying with the updated addr+size would Just Work?  E.g.

Looks good to me. Thanks!

Chao
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 55b07aae67cc..f1de592a1a06 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1015,15 +1015,12 @@ static int kvm_vm_ioctl_set_mem_attr(struct kvm *kvm, gpa_t gpa, gpa_t size,
>  
>         kvm_unmap_mem_range(kvm, start, end, attr);
>  
> -       goto ret;
> -err:
> -       for (; i > start; i--)
> -               xa_erase(&kvm->mem_attr_array, i);
> -ret:
>         kvm_mmu_invalidate_end(kvm, start, end);
>         KVM_MMU_UNLOCK(kvm);
>         srcu_read_unlock(&kvm->srcu, idx);
>  
> +       <update gpa and size>
> +
>         return r;
>  }
>  #endif /* CONFIG_KVM_GENERIC_PRIVATE_MEM */
> @@ -4989,6 +4986,8 @@ static long kvm_vm_ioctl(struct file *filp,
>  
>                 r = kvm_vm_ioctl_set_mem_attr(kvm, region.addr,
>                                               region.size, set);
> +               if (copy_to_user(argp, &region, sizeof(region)) && !r)
> +                       r = -EFAULT
>                 break;
>         }
>  #endif