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

Chao Peng
2022-11-17T13:25:17Z

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