linux-kernel - Re: [PATCH v10 10/16] KVM: x86: Introduce KVM_GET_SHARED_PAGES

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Date:   Thu, 4 Feb 2021 16:34:52 +0000
From:   Ashish Kalra <ashish.kalra@....com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, rkrcmar@...hat.com, joro@...tes.org, bp@...e.de,
        x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        srutherford@...gle.com, seanjc@...gle.com,
        venu.busireddy@...cle.com, brijesh.singh@....com
Subject: Re: [PATCH v10 10/16] KVM: x86: Introduce KVM_GET_SHARED_PAGES_LIST
 ioctl

Hello Tom,

On Thu, Feb 04, 2021 at 10:14:37AM -0600, Tom Lendacky wrote:
> On 2/3/21 6:39 PM, Ashish Kalra wrote:
> > From: Brijesh Singh <brijesh.singh@....com>
> > 
> > The ioctl is used to retrieve a guest's shared pages list.
> > 
> 
> ...
> 
> > +int svm_get_shared_pages_list(struct kvm *kvm,
> > +			      struct kvm_shared_pages_list *list)
> > +{
> > +	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> > +	struct shared_region_array_entry *array;
> > +	struct shared_region *pos;
> > +	int ret, nents = 0;
> > +	unsigned long sz;
> > +
> > +	if (!sev_guest(kvm))
> > +		return -ENOTTY;
> > +
> > +	if (!list->size)
> > +		return -EINVAL;
> > +
> > +	if (!sev->shared_pages_list_count)
> > +		return put_user(0, list->pnents);
> > +
> > +	sz = sev->shared_pages_list_count * sizeof(struct shared_region_array_entry);
> > +	if (sz > list->size)
> > +		return -E2BIG;
> > +
> > +	array = kmalloc(sz, GFP_KERNEL);
> > +	if (!array)
> > +		return -ENOMEM;
> > +
> > +	mutex_lock(&kvm->lock);
> 
> I think this lock needs to be taken before the memory size is calculated. If
> the list is expanded after obtaining the size and before taking the lock,
> you will run off the end of the array.
> 

Yes, as the page encryption status hcalls can happen simultaneously to
this ioctl, therefore this is an issue, so i will take the lock before
the memory size is calculated.

Thanks,
Ashish

> 
> > +	list_for_each_entry(pos, &sev->shared_pages_list, list) {
> > +		array[nents].gfn_start = pos->gfn_start;
> > +		array[nents++].gfn_end = pos->gfn_end;
> > +	}
> > +	mutex_unlock(&kvm->lock);
> > +
> > +	ret = -EFAULT;
> > +	if (copy_to_user(list->buffer, array, sz))
> > +		goto out;
> > +	if (put_user(nents, list->pnents))
> > +		goto out;
> > +	ret = 0;
> > +out:
> > +	kfree(array);
> > +	return ret;
> > +}
> > +