linux-kernel - Re: [PATCH 3/3] KVM: Expose MSRs to userspace

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Date:	Fri, 17 Nov 2006 09:20:49 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Andrew Morton <akpm@...l.org>
CC:	kvm-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	uril@...ranet.com
Subject: Re: [PATCH 3/3] KVM: Expose MSRs to userspace

Andrew Morton wrote:
> On Thu, 16 Nov 2006 18:04:22 -0000
> Avi Kivity <avi@...ranet.com> wrote:
>
>   
>> +static int kvm_dev_ioctl_set_msrs(struct kvm *kvm, struct kvm_msrs *msrs)
>> +{
>> +	struct kvm_vcpu *vcpu;
>> +	struct kvm_msr_entry *entry, *entries;
>> +	int rc;
>> +	u32 size, num_entries, i;
>> +
>> +	if (msrs->vcpu < 0 || msrs->vcpu >= KVM_MAX_VCPUS)
>> +		return -EINVAL;
>> +
>> +	num_entries = ARRAY_SIZE(msrs_to_save);
>> +	if (msrs->nmsrs < num_entries) {
>> +		msrs->nmsrs = num_entries; /* inform actual size */
>> +		return -EINVAL;
>> +	}
>> +
>> +	vcpu = vcpu_load(kvm, msrs->vcpu);
>> +	if (!vcpu)
>> +		return -ENOENT;
>> +
>> +	size = msrs->nmsrs * sizeof(struct kvm_msr_entry);
>> +	rc = -E2BIG;
>> +	if (size > 4096)
>> +		goto out_vcpu;
>>     
>
> Classic mutiplicative overflow bug.  

Right, will fix.  The 4096 limit is arbitrary anyway, and can be 
replaced by an arbitrary limit on nmsrs.


> Only msrs->nmsrs doesn't get used
> again, so there is no bug here.  Yet.
>
>   

But why isn't it used again?  Looks like the kernel is forcing the user 
to send at least num_entries for no good reason, and ignoring any 
entries beyond num_entries.

>> +	rc = -ENOMEM;
>> +	entries = vmalloc(size);
>> +	if (entries == NULL)
>> +		goto out_vcpu;
>> +
>> +	rc = -EFAULT;
>> +	if (copy_from_user(entries, msrs->entries, size))
>> +		goto out_free;
>> +
>> +	rc = -EINVAL;
>> +	for (i=0; i<num_entries; i++) {
>> +		entry = &entries[i];
>> +		if (set_msr(vcpu, entry->index,  entry->data))
>> +			goto out_free;
>> +	}
>> +
>> +	rc = 0;
>> +out_free:
>> +	vfree(entries);
>> +
>> +out_vcpu:
>> +	vcpu_put(vcpu);
>> +
>> +	return rc;
>> +}
>>     
>
> This function returns no indication of how many msrs it actually did set. 
> Should it?
>   

It can't hurt.  Is returning the number of msrs set in the return code 
(ala short write) acceptable, or do I need to make this a read/write ioctl?

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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