Re: [Xen-devel] [PATCH] xen: add new hypercall buffer mapping device

[Juergen Gross]

On 15/06/18 16:15, Andrew Cooper wrote:
> On 15/06/18 14:17, Juergen Gross wrote:
>> +MODULE_LICENSE("GPL");
>> +
>> +static int limit = 64;
>> +module_param(limit, int, 0644);
>> +MODULE_PARM_DESC(limit, "Maximum number of pages that may be allocated by "
>> +			"the privcmd-buf device per open file");
> 
> I have a feeling that, once we try and remove some of the bounce
> buffering, 64 pages will be somewhat restricting. In particular,
> migration performance will benefit by keeping the logdirty bitmap buffer
> persistently mapped, rather than allocated/bounced/deallocated on each
> iteration.
> 
> However, perhaps 64 is fine for now.
> 
>> +static int privcmd_buf_mmap(struct file *file, struct vm_area_struct *vma)
>> +{
>> +	struct privcmd_buf_private *file_priv = file->private_data;
>> +	struct privcmd_buf_vma_private *vma_priv;
>> +	unsigned int count = vma_pages(vma);
> 
> This will truncate to 0 if anyone tried mmap()ing 8T (if I've done my
> calculations correctly) of virtual address space.

Okay, I'll change the type to unsigned long.

> 
>> +	unsigned int i;
>> +	int ret = 0;
>> +
>> +	if (!(vma->vm_flags & VM_SHARED)) {
>> +		pr_err("Mapping must be shared\n");
>> +		return -EINVAL;
>> +	}
>> +
>> +	if (file_priv->allocated + count > limit) {
> 
> cout > limit || (allocated + count) > limit to avoid overflows.

unsigned long again.

> 
>> +		pr_err("Mapping limit reached!\n");
>> +		return -ENOSPC;
>> +	}
>> +
>> +	vma_priv = kzalloc(sizeof(*vma_priv) + count * sizeof(void *),
>> +			   GFP_KERNEL);
>> +	if (!vma_priv)
>> +		return -ENOMEM;
>> +
>> +	vma_priv->n_pages = count;
>> +	count = 0;
>> +	for (i = 0; i < vma_priv->n_pages; i++) {
>> +		vma_priv->pages[i] = alloc_page(GFP_KERNEL | __GFP_ZERO);
>> +		if (!vma_priv->pages[i])
>> +			break;
>> +		count++;
>> +	}
>> +
>> +	mutex_lock(&file_priv->lock);
>> +
>> +	file_priv->allocated += count;
>> +
>> +	vma_priv->file_priv = file_priv;
>> +	vma_priv->users = 1;
>> +
>> +	vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
> 
> Why DONTDUMP? Its just data, and stands a reasonable chance of being
> related to the cause of a crash.

Hmm, yes. I'll drop it.

> 
>> +	vma->vm_ops = &privcmd_buf_vm_ops;
>> +	vma->vm_private_data = vma_priv;
>> +
>> +	list_add(&vma_priv->list, &file_priv->list);
>> +
>> +	if (vma_priv->n_pages != count)
>> +		ret = -ENOMEM;
>> +	else
>> +		for (i = 0; i < vma_priv->n_pages; i++) {
>> +			ret = vm_insert_page(vma, vma->vm_start + i * PAGE_SIZE,
>> +					     vma_priv->pages[i]);
>> +			if (ret)
>> +				break;
>> +		}
>> +
>> +	if (ret)
>> +		privcmd_buf_vmapriv_free(vma_priv);
>> +
>> +	mutex_unlock(&file_priv->lock);
>> +
>> +	return ret;
>> +}
>> +
>> +const struct file_operations xen_privcmdbuf_fops = {
>> +	.owner = THIS_MODULE,
>> +	.open = privcmd_buf_open,
>> +	.release = privcmd_buf_release,
>> +	.mmap = privcmd_buf_mmap,
>> +};
>> +EXPORT_SYMBOL_GPL(xen_privcmdbuf_fops);
>> +
>> +struct miscdevice xen_privcmdbuf_dev = {
>> +	.minor = MISC_DYNAMIC_MINOR,
>> +	.name = "xen/privcmd-buf",
> 
> Sorry to nitpick, but how about naming this just "xen/hypercall" ?

I really have no special preferences here.

> privcmd is currently a rather large security hole because it allows
> userspace to have access to all the hypercalls, including the ones which
> should be restricted to just the kernel. In the past, a plan has been
> floated to slowly replace the use of the raw ioctl() with proper ioctls
> for the hypercalls which userspace might reasonably use.

I'd rather let the privcmd driver either ask the hypervisor which
hypercalls are fine to call from user mode, or let it encapsulate
the hypercall in a new "user hypercall" which the hypervisor can
verify then.

>> diff --git a/drivers/xen/xenfs/super.c b/drivers/xen/xenfs/super.c
>> index 71ddfb4cf61c..d752d0dd3d1d 100644
>> --- a/drivers/xen/xenfs/super.c
>> +++ b/drivers/xen/xenfs/super.c
>> @@ -48,6 +48,7 @@ static int xenfs_fill_super(struct super_block *sb, void *data, int silent)
>>  		[2] = { "xenbus", &xen_xenbus_fops, S_IRUSR|S_IWUSR },
>>  		{ "capabilities", &capabilities_file_ops, S_IRUGO },
>>  		{ "privcmd", &xen_privcmd_fops, S_IRUSR|S_IWUSR },
>> +		{ "privcmd-buf", &xen_privcmdbuf_fops, S_IRUSR|S_IWUSR },
> 
> Do we really need to provide the fallback here? /dev/xen has been
> around for ages, and it would really be a good thing if we can
> eventually retire xenfs.

I'd be fine dropping it.

Just did some archaeology: /dev/xen is supported since Xen 4.5. Do we
really want to drop support of older Xen versions in the Linux kernel?


Juergen