RE: [PATCH v2 2/3] xen/privcmd: Add IOCTL_PRIVCMD_DM_OP

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Boris Ostrovsky [mailto:boris.ostrovsky@oracle.com]
> Sent: 10 February 2017 16:18
> To: Paul Durrant <Paul.Durrant@citrix.com>; xen-devel@lists.xenproject.org;
> linux-kernel@vger.kernel.org
> Cc: Juergen Gross <jgross@suse.com>
> Subject: Re: [PATCH v2 2/3] xen/privcmd: Add IOCTL_PRIVCMD_DM_OP
> 
> On 02/10/2017 09:24 AM, Paul Durrant wrote:
> > +static long privcmd_ioctl_dm_op(void __user *udata)
> > +{
> > +	struct privcmd_dm_op kdata;
> > +	struct privcmd_dm_op_buf *kbufs;
> > +	unsigned int nr_pages = 0;
> > +	struct page **pages = NULL;
> > +	struct xen_dm_op_buf *xbufs = NULL;
> > +	unsigned int i;
> > +	long rc;
> > +
> > +	if (copy_from_user(&kdata, udata, sizeof(kdata)))
> > +		return -EFAULT;
> > +
> > +	if (kdata.num == 0)
> > +		return 0;
> > +
> > +	/*
> > +	 * Set a tolerable upper limit on the number of buffers
> > +	 * without being overly restrictive, since we can't easily
> > +	 * predict what future dm_ops may require.
> > +	 */
> 
> I think this deserves its own macro since it really has nothing to do
> with page size, has it? Especially since you are referencing it again
> below too.
> 
> 
> > +	if (kdata.num * sizeof(*kbufs) > PAGE_SIZE)
> > +		return -E2BIG;
> > +
> > +	kbufs = kcalloc(kdata.num, sizeof(*kbufs), GFP_KERNEL);
> > +	if (!kbufs)
> > +		return -ENOMEM;
> > +
> > +	if (copy_from_user(kbufs, kdata.ubufs,
> > +			   sizeof(*kbufs) * kdata.num)) {
> > +		rc = -EFAULT;
> > +		goto out;
> > +	}
> > +
> > +	for (i = 0; i < kdata.num; i++) {
> > +		if (!access_ok(VERIFY_WRITE, kbufs[i].uptr,
> > +			       kbufs[i].size)) {
> > +			rc = -EFAULT;
> > +			goto out;
> > +		}
> > +
> > +		nr_pages += DIV_ROUND_UP(
> > +			offset_in_page(kbufs[i].uptr) + kbufs[i].size,
> > +			PAGE_SIZE);
> > +	}
> > +
> > +	/*
> > +	 * Again, set a tolerable upper limit on the number of pages
> > +	 * needed to lock all the buffers without being overly
> > +	 * restrictive, since we can't easily predict the size of
> > +	 * buffers future dm_ops may use.
> > +	 */
> 
> OTOH, these two cases describe different types of copying (the first one
> is for buffer descriptors and the second is for buffers themselves). And
> so should they be limited by the same value?
> 

I think there needs to be some limit and limiting the allocation to a page was the best I came up with. Can you think of a better one?

> > +	if (nr_pages * sizeof(*pages) > PAGE_SIZE) {
> > +		rc = -E2BIG;
> > +		goto out;
> > +	}
> > +
> > +	pages = kcalloc(nr_pages, sizeof(*pages), GFP_KERNEL);
> > +	if (!pages) {
> > +		rc = -ENOMEM;
> > +		goto out;
> > +	}
> > +
> > +	xbufs = kcalloc(kdata.num, sizeof(*xbufs), GFP_KERNEL);
> > +	if (!xbufs) {
> > +		rc = -ENOMEM;
> > +		goto out;
> > +	}
> > +
> > +	rc = lock_pages(kbufs, kdata.num, pages, nr_pages);
> 
> 
> Aren't those buffers already locked (as Andrew mentioned)? They are
> mmapped with MAP_LOCKED.

No, they're not. The new libxendevicemodel code I have does not make any use of xencall or guest handles when privcmd supports the DM_OP ioctl, so the caller buffers will not be locked.

> 
> And I also wonder whether we need to take rlimit(RLIMIT_MEMLOCK) into
> account.
> 

Maybe. I'll look at that.

  Paul

> -boris
> 
>