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 17:45
> 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 11:28 AM, Paul Durrant wrote:
> >> -----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?
> 
> How about something like (with rather arbitrary values)
> 
> #define PRIVCMD_DMOP_MAX_NUM_BUFFERS       16
> #define PRIVCMD_DMOP_MAX_TOT_BUFFER_SZ     4096
> 
> and make them part of the interface (i.e. put them into privcmd.h)?

Given that the values are arbitrary, I think it may be better to make them module params. They can then at least be tweaked if privcmd becomes a problem with later dm_ops.

  Paul

> 
> -boris