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

From: Paul Durrant
Date: Mon Feb 13 2017 - 04:04:07 EST


> -----Original Message-----
> From: Boris Ostrovsky [mailto:boris.ostrovsky@xxxxxxxxxx]
> Sent: 10 February 2017 17:45
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx
> Cc: Juergen Gross <jgross@xxxxxxxx>
> 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@xxxxxxxxxx]
> >> Sent: 10 February 2017 16:18
> >> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxxxxxxxxx;
> >> linux-kernel@xxxxxxxxxxxxxxx
> >> Cc: Juergen Gross <jgross@xxxxxxxx>
> >> 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