Re: [PATCH v4] tee: Use iov_iter to better support shared buffer registration

[Sumit Garg]

Hi Arnaud,

On Mon, 4 Dec 2023 at 22:32, Arnaud POULIQUEN
<arnaud.pouliquen@xxxxxxxxxxx> wrote:
>
> Hi,
>
> On 12/4/23 17:40, Jens Axboe wrote:
> > On 12/4/23 9:36 AM, Jens Axboe wrote:
> >> On 12/4/23 5:42 AM, Sumit Garg wrote:
> >>> IMO, access_ok() should be the first thing that import_ubuf() or
> >>> import_single_range() should do, something as follows:
> >>>
> >>> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> >>> index 8ff6824a1005..4aee0371824c 100644
> >>> --- a/lib/iov_iter.c
> >>> +++ b/lib/iov_iter.c
> >>> @@ -1384,10 +1384,10 @@ EXPORT_SYMBOL(import_single_range);
> >>>
> >>>  int import_ubuf(int rw, void __user *buf, size_t len, struct iov_iter *i)
> >>>  {
> >>> -       if (len > MAX_RW_COUNT)
> >>> -               len = MAX_RW_COUNT;
> >>>         if (unlikely(!access_ok(buf, len)))
> >>>                 return -EFAULT;
> >>> +       if (len > MAX_RW_COUNT)
> >>> +               len = MAX_RW_COUNT;
> >>>
> >>>         iov_iter_ubuf(i, rw, buf, len);
> >>>         return 0;
> >>>
> >>> Jens A., Al Viro,
> >>>
> >>> Was there any particular reason which I am unaware of to perform
> >>> access_ok() check on modified input length?
> >>
> >> This change makes sense to me, and seems consistent with what is done
> >> elsewhere too.
> >
> > For some reason I missed import_single_range(), which does it the same
> > way as import_ubuf() currently does - cap the range before the
> > access_ok() check. The vec variants sum as they go, but access_ok()
> > before the range.
> >
> > I think part of the issue here is that the single range imports return 0
> > for success and -ERROR otherwise. This means that the caller does not
> > know if the full range was imported or not. OTOH, we always cap any data
> > transfer at MAX_RW_COUNT, so may make more sense to fix up the caller
> > here.
> >
>
> Should we limit to MAX_RW_COUNT or return an error?
> Seems to me that limiting could generate side effect later that could be not
> simple to debug.
>
>
> >>>  int import_ubuf(int rw, void __user *buf, size_t len, struct iov_iter *i)
> >>>  {
> >>> -       if (len > MAX_RW_COUNT)
> >>> +               return -EFAULT;
> >>>         if (unlikely(!access_ok(buf, len)))
> >>>                 return -EFAULT;
> >>>
> >>>         iov_iter_ubuf(i, rw, buf, len);
> >>>         return 0;
>
> or perhaps just remove the test as __access_ok() already tests that the
> size < TASK_SIZE
>
> https://elixir.bootlin.com/linux/v6.7-rc4/source/include/asm-generic/access_ok.h#L31
>

It looks like there are predefined constraints for using import_ubuf()
which doesn't properly match our needs. So let's directly use:
iov_iter_ubuf() instead.

-Sumit

>
> Thanks,
> Arnaud
>