RE: [PATCH] usb: gadget: f_fs: report error if excess data received

[Du, Changbin]

> Hi,
> 
> "Du, Changbin" <changbin.du@xxxxxxxxx> writes:
> >> >> > These all can lead host send more than device wanted bytes. For
> sure
> >> >> > it wrong at host side, but device side don't know.
> >> >>
> >> >> but none of this means we have a bug at device side. In fact, by
> >> >> allowing these extra bytes to reach userspace, we could be creating a
> >> >> possible attack vector.
> >> >>
> >> >> Your explanation is unsatisfactory, so I won't apply your patch, sorry.
> >> >>
> >> >> --
> >> >> balbi
> >> > It is fine. Then need userspace take care of all the data it received.
> Because
> >> > Kernel may drop some data for it. Kernel ffs driver is unauthentic
> >> sometimes.
> >>
> >> I really cannot understand what you mean sometimes. You're saying that
> >> userspace needs to take care of all the data it received because kernel
> >> can drop data. If kernel is dropping data, there's no extra data
> >> reaching userspace, right?
> >>
> > For sure, maybe I didn't describe it well so let you confused. :)
> 
> okay
> 
> >> Is the problem that we *are* giving more data than expected to
> >> userspace? Are we overflowing some userspace buffer? If that's the case,
> >> then below should be enough for the time being:
> >>
> > No, the problem is we drop data but silently. We cannot give more data to
> 
> okay, but does that create any problems for device side userspace? What
> problem is that?
> 
> > userspace since buffer is limited.
> 
> right, and that was my point: if we copy more to userspace, then we have
> a real big problem.
> 
Yes, we drop the data because we userspace buffer is not enough this time.
The problem here is that really can we just drop it silently? Maybe not.

> >> @@ -811,7 +815,12 @@ static ssize_t ffs_epfile_io(struct file *file, struct
> >> ffs_io_data *io_data)
> >>  		 */
> >>  		ret = interrupted ? -EINTR : ep->status;
> >>  		if (io_data->read && ret > 0) {
> >> -			ret = copy_to_iter(data, ret, &io_data->data);
> >> +			if (ret > io_data->expected_len)
> >> +				pr_debug("FFS: size mismatch: %zd for %zd",
> >> +						ret, io_data->expected_len);
> >> +
> >> +			ret = copy_to_iter(data, io_data->expected_len,
> >> +					&io_data->data);
> >>  			if (!ret)
> >>  				ret = -EFAULT;
> >>  		}
> >>
> >> that we can get merged during v4.7-rc and Cc stable and backport this to
> >> anything containing Al's commit c993c39b8639 ("gadget/function/f_fs.c:
> >> use put iov_iter into io_data").
> >>
> >
> > The different for this code is just give warning but not return
> > error. It is also fine for me that at least this let development can
> > find some key message to find What happed under kernel. But the
> > message should be *error* I think.
> 
> I'm fine with pr_error()
> 
> > And this missed AIO path. This is identify to my patch after remove the
> 
> right, it's more of a debug patch since I don't have the setup to
> trigger this (I'm assuming you're using adb?)
> 
Right. And adb can detect this unexpected behavior(data mismatch) quickly
because it has some selfcheck for the data content.

> > "return -EOVERFLOW;" line.
> 
> there's one key difference, see below
> 
> > Byw, we not need add the field "expected_len", we can get it from the
> > struct ffs_io_data.
> 
> without expected_len we can copy more data to userspace, right ? If
> req->actual > data_len_before_aligning_to_maxpacket, then we will copy
> more data then we should to userspace and this was a regression caused
> by Al's commit, AFAICT.
> 
No, expected_len equals to iov_iter_count(&io_data->data), right? So we
do not need a new field.

> > If this is fine for you, I can publish a new patch.
> >
> >> --
> >> Balbi
> >
> > Best Regards,
> > Du, Changbin
> 
> --
> balbi