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

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Date:	Thu, 12 May 2016 13:14:34 +0300
From:	Felipe Balbi <felipe.balbi@...ux.intel.com>
To:	"Du\, Changbin" <changbin.du@...el.com>,
	Al Viro <viro@...iv.linux.org.uk>
Cc:	"gregkh\@linuxfoundation.org" <gregkh@...uxfoundation.org>,
	"mina86\@mina86.com" <mina86@...a86.com>,
	"rui.silva\@linaro.org" <rui.silva@...aro.org>,
	"k.opasiak\@samsung.com" <k.opasiak@...sung.com>,
	"lars\@metafoo.de" <lars@...afoo.de>,
	"linux-usb\@vger.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] usb: gadget: f_fs: report error if excess data received


Hi,

"Du, Changbin" <changbin.du@...el.com> 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.

>> @@ -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?)

> "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.

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

-- 
balbi