Re: [Qemu-devel] [PATCH 0/2] virtio len fixes for qemu.

[Rusty Russell]

"Michael S. Tsirkin" <address@hidden> writes:
> On Mon, Mar 16, 2015 at 01:44:22PM +1030, Rusty Russell wrote:
>> diff --git a/content.tex b/content.tex
>> index 6ba079d..2c946a5 100644
>> --- a/content.tex
>> +++ b/content.tex
>> @@ -600,10 +600,19 @@ them: it is only written to by the device, and read by 
>> the driver.
>>  Each entry in the ring is a pair: \field{id} indicates the head entry of the
>>  descriptor chain describing the buffer (this matches an entry
>>  placed in the available ring by the guest earlier), and \field{len} the 
>> total
>> -of bytes written into the buffer. The latter is extremely useful
>> -for drivers using untrusted buffers: if you do not know exactly
>> -how much has been written by the device, you usually have to zero
>> -the buffer to ensure no data leakage occurs.
>> +of bytes written into the buffer. 
>> +
>> +\begin{note}
>> +\field{len} is useful
>> +for drivers using untrusted buffers: if a driver does not know exactly
>> +how much has been written by the device, the driver would have to zero
>> +the buffer in advance to ensure no data leakage occurs.
>> +
>> +For example, a network driver may hand a received buffer directly to
>> +an unprivileged userspace application.  If the network device has not
>> +overwritten the bytes which were in that buffer, this may leak the
>> +contents of freed memory from other processes to the application.
>> +\end{note}
>>  
>>  \begin{note}
>>  The legacy \hyperref[intro:Virtio PCI Draft]{[Virtio PCI Draft]}
>> @@ -612,6 +621,28 @@ the constant as VRING_USED_F_NO_NOTIFY, but the layout 
>> and value were
>>  identical.
>>  \end{note}
>>  
>> +\devicenormative{\subsubsection}{Virtqueue Notification Suppression}{Basic 
>> Facilities of a Virtio Device / Virtqueues / The Virtqueue Used Ring}
>> +
>> +The device MUST set \field{len} prior to updating the used \field{idx}.
>> +
>> +The device MUST write at least \field{len} bytes to descriptor,
>> +beginning at the first device-writable buffer,
>> +prior to updating the used \field{idx}.
>> +
>> +The device MAY write more than \field{len} bytes to descriptor.
>> +
>> +\begin{note}
>> +There are potential error cases where a device might not know what
>> +parts of the buffers have been written.  This is why \field{len} may
>> +be an underestimate, but that's preferable to the driver believing
>> +that uninitialized memory has been overwritten when it has not.
>> +\end{note}
>> +
>> +\drivernormative{\subsubsection}{Virtqueue Notification Suppression}{Basic 
>> Facilities of a Virtio Device / Virtqueues / The Virtqueue Used Ring}
>> +
>> +The driver MUST NOT make assumptions about data in device-writable buffers
>> +beyond the first \field{len} bytes, and SHOULD ignore it.
>
> it -> this data.
>
> Otherwise on first reading I thought "it" refers to len field.

Thanks, fixed.

>> +
>>  \subsection{Virtqueue Notification Suppression}\label{sec:Basic Facilities 
>> of a Virtio Device / Virtqueues / Virtqueue Notification Suppression}
>>  
>>  The device can suppress notifications in a manner analogous to the way
>
> Sounds good, let's move discussion to virtio/virtio-dev now?
> I think it's 1.1 material - agree?

Moving.  My intent was not to change the spec, but to clarify it.  It's a
bug in the spec that we didn't spell out what len means, but left it by
implication.

I'm OK with leaving it for 1.1 if you think this fix is too much, as
long as implementations don't leave fixing it until 1.1 :)

Thanks,
Rusty.