virtio — archive

[PATCH] used ring: define the meaning and requirements of the len field.

From: Rusty Russell
To: [email protected]
Date: Fri, 20 Mar 2015 01:46:00 -0000

We said what it was for, and noted why. We didn't place any requirements on it, nor clearly spell out the implications of its use. This clarification comes particularly from noticing that QEMU didn't set len correctly, and philosophising over the correct value when an error has occurred. (Wording precision feedback from Michael and Cornelia - Thanks!) Signed-off-by: Rusty Russell <[email protected]> diff --git a/content.tex b/content.tex index 6ba079d..2ff8c65 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. + +egin{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 could leak the +contents of freed memory from other processes to the application. +end{note} egin{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. + +egin{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} is +permitted to be an underestimate: 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 this data. + 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