Re: [RFC PATCH] ring: Forbid using descriptors in two chains at once

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Message-ID: <[email protected]>
Date: Sun, 17 May 2026 22:37:56 -0400
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Subject: Re: [RFC PATCH] ring: Forbid using descriptors in two chains at once
To: "Michael S. Tsirkin" <[email protected]>
Cc: [email protected]
References: <[email protected]>
 <[email protected]>
 <[email protected]>
 <[email protected]>
Content-Language: en-US
From: Demi Marie Obenour <[email protected]>
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In-Reply-To: <[email protected]>

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On 5/17/26 20:59, Michael S. Tsirkin wrote:
> On Sun, May 17, 2026 at 04:05:28PM -0400, Demi Marie Obenour wrote:
>> On 5/7/26 01:50, Michael S. Tsirkin wrote:
>>> On Thu, May 07, 2026 at 12:34:19AM -0400, Demi Marie Obenour wrote:
>>>> Using the same descriptor in multiple chains at once creates
>>>> difficulties.  If the descriptor is device-writeable, this is beggin=
g
>>>> for data corruption due to conflicting writes from the device.  Even=
 if
>>>> the descriptor is device-readable, the mere possibility requires dev=
ices
>>>> to use more sophisticated internal data structures.
>>>
>>> The natural way to index requests is by using the avail ring head,
>>> not descriptors, though? I suspect it's about device memory being
>>> limited, etc. But pls make it more explicit.
>>
>> It's about being able to *not* use scatter-gather I/O, and instead use=

>> a single I/O request for each descriptor.  io_uring makes this very
>> cheap.  However, it requires that the lookup key be (avail ring head,
>> descriptor index) rather than just (avail ring head).
>>
>> The latter has sufficiently few possibilities one can just use
>> a bitmap.  The former isn't.  This means using a memory pool, heap
>> allocation, or associative container.
>>
>>>> Require descriptors to be used in at most one chain at any given tim=
e.
>>>> Indirect descriptors can be used if support for huge batches is need=
ed.
>>>
>>> does this imply indirect are exempt from this rule? the text
>>> you propose makes no such exemptions.
>>
>> It means that indirect descriptors can point to huge numbers of
>> descriptors, rather than just what fits in the descriptor ring.
>=20
> Nope, the spec limits the chain size to ring size.

Oh, I see.

>>>> Signed-off-by: Demi Marie Obenour <[email protected]>
>>>> ---
>>>> I have not checked whether this will break existing drivers, so I'm
>>>> marking this as RFC for now.
>>>> ---
>>>>  packed-ring.tex | 6 ++++++
>>>>  split-ring.tex  | 6 ++++++
>>>>  2 files changed, 12 insertions(+)
>>>>
>>>> diff --git a/packed-ring.tex b/packed-ring.tex
>>>> index 3ee55a18d024d4da1a48d522e98ea563b9809b1c..8fd5a770c7ca4e6f6091=
1b2861e00a273a69ea73 100644
>>>> --- a/packed-ring.tex
>>>> +++ b/packed-ring.tex
>>>> @@ -473,6 +473,12 @@ \subsection{Event Suppression Structure Format}=
\label{sec:Basic
>>>> =20
>>>>  This implies that loops in the descriptor list are forbidden!
>>>> =20
>>>> +Drivers MUST NOT include a descriptor in a chain if the
>>>> +descriptor is already part of a previous chain whose processing
>>>> +has not completed.  This implies that the total number of
>>>> +descriptors submitted to the device, but not yet processed, is
>>>> +limited by the queue size.
>>>
>>> I do not understand what is this trying to say for the packed ring.
>>> How do you include a descriptor in a chain twice - the ring has to
>>> wrap around, no?
>>
>> I don't think you can.
>>
>>> For that matter, I do not think "This implies that loops in the
>>> descriptor list are forbidden!" makes sense for packed either -
>>> I think we copied it from split by mistake.
>>
>> Fair.
>>
>>>> +
>>>>  The driver MUST place any device-writable descriptor elements after=

>>>>  any device-readable descriptor elements.
>>>> =20
>>>> diff --git a/split-ring.tex b/split-ring.tex
>>>> index de9403882df1e6d20e16ec983ecf0a46d39efa6b..014e279ef1da7653c705=
1e3a24c6169fe8d51983 100644
>>>> --- a/split-ring.tex
>>>> +++ b/split-ring.tex
>>>> @@ -223,6 +223,12 @@ \subsection{The Virtqueue Descriptor Table}\lab=
el{sec:Basic Facilities of a Virt
>>>>  Drivers MUST NOT add a descriptor chain longer than $2^{32}$ bytes =
in total;
>>>>  this implies that loops in the descriptor chain are forbidden!
>>>> =20
>>>> +Drivers MUST NOT include a descriptor in a chain if the
>>>> +descriptor is already part of a previous chain whose processing
>>>> +has not completed.
>>>
>>> what specifically is "include a descriptor in a chain"? something to =
do
>>> with making a head available? and what is "whose processing has not
>>> completed"? something to do with a head not used?
>>>
>>>>  This implies that the total number of
>>>> +descriptors submitted to the device, but not yet processed, is
>>>> +limited by the queue size.
>>>> +
>>>>  If VIRTIO_F_IN_ORDER has been negotiated, and when making a
>>>>  descriptor with VRING_DESC_F_NEXT set in \field{flags} at offset
>>>>  $x$ in the table available to the device, driver MUST set
>>>
>>> We can't really introduce new mandatory requirements without
>>> a feature bit.
>>
>> I'm making the assumption that no existing driver actually violates
>> this rule.  That said, another option would be to force any driver
>> that does violate it onto a slow path, perhaps by serializing the
>> I/O operations.
>>
>> As an aside, this is why the spec I am working on for the vhost-user
>> device backend has seemingly arbitrary restrictions around MSI-X
>> interrupts.  It makes life easier for device authors.
>>
>> Finally, I understand if this isn't viable.  It can be worked
>> around in devices without *too* much complexity.
>> --=20
>> Sincerely,
>> Demi Marie Obenour (she/her/hers)
>=20
> We have two options:
> - a new feature bit + if negotiated, driver MUST.
>   device can decide if it wants to support drivers without a feature bi=
t
> - a recommendation (driver SHOULD)

I'm going to drop this.  Neither removes any complexity from device
implementations that need to work with existing drivers.

My hope was that no existing driver actually relied on being able to
use a descriptor in two chains at once.  If that is true, disallowing
it would not break any code in the wild.

However, that only works if one can enumerate the existing drivers
that are out there.  There might be proprietary drivers deployed
in closed environments that never see the light of day.  So this
reasoning doesn't work.

This leaves otherwise-unfixable security holes as the only
justification I can think of for this kind of change.
--=20
Sincerely,
Demi Marie Obenour (she/her/hers)
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