Re: [PATCH] xen-blkback: fix compatibility bug with single page rings

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.01.2021 12:09, Paul Durrant wrote:
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@xxxxxxxx>
>> Sent: 27 January 2021 10:57
>> To: Paul Durrant <paul@xxxxxxx>
>> Cc: Paul Durrant <pdurrant@xxxxxxxxxx>; Konrad Rzeszutek Wilk 
>> <konrad.wilk@xxxxxxxxxx>; Roger Pau
>> Monné <roger.pau@xxxxxxxxxx>; Jens Axboe <axboe@xxxxxxxxx>; Dongli Zhang 
>> <dongli.zhang@xxxxxxxxxx>;
>> linux-kernel@xxxxxxxxxxxxxxx; linux-block@xxxxxxxxxxxxxxx; 
>> xen-devel@xxxxxxxxxxxxxxxxxxxx
>> Subject: Re: [PATCH] xen-blkback: fix compatibility bug with single page 
>> rings
>>
>> On 27.01.2021 11:30, Paul Durrant wrote:
>>> From: Paul Durrant <pdurrant@xxxxxxxxxx>
>>>
>>> Prior to commit 4a8c31a1c6f5 ("xen/blkback: rework connect_ring() to avoid
>>> inconsistent xenstore 'ring-page-order' set by malicious blkfront"), the
>>> behaviour of xen-blkback when connecting to a frontend was:
>>>
>>> - read 'ring-page-order'
>>> - if not present then expect a single page ring specified by 'ring-ref'
>>> - else expect a ring specified by 'ring-refX' where X is between 0 and
>>>   1 << ring-page-order
>>>
>>> This was correct behaviour, but was broken by the afforementioned commit to
>>> become:
>>>
>>> - read 'ring-page-order'
>>> - if not present then expect a single page ring
>>> - expect a ring specified by 'ring-refX' where X is between 0 and
>>>   1 << ring-page-order
>>> - if that didn't work then see if there's a single page ring specified by
>>>   'ring-ref'
>>>
>>> This incorrect behaviour works most of the time but fails when a frontend
>>> that sets 'ring-page-order' is unloaded and replaced by one that does not
>>> because, instead of reading 'ring-ref', xen-blkback will read the stale
>>> 'ring-ref0' left around by the previous frontend will try to map the wrong
>>> grant reference.
>>>
>>> This patch restores the original behaviour.
>>
>> Isn't this only the 2nd of a pair of fixes that's needed, the
>> first being the drivers, upon being unloaded, to fully clean up
>> after itself? Any stale key left may lead to confusion upon
>> re-use of the containing directory.
> 
> In a backend we shouldn't be relying on, nor really expect IMO, a frontend to 
> clean up after itself. Any backend should know *exactly* what xenstore nodes 
> it’s looking for from a frontend.

But the backend can't know whether a node exists because the present
frontend has written it, or because an earlier instance forgot to
delete it. It can only honor what's there. (In fact the other day I
was wondering whether some of the writes of boolean "false" nodes
wouldn't better be xenbus_rm() instead.)

Jan