Re: [PATCH v2] xen-blkback: fix compatibility bug with single page rings

[Dongli Zhang]

On 1/29/21 12:13 AM, Paul Durrant wrote:
>> -----Original Message-----
>> From: Jürgen Groß <jgross@xxxxxxxx>
>> Sent: 29 January 2021 07:35
>> To: Dongli Zhang <dongli.zhang@xxxxxxxxxx>; Paul Durrant <paul@xxxxxxx>; xen-
>> devel@xxxxxxxxxxxxxxxxxxxx; linux-block@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
>> Cc: Paul Durrant <pdurrant@xxxxxxxxxx>; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Roger Pau
>> Monné <roger.pau@xxxxxxxxxx>; Jens Axboe <axboe@xxxxxxxxx>
>> Subject: Re: [PATCH v2] xen-blkback: fix compatibility bug with single page rings
>>
>> On 29.01.21 07:20, Dongli Zhang wrote:
>>>
>>>
>>> On 1/28/21 5:04 AM, 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 (i.e. ring-page-order = 0)
>>>> - 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.
>>>>
>>>> Fixes: 4a8c31a1c6f5 ("xen/blkback: rework connect_ring() to avoid inconsistent xenstore 'ring-page-
>> order' set by malicious blkfront")
>>>> Signed-off-by: Paul Durrant <pdurrant@xxxxxxxxxx>
>>>> ---
>>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>>>> Cc: "Roger Pau Monné" <roger.pau@xxxxxxxxxx>
>>>> Cc: Jens Axboe <axboe@xxxxxxxxx>
>>>> Cc: Dongli Zhang <dongli.zhang@xxxxxxxxxx>
>>>>
>>>> v2:
>>>>   - Remove now-spurious error path special-case when nr_grefs == 1
>>>> ---
>>>>   drivers/block/xen-blkback/common.h |  1 +
>>>>   drivers/block/xen-blkback/xenbus.c | 38 +++++++++++++-----------------
>>>>   2 files changed, 17 insertions(+), 22 deletions(-)
>>>>
>>>> diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
>>>> index b0c71d3a81a0..524a79f10de6 100644
>>>> --- a/drivers/block/xen-blkback/common.h
>>>> +++ b/drivers/block/xen-blkback/common.h
>>>> @@ -313,6 +313,7 @@ struct xen_blkif {
>>>>
>>>>   	struct work_struct	free_work;
>>>>   	unsigned int 		nr_ring_pages;
>>>> +	bool                    multi_ref;
>>>
>>> Is it really necessary to introduce 'multi_ref' here or we may just re-use
>>> 'nr_ring_pages'?
>>>
>>> According to blkfront code, 'ring-page-order' is set only when it is not zero,
>>> that is, only when (info->nr_ring_pages > 1).
>>
> 
> That's how it is *supposed* to be. Windows certainly behaves that way too.
> 
>> Did you look into all other OS's (Windows, OpenBSD, FreebSD, NetBSD,
>> Solaris, Netware, other proprietary systems) implementations to verify
>> that claim?
>>
>> I don't think so. So better safe than sorry.
>>
> 
> Indeed. It was unfortunate that the commit to blkif.h documenting multi-page (829f2a9c6dfae) was not crystal clear and (possibly as a consequence) blkback was implemented to read ring-ref0 rather than ring-ref if ring-page-order was present and 0. Hence the only safe thing to do is to restore that behaviour.
> 

Thank you very much for the explanation!

Reviewed-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>

Dongli ZHang