diff mbox series

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

Message ID 20210128130441.11744-1-paul@xen.org (mailing list archive)
State New, archived
Headers show
Series [v2] xen-blkback: fix compatibility bug with single page rings | expand

Commit Message

Paul Durrant Jan. 28, 2021, 1:04 p.m. UTC
From: Paul Durrant <pdurrant@amazon.com>

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@amazon.com>
---
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Dongli Zhang <dongli.zhang@oracle.com>

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(-)

Comments

Dongli Zhang Jan. 29, 2021, 6:20 a.m. UTC | #1
On 1/28/21 5:04 AM, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@amazon.com>
> 
> 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@amazon.com>
> ---
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: "Roger Pau Monné" <roger.pau@citrix.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Dongli Zhang <dongli.zhang@oracle.com>
> 
> 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).

1819         if (info->nr_ring_pages > 1) {
1820                 err = xenbus_printf(xbt, dev->nodename, "ring-page-order",
"%u",
1821                                     ring_page_order);
1822                 if (err) {
1823                         message = "writing ring-page-order";
1824                         goto abort_transaction;
1825                 }
1826         }

Therefore, can we assume 'ring-page-order' can never be 0? Once we have
'ring-page-order' set, it should be >= 1 and we should read from "ring-ref%u"?

If the specification allows 'ring-page-order' to be zero with "ring-ref%u"
available, we should introduce 'multi_ref'.

Thank you very much!

Dongli Zhang


>  	/* All rings for this device. */
>  	struct xen_blkif_ring	*rings;
>  	unsigned int		nr_rings;
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index 9860d4842f36..6c5e9373e91c 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -998,14 +998,17 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
>  	for (i = 0; i < nr_grefs; i++) {
>  		char ring_ref_name[RINGREF_NAME_LEN];
>  
> -		snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
> +		if (blkif->multi_ref)
> +			snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
> +		else {
> +			WARN_ON(i != 0);
> +			snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref");
> +		}
> +
>  		err = xenbus_scanf(XBT_NIL, dir, ring_ref_name,
>  				   "%u", &ring_ref[i]);
>  
>  		if (err != 1) {
> -			if (nr_grefs == 1)
> -				break;
> -
>  			err = -EINVAL;
>  			xenbus_dev_fatal(dev, err, "reading %s/%s",
>  					 dir, ring_ref_name);
> @@ -1013,18 +1016,6 @@ static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
>  		}
>  	}
>  
> -	if (err != 1) {
> -		WARN_ON(nr_grefs != 1);
> -
> -		err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u",
> -				   &ring_ref[0]);
> -		if (err != 1) {
> -			err = -EINVAL;
> -			xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
> -			return err;
> -		}
> -	}
> -
>  	err = -ENOMEM;
>  	for (i = 0; i < nr_grefs * XEN_BLKIF_REQS_PER_PAGE; i++) {
>  		req = kzalloc(sizeof(*req), GFP_KERNEL);
> @@ -1129,10 +1120,15 @@ static int connect_ring(struct backend_info *be)
>  		 blkif->nr_rings, blkif->blk_protocol, protocol,
>  		 blkif->vbd.feature_gnt_persistent ? "persistent grants" : "");
>  
> -	ring_page_order = xenbus_read_unsigned(dev->otherend,
> -					       "ring-page-order", 0);
> -
> -	if (ring_page_order > xen_blkif_max_ring_order) {
> +	err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-page-order", "%u",
> +			   &ring_page_order);
> +	if (err != 1) {
> +		blkif->nr_ring_pages = 1;
> +		blkif->multi_ref = false;
> +	} else if (ring_page_order <= xen_blkif_max_ring_order) {
> +		blkif->nr_ring_pages = 1 << ring_page_order;
> +		blkif->multi_ref = true;
> +	} else {
>  		err = -EINVAL;
>  		xenbus_dev_fatal(dev, err,
>  				 "requested ring page order %d exceed max:%d",
> @@ -1141,8 +1137,6 @@ static int connect_ring(struct backend_info *be)
>  		return err;
>  	}
>  
> -	blkif->nr_ring_pages = 1 << ring_page_order;
> -
>  	if (blkif->nr_rings == 1)
>  		return read_per_ring_refs(&blkif->rings[0], dev->otherend);
>  	else {
>
Juergen Gross Jan. 29, 2021, 7:35 a.m. UTC | #2
On 29.01.21 07:20, Dongli Zhang wrote:
> 
> 
> On 1/28/21 5:04 AM, Paul Durrant wrote:
>> From: Paul Durrant <pdurrant@amazon.com>
>>
>> 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@amazon.com>
>> ---
>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>> Cc: "Roger Pau Monné" <roger.pau@citrix.com>
>> Cc: Jens Axboe <axboe@kernel.dk>
>> Cc: Dongli Zhang <dongli.zhang@oracle.com>
>>
>> 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).

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.


Juergen
Dongli Zhang Jan. 30, 2021, 5:09 a.m. UTC | #3
On 1/29/21 12:13 AM, Paul Durrant wrote:
>> -----Original Message-----
>> From: Jürgen Groß <jgross@suse.com>
>> Sent: 29 January 2021 07:35
>> To: Dongli Zhang <dongli.zhang@oracle.com>; Paul Durrant <paul@xen.org>; xen-
>> devel@lists.xenproject.org; linux-block@vger.kernel.org; linux-kernel@vger.kernel.org
>> Cc: Paul Durrant <pdurrant@amazon.com>; Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>; Roger Pau
>> Monné <roger.pau@citrix.com>; Jens Axboe <axboe@kernel.dk>
>> 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@amazon.com>
>>>>
>>>> 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@amazon.com>
>>>> ---
>>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>>> Cc: "Roger Pau Monné" <roger.pau@citrix.com>
>>>> Cc: Jens Axboe <axboe@kernel.dk>
>>>> Cc: Dongli Zhang <dongli.zhang@oracle.com>
>>>>
>>>> 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@oracle.com>

Dongli ZHang
Paul Durrant Feb. 2, 2021, 9:36 a.m. UTC | #4
> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@lists.xenproject.org> On Behalf Of Dongli Zhang
> Sent: 30 January 2021 05:09
> To: paul@xen.org; 'Jürgen Groß' <jgross@suse.com>; xen-devel@lists.xenproject.org; linux-
> block@vger.kernel.org; linux-kernel@vger.kernel.org
> Cc: 'Paul Durrant' <pdurrant@amazon.com>; 'Konrad Rzeszutek Wilk' <konrad.wilk@oracle.com>; 'Roger Pau
> Monné' <roger.pau@citrix.com>; 'Jens Axboe' <axboe@kernel.dk>
> Subject: Re: [PATCH v2] xen-blkback: fix compatibility bug with single page rings
> 
> 
> 
> On 1/29/21 12:13 AM, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Jürgen Groß <jgross@suse.com>
> >> Sent: 29 January 2021 07:35
> >> To: Dongli Zhang <dongli.zhang@oracle.com>; Paul Durrant <paul@xen.org>; xen-
> >> devel@lists.xenproject.org; linux-block@vger.kernel.org; linux-kernel@vger.kernel.org
> >> Cc: Paul Durrant <pdurrant@amazon.com>; Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>; Roger Pau
> >> Monné <roger.pau@citrix.com>; Jens Axboe <axboe@kernel.dk>
> >> 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@amazon.com>
> >>>>
> >>>> 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@amazon.com>
> >>>> ---
> >>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> >>>> Cc: "Roger Pau Monné" <roger.pau@citrix.com>
> >>>> Cc: Jens Axboe <axboe@kernel.dk>
> >>>> Cc: Dongli Zhang <dongli.zhang@oracle.com>
> >>>>
> >>>> 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@oracle.com>
> 

Thanks.

Roger, Konrad, can I get a maintainer ack or otherwise, please?

  Paul
Roger Pau Monné Feb. 2, 2021, 4:28 p.m. UTC | #5
On Thu, Jan 28, 2021 at 01:04:41PM +0000, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@amazon.com>
> 
> 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@amazon.com>
> ---
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: "Roger Pau Monné" <roger.pau@citrix.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Dongli Zhang <dongli.zhang@oracle.com>
> 
> 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;

You seem to have used spaces between the type and the variable name
here, while neighbors also use hard tabs.

The rest LGTM:

Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

We should have forbidden the usage of ring-page-order = 0 and we could
have avoided having to add the multi_ref variable, but that's too late
now.

Thanks, Roger.
Paul Durrant Feb. 2, 2021, 4:42 p.m. UTC | #6
> -----Original Message-----
> From: Roger Pau Monné <roger.pau@citrix.com>
> Sent: 02 February 2021 16:29
> To: Paul Durrant <paul@xen.org>
> Cc: xen-devel@lists.xenproject.org; linux-block@vger.kernel.org; linux-kernel@vger.kernel.org; Paul
> Durrant <pdurrant@amazon.com>; Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>; Jens Axboe
> <axboe@kernel.dk>; Dongli Zhang <dongli.zhang@oracle.com>
> Subject: Re: [PATCH v2] xen-blkback: fix compatibility bug with single page rings
> 
> On Thu, Jan 28, 2021 at 01:04:41PM +0000, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@amazon.com>
> >
> > 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@amazon.com>
> > ---
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> > Cc: "Roger Pau Monné" <roger.pau@citrix.com>
> > Cc: Jens Axboe <axboe@kernel.dk>
> > Cc: Dongli Zhang <dongli.zhang@oracle.com>
> >
> > 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;
> 
> You seem to have used spaces between the type and the variable name
> here, while neighbors also use hard tabs.
> 

Oops. Xen vs. Linux coding style :-( I'll send a v3 with the whitespace fixed.

> The rest LGTM:
> 
> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
> 
> We should have forbidden the usage of ring-page-order = 0 and we could
> have avoided having to add the multi_ref variable, but that's too late
> now.

Thanks. Yes, that cat is out of the bag and has been for a while unfortunately.

  Paul

> 
> Thanks, Roger.
diff mbox series

Patch

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;
 	/* All rings for this device. */
 	struct xen_blkif_ring	*rings;
 	unsigned int		nr_rings;
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 9860d4842f36..6c5e9373e91c 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -998,14 +998,17 @@  static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
 	for (i = 0; i < nr_grefs; i++) {
 		char ring_ref_name[RINGREF_NAME_LEN];
 
-		snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
+		if (blkif->multi_ref)
+			snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref%u", i);
+		else {
+			WARN_ON(i != 0);
+			snprintf(ring_ref_name, RINGREF_NAME_LEN, "ring-ref");
+		}
+
 		err = xenbus_scanf(XBT_NIL, dir, ring_ref_name,
 				   "%u", &ring_ref[i]);
 
 		if (err != 1) {
-			if (nr_grefs == 1)
-				break;
-
 			err = -EINVAL;
 			xenbus_dev_fatal(dev, err, "reading %s/%s",
 					 dir, ring_ref_name);
@@ -1013,18 +1016,6 @@  static int read_per_ring_refs(struct xen_blkif_ring *ring, const char *dir)
 		}
 	}
 
-	if (err != 1) {
-		WARN_ON(nr_grefs != 1);
-
-		err = xenbus_scanf(XBT_NIL, dir, "ring-ref", "%u",
-				   &ring_ref[0]);
-		if (err != 1) {
-			err = -EINVAL;
-			xenbus_dev_fatal(dev, err, "reading %s/ring-ref", dir);
-			return err;
-		}
-	}
-
 	err = -ENOMEM;
 	for (i = 0; i < nr_grefs * XEN_BLKIF_REQS_PER_PAGE; i++) {
 		req = kzalloc(sizeof(*req), GFP_KERNEL);
@@ -1129,10 +1120,15 @@  static int connect_ring(struct backend_info *be)
 		 blkif->nr_rings, blkif->blk_protocol, protocol,
 		 blkif->vbd.feature_gnt_persistent ? "persistent grants" : "");
 
-	ring_page_order = xenbus_read_unsigned(dev->otherend,
-					       "ring-page-order", 0);
-
-	if (ring_page_order > xen_blkif_max_ring_order) {
+	err = xenbus_scanf(XBT_NIL, dev->otherend, "ring-page-order", "%u",
+			   &ring_page_order);
+	if (err != 1) {
+		blkif->nr_ring_pages = 1;
+		blkif->multi_ref = false;
+	} else if (ring_page_order <= xen_blkif_max_ring_order) {
+		blkif->nr_ring_pages = 1 << ring_page_order;
+		blkif->multi_ref = true;
+	} else {
 		err = -EINVAL;
 		xenbus_dev_fatal(dev, err,
 				 "requested ring page order %d exceed max:%d",
@@ -1141,8 +1137,6 @@  static int connect_ring(struct backend_info *be)
 		return err;
 	}
 
-	blkif->nr_ring_pages = 1 << ring_page_order;
-
 	if (blkif->nr_rings == 1)
 		return read_per_ring_refs(&blkif->rings[0], dev->otherend);
 	else {