[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: [PATCH v2] xen-blkback: fix compatibility bug with single page rings
> -----Original Message----- > From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of Dongli > Zhang > Sent: 30 January 2021 05:09 > To: paul@xxxxxxx; 'Jürgen Groß' <jgross@xxxxxxxx>; > 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 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> > Thanks. Roger, Konrad, can I get a maintainer ack or otherwise, please? Paul
|
Lists.xenproject.org is hosted with RackSpace, monitoring our |