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| State | Superseded |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