Re: [PATCH v2] xen/arm: fix gnttab_need_iommu_mapping

[Rahul Singh <Rahul.Singh@xxxxxxx>]

Hello Stefano,

> On 10 Feb 2021, at 9:13 pm, Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:
> 
> On Wed, 10 Feb 2021, Rahul Singh wrote:
>>> On 9 Feb 2021, at 8:36 pm, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
>>> wrote:
>>> On Tue, 9 Feb 2021, Rahul Singh wrote:
>>>>> On 8 Feb 2021, at 6:49 pm, Stefano Stabellini <sstabellini@xxxxxxxxxx> 
>>>>> wrote:
>>>>> 
>>>>> Commit 91d4eca7add broke gnttab_need_iommu_mapping on ARM.
>>>>> The offending chunk is:
>>>>> 
>>>>> #define gnttab_need_iommu_mapping(d)                    \
>>>>> -    (is_domain_direct_mapped(d) && need_iommu(d))
>>>>> +    (is_domain_direct_mapped(d) && need_iommu_pt_sync(d))
>>>>> 
>>>>> On ARM we need gnttab_need_iommu_mapping to be true for dom0 when it is
>>>>> directly mapped and IOMMU is enabled for the domain, like the old check
>>>>> did, but the new check is always false.
>>>>> 
>>>>> In fact, need_iommu_pt_sync is defined as dom_iommu(d)->need_sync and
>>>>> need_sync is set as:
>>>>> 
>>>>>  if ( !is_hardware_domain(d) || iommu_hwdom_strict )
>>>>>      hd->need_sync = !iommu_use_hap_pt(d);
>>>>> 
>>>>> iommu_use_hap_pt(d) means that the page-table used by the IOMMU is the
>>>>> P2M. It is true on ARM. need_sync means that you have a separate IOMMU
>>>>> page-table and it needs to be updated for every change. need_sync is set
>>>>> to false on ARM. Hence, gnttab_need_iommu_mapping(d) is false too,
>>>>> which is wrong.
>>>>> 
>>>>> As a consequence, when using PV network from a domU on a system where
>>>>> IOMMU is on from Dom0, I get:
>>>>> 
>>>>> (XEN) smmu: /smmu@fd800000: Unhandled context fault: fsr=0x402, 
>>>>> iova=0x8424cb148, fsynr=0xb0001, cb=0
>>>>> [   68.290307] macb ff0e0000.ethernet eth0: DMA bus error: HRESP not OK
>>>>> 
>>>>> The fix is to go back to something along the lines of the old
>>>>> implementation of gnttab_need_iommu_mapping.
>>>>> 
>>>>> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
>>>>> Fixes: 91d4eca7add
>>>>> Backport: 4.12+
>>>>> 
>>>>> ---
>>>>> 
>>>>> Given the severity of the bug, I would like to request this patch to be
>>>>> backported to 4.12 too, even if 4.12 is security-fixes only since Oct
>>>>> 2020.
>>>>> 
>>>>> For the 4.12 backport, we can use iommu_enabled() instead of
>>>>> is_iommu_enabled() in the implementation of gnttab_need_iommu_mapping.
>>>>> 
>>>>> Changes in v2:
>>>>> - improve commit message
>>>>> - add is_iommu_enabled(d) to the check
>>>>> ---
>>>>> xen/include/asm-arm/grant_table.h | 2 +-
>>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>> 
>>>>> diff --git a/xen/include/asm-arm/grant_table.h 
>>>>> b/xen/include/asm-arm/grant_table.h
>>>>> index 6f585b1538..0ce77f9a1c 100644
>>>>> --- a/xen/include/asm-arm/grant_table.h
>>>>> +++ b/xen/include/asm-arm/grant_table.h
>>>>> @@ -89,7 +89,7 @@ int replace_grant_host_mapping(unsigned long gpaddr, 
>>>>> mfn_t mfn,
>>>>>   (((i) >= nr_status_frames(t)) ? INVALID_GFN : (t)->arch.status_gfn[i])
>>>>> 
>>>>> #define gnttab_need_iommu_mapping(d)                    \
>>>>> -    (is_domain_direct_mapped(d) && need_iommu_pt_sync(d))
>>>>> +    (is_domain_direct_mapped(d) && is_iommu_enabled(d))
>>>>> 
>>>>> #endif /* __ASM_GRANT_TABLE_H__ */
>>>> 
>>>> I tested the patch and while creating the guest I observed the below 
>>>> warning from Linux for block device.
>>>> https://elixir.bootlin.com/linux/v4.3/source/drivers/block/xen-blkback/xenbus.c#L258
>>> 
>>> So you are creating a guest with "xl create" in dom0 and you see the
>>> warnings below printed by the Dom0 kernel? I imagine the domU has a
>>> virtual "disk" of some sort.
>> 
>> Yes you are right I am trying to create the guest with "xl create” and 
>> before that, I created the logical volume and trying to attach the logical 
>> volume
>> block to the domain with “xl block-attach”. I observed this error with the 
>> "xl block-attach” command.
>> 
>> This issue occurs after applying this patch as what I observed this patch 
>> introduce the calls to iommu_legacy_{, un}map() to map the grant pages for
>> IOMMU that touches the page-tables. I am not sure but what I observed is 
>> that something is written wrong when iomm_unmap calls unmap the pages
>> because of that issue is observed.
>> 
>> You can reproduce the issue by just creating the dummy image file and try to 
>> attach it with “xl block-attach”
>> 
>> dd if=/dev/zero of=test.img bs=1024 count=0 seek=1024
>> xl block-attach 0 phy:test.img xvda w
>> 
>> Sequence of command that I follow is as follows to reproduce the issue:  
>> 
>> lvs vg-xen/myguest
>> lvcreate -y -L 4G -n myguest vg-xen
>> parted -s /dev/vg-xen/myguest mklabel msdos
>> parted -s /dev/vg-xen/myguest unit MB mkpart primary 1 4096
>> sync
>> xl block-attach 0 phy:/dev/vg-xen/myguest xvda w
> 
> Ah! You are block-attaching a device in dom0 to dom0! So frontend and
> backend are both in the same domain, dom0. Yeah that is not supposed to
> work, and if it did before it was just by coincidence :-)

I think this is very common to attach a block device to DOM0 where 
frontend/backend is in same domain. 
I found two reference that use the same concept

https://wiki.xenproject.org/wiki/Access_a_LVM-based_DomU_disk_outside_of_the_domU
https://www.suse.com/support/kb/doc/?id=000016154

Regards,
Rahul

> 
> There are a number of checks in Linux that wouldn't work as intended if
> the page is coming from itself. This is not an ARM-only issue either.
> 
> I tried it with dom0, like you did, and I am seeing the same warning.
> I
> did try to do block-attach to a domU and it works fine.
> 
> I don't think this is a concern.