Re: [Xen-devel] [PATCH 1/2] IOMMU/MMU: Adjust top level functions for VT-d Device-TLB flush error.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.03.16 at 05:33, <quan.xu@xxxxxxxxx> wrote:
> On March 18, 2016 1:15am, <JBeulich@xxxxxxxx> wrote:
>> >>> On 17.03.16 at 07:54, <quan.xu@xxxxxxxxx> wrote:
>> > --- a/xen/common/grant_table.c
>> > +++ b/xen/common/grant_table.c
>> > @@ -932,8 +932,9 @@ __gnttab_map_grant_ref(
>> >              {
>> >                  nr_gets++;
>> >                  (void)get_page(pg, rd);
>> > -                if ( !(op->flags & GNTMAP_readonly) )
>> > -                    get_page_type(pg, PGT_writable_page);
>> > +                if ( !(op->flags & GNTMAP_readonly) &&
>> > +                     !get_page_type(pg, PGT_writable_page) )
>> > +                        goto could_not_pin;
>> 
>> This needs explanation, as it doesn't look related to what your actual goal 
>> is: If
>> an error was possible here, I think this would be a security issue. However, 
>> as
>> also kind of documented by the explicitly ignored return value from 
>> get_page(),
>> it is my understanding there here we only obtain an _extra_ reference.
>> 
> 
> For this point, I inferred from:
> map_vcpu_info()
> {
> ...
>     if ( !get_page_type(page, PGT_writable_page) )
>     {
>         put_page(page);
>         return -EINVAL;
>     }
> ...
> }
> , then for get_page_type(), I think the return value:
>      0 -- error, 
>      1-- right.
> 
> So if get_page_type() is failed, we should goto could_not_pin.

Did you read my reply at all? The explanation I'm expecting here is
why error checking is all of the sudden needed _at all_.

> btw, there is another issue in the call path:
>     iommu_{,un}map_page() -- __get_page_type() -- get_page_type()---
> 
> 
> I tried to return iommu_{,un}map_page() error code in __get_page_type(), is 
> it right?

If the operation got fully rolled back - yes. Whether fully rolling back
is feasible there though is - see the respective discussion - an open
question.

>> > --- a/xen/drivers/passthrough/x86/iommu.c
>> > +++ b/xen/drivers/passthrough/x86/iommu.c
>> > @@ -104,7 +104,11 @@ int arch_iommu_populate_page_table(struct
>> domain *d)
>> >      this_cpu(iommu_dont_flush_iotlb) = 0;
>> >
>> >      if ( !rc )
>> > -        iommu_iotlb_flush_all(d);
>> > +    {
>> > +        rc = iommu_iotlb_flush_all(d);
>> > +        if ( rc )
>> > +            iommu_teardown(d);
>> > +    }
>> >      else if ( rc != -ERESTART )
>> >          iommu_teardown(d);
>> 
>> Why can't you just use the existing call to iommu_teardown(), by simply 
> deleting
>> the "else"?
>> 
> 
> Just check it, could I modify it as below:
> --- a/xen/drivers/passthrough/x86/iommu.c
> +++ b/xen/drivers/passthrough/x86/iommu.c
> @@ -105,7 +105,8 @@ int arch_iommu_populate_page_table(struct domain *d)
> 
>      if ( !rc )
>          iommu_iotlb_flush_all(d);
> -    else if ( rc != -ERESTART )
> +
> +    if ( rc != -ERESTART )
>          iommu_teardown(d);

Clearly not - not only are you losing the return value of
iommu_iotlb_flush_all() now, you would then also call
iommu_teardown() in the "success" case. My comment was
related to code structure, yet you seem to have taken it
literally.

Jan


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