Re: [for-4.15][PATCH v2 3/5] xen/iommu: iommu_map: Don't crash the domain if it is dying

[Julien Grall <julien@xen.org>]

Hi Jan,

On 10/02/2021 14:14, Jan Beulich wrote:
> On 09.02.2021 22:14, Julien Grall wrote:
>> On Tue, 9 Feb 2021 at 20:28, Paul Durrant <xadimgnik@gmail.com> wrote:
>>>> From: Julien Grall <julien@xen.org>
>>>> Sent: 09 February 2021 15:28
>>>>
>>>> It is a bit pointless to crash a domain that is already dying. This will
>>>> become more an annoyance with a follow-up change where page-table
>>>> allocation will be forbidden when the domain is dying.
>>>>
>>>> Security wise, there is no change as the devices would still have access
>>>> to the IOMMU page-tables even if the domain has crashed until Xen
>>>> start to relinquish the resources.
>>>>
>>>> For x86, we rely on dom_iommu(d)->arch.mapping.lock to ensure
>>>> d->is_dying is correctly observed (a follow-up patch will held it in the
>>>> relinquish path).
> 
> Am I to understand this to mean that at this point of the series
> things aren't really correct yet in this regard? If so, wouldn't
> it be better to re-order?

You asked this specific order... So are you saying you want me to use 
the original ordering?

> 
>>>> For Arm, there is still a small race possible. But there is so far no
>>>> failure specific to a domain dying.
>>>>
>>>> Signed-off-by: Julien Grall <jgrall@amazon.com>
>>>>
>>>> ---
>>>>
>>>> This was spotted when trying to destroy IOREQ servers while the domain
>>>> is dying. The code will try to add the entry back in the P2M and
>>>> therefore update the P2M (see arch_ioreq_server_disable() ->
>>>> hvm_add_ioreq_gfn()).
>>>>
>>>> It should be possible to skip the mappin in hvm_add_ioreq_gfn(), however
>>>> I didn't try a patch yet because checking d->is_dying can be racy (I
>>>> can't find a proper lock).
> 
> I understand the concern. I find it odd though that we permit
> iommu_map() to do anything at all when the domain is already
> dying. So irrespective of the remark below, how about bailing
> from iommu_map() earlier when the domain is dying?

I felt this was potentially too racy to use it. But it should be fine if 
keep the !d->is_dying below.

> 
>>>> --- a/xen/drivers/passthrough/iommu.c
>>>> +++ b/xen/drivers/passthrough/iommu.c
>>>> @@ -272,7 +272,7 @@ int iommu_map(struct domain *d, dfn_t dfn, mfn_t mfn,
>>>>                               flush_flags) )
>>>>                   continue;
>>>>
>>>> -        if ( !is_hardware_domain(d) )
>>>> +        if ( !is_hardware_domain(d) && !d->is_dying )
>>>>               domain_crash(d);
>>>
>>> Would it make more sense to check is_dying inside domain_crash() (and turn it into a no-op in that case)?
>>
>> Jan also suggested moving the check in domain_crash(). However, I felt
>> it is potentially a too risky change for 4.15 as there are quite a few
>> callers.
> 
> This is a fair point. However, in such a case I'd prefer symmetry
> at least throughout this one source file (there are three more
> places), unless there are strong reasons against doing so.

I can have a look and see if the decision is easy to make.

Cheers,

-- 
Julien Grall