Re: [PATCH v1 2/3] xen/domain: fix UBSAN null pointer dereference in vcpu_info_reset()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.05.2026 13:32, Andrew Cooper wrote:
> On 19/05/2026 12:22 pm, Oleksii Kurochko wrote:
>> On 5/19/26 12:55 PM, Oleksii Kurochko wrote:
>>> On 5/19/26 11:37 AM, Jan Beulich wrote:
>>>> On 19.05.2026 10:39, Oleksii Kurochko wrote:
>>>>> vcpu_info_reset() maps v->vcpu_info_area.map to the per-vcpu slot
>>>>> inside
>>>>> the domain's shared_info page for vcpus with id <
>>>>> XEN_LEGACY_MAX_VCPUS,
>>>>> and falls back to dummy_vcpu_info for vcpus beyond that limit.
>>>>>
>>>>> However, it does not guard against d->shared_info being NULL.  The
>>>>> shared_info() macro expands to a member access through d->shared_info,
>>>>> so when an architecture does not allocate a shared_info page the
>>>>> dereference triggers UBSAN:
>>>>>    UBSAN: Undefined behaviour in common/domain.c:325:10
>>>>>    member access within null pointer of type 'struct shared_info_t'
>>>>>
>>>>> Extend the existing fallback condition to also cover the case where no
>>>>> shared_info page has been allocated, mapping the vcpu to
>>>>> dummy_vcpu_info
>>>>> instead. This is the correct behaviour: dummy_vcpu_info already serves
>>>>> as the safe stand-in for vcpus that have no usable shared_info slot.
>>>>>
>>>>> Fixes: 295514ff75506 ("common: convert vCPU info area registration")
>>>>
>>>> I question this, largely (but not only) because I also ...
>>>>
>>>>> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
>>>>> Reviewed-by: Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>
>>>>> ---
>>>>> RISC-V does not allocate a shared_info page at the momemnt because its
>>>>> guests run in dom0less mode and do not use the Xen PV ABI, so
>>>>> d->shared_info remains NULL throughout domain lifetime.
>>>>
>>>> ... question this mode of operation. Yes, you may (for now) be able
>>>> to get
>>>> away without, but e.g. event channels will want supporting at some
>>>> point.
>>>> Which will require a shared info page. Better put that in place
>>>> right away,
>>>> even if the guests you test with don't use it (yet). Certain other
>>>> common
>>>> code also assumes d->shared_info to never be NULL for an alive domain.
>>>>
>>>
>>> Would it be fine than to allocate it in arch_domain_create() ... :
>>>
>>>      if ( (d->shared_info = alloc_xenheap_pages(0, 0)) == NULL )
>>>          goto fail;
>>>
>>>      clear_page(d->shared_info);
>>>
>>> ... but without calling share_xen_page_with_guest() after that
>>> allocation as share_xen_page_with_guest() isn't implemented at the
>>> moment?
>>
>> Or could it be an option for all arch-s move allocation of
>> d->shared_info to domain_create() in common just after
>> arch_domain_create()?
>>
>> The only question if share_xen_page_with_guest() could be ifdef-ed
>> somehow so not to block new ports to implement it from the start.
> 
> shared_info is an x86-PV-ism which escaped into HVM and then infected
> ARM too.
> 
> Sadly it's ABI there, but this is one of many areas where I really want
> RISC-V not to inherit the mistakes of prior ports.

In which case, how do you propose e.g. event channels to be handled in
whatever is going to be the alternative?

Jan