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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 19/05/2026 12:51 pm, Jan Beulich wrote:
> 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?

Implement proper enumeration of virtual capabilities (to be retrofitted
to x86/ARM too), and only offer the FIFO ABI (which is superior in every
way to the 2L ABI).

~Andrew