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

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

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.
~ Oleksii