xen/sched: core: skip missing vcpu slots in sched_move_domain()

sched_move_domain() derives the number of units to rebuild from
d->max_vcpus, which is fixed at domain creation and never rolled
back if vcpu_create() fails partway through building a domain. So
d->vcpu[i] can be NULL for some i even though max_vcpus still
counts it - this happens if sched_alloc_udata() returns NULL.

The per-unit loop doesn't check for this: it sets
unit->vcpu_list = d->vcpu[unit_id] (NULL) and hands that broken
unit straight to the destination scheduler's alloc_udata(),
which assumes vcpu_list is always valid and crashes Xen when
it is not.

Reproduced by building a domain in a non-default cpupool where
vcpu creation fails partway through, then destroying it.
domain_kill() moves the domain back to the default cpupool via
sched_move_domain() before actually destroying it, crashing
inside the destination scheduler's alloc_udata() (seen in
Credit2's csched2_alloc_udata() -> is_idle_unit() -> NULL deref).

Before building a unit in sched_move_domain(), check whether all
vpcu slots belonging to that unit are populated. If any of its
vpcus is missing:
 - For a dying domain, skip the unit allocation.
 - For an active domain, abort the move and return -EINVAL to
   prevent running with dropped vCPUs.

Fixes: 70fadc4163 ("xen/cpupool: support moving domain between cpupools with different granularity")
Signed-off-by: Furkan Caliskan <frn1furkan10@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>

xen/common/sched/core.c

@@ -745,6 +745,38 @@ int sched_move_domain(struct domain *d, struct cpupool *c)
 
     for ( unit_idx = 0; unit_idx < n_units; unit_idx++ )
     {
+        /*
+         * A vcpu slot can be missing if creation failed partway
+         * through. A dying domain is being torn down regardless, so
+         * skip the unit -- but a domain that isn't dying still needs
+         * every vcpu it has schedulable, so fail instead of silently
+         * dropping some of them.
+         */
+        bool vcpu_failed = false;
+
+        for ( unsigned int i = 0;
+              i < gran && unit_idx * gran + i < d->max_vcpus; i++ )
+        {
+            if ( !d->vcpu[unit_idx * gran + i] )
+            {
+                vcpu_failed = true;
+                break;
+            }
+        }
+
+        if ( vcpu_failed )
+        {
+            if ( !d->is_dying )
+            {
+                sched_move_domain_cleanup(c->sched, new_units, domdata);
+                rcu_read_unlock(&sched_res_rculock);
+
+                return -EINVAL;
+            }
+
+            continue;
+        }
+
         unit = sched_alloc_unit_mem();
         if ( unit )
         {