x86/shadow: drop (replace) bogus assertions

The addition of a call to shadow_blow_tables() from shadow_teardown()
has resulted in the "no vcpus" related assertion becoming triggerable:
If domain_create() fails with at least one page successfully allocated
in the course of shadow_enable(), or if domain_create() succeeds and
the domain is then killed without ever invoking XEN_DOMCTL_max_vcpus.
Note that in-tree tests (test-resource and test-tsx) do exactly the
latter of these two.

The assertion's comment was bogus anyway: Shadow mode has been getting
enabled before allocation of vCPU-s for quite some time. Convert the
assertion to a conditional: As long as there are no vCPU-s, there's
nothing to blow away.

Fixes: e7aa55c0aa ("x86/p2m: free the paging memory pool preemptively")
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>

A similar assertion/comment pair exists in _shadow_prealloc(); the
comment is similarly bogus, and the assertion could in principle trigger
e.g. when shadow_alloc_p2m_page() is called early enough. Replace those
at the same time by a similar early return, here indicating failure to
the caller (which will generally lead to the domain being crashed in
shadow_prealloc()).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: a92dc2bb30
master date: 2022-10-24 15:46:11 +0200

xen/arch/x86/mm/shadow/common.c

@@ -942,8 +942,9 @@ static bool __must_check _shadow_prealloc(struct domain *d, unsigned int pages)
         /* No reclaim when the domain is dying, teardown will take care of it. */
         return false;
 
-    /* Shouldn't have enabled shadows if we've no vcpus. */
-    ASSERT(d->vcpu && d->vcpu[0]);
+    /* Nothing to reclaim when there are no vcpus yet. */
+    if ( !d->vcpu[0] )
+        return false;
 
     /* Stage one: walk the list of pinned pages, unpinning them */
     perfc_incr(shadow_prealloc_1);
@@ -1031,8 +1032,9 @@ static void shadow_blow_tables(struct domain *d)
     mfn_t smfn;
     int i;
 
-    /* Shouldn't have enabled shadows if we've no vcpus. */
-    ASSERT(d->vcpu && d->vcpu[0]);
+    /* Nothing to do when there are no vcpus yet. */
+    if ( !d->vcpu[0] )
+        return;
 
     /* Pass one: unpin all pinned pages */
     foreach_pinned_shadow(d, sp, t)