KVM: s390: Fix _gaccess_shadow_fault()

In some circumstances, it is possible that the page of nested guest
memory that is being shadowed is not present at all in the parent guest
gmap. dat_entry_walk() will not find any leaf entry and return with
-ENOENT, which will erroneously be propagated all the way to userspace.

Fix by manually calling gmap_link() on the memory of the nested guest
that is being shadowed if the mapping was not already present.

Fixes: e38c884df921 ("KVM: s390: Switch to new gmap")
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-ID: <20260828115439.145885-4-imbrenda@linux.ibm.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/arch/s390/kvm/s390/gaccess.c b/arch/s390/kvm/s390/gaccess.c
index e5c064f263df..405345ccc4f1 100644
--- a/arch/s390/kvm/s390/gaccess.c
+++ b/arch/s390/kvm/s390/gaccess.c
@@ -1589,12 +1589,25 @@ static inline int ___gaccess_shadow_fault(struct kvm_vcpu *vcpu, struct gmap *sg
 	parent = READ_ONCE(sg->parent);
 	if (!parent)
 		return -EAGAIN;
+retry:
 	scoped_guard(spinlock, &parent->children_lock) {
 		if (READ_ONCE(sg->parent) != parent)
 			return -EAGAIN;
 		sg->invalidated = false;
 		rc = _gaccess_do_shadow(vcpu->arch.mc, sg, saddr, walk);
 	}
+	if (rc == -ENOENT) {
+		struct kvm_memory_slot *slot;
+		struct guest_fault *entries;
+
+		entries = get_entries(walk);
+		slot = kvm_vcpu_gfn_to_memslot(vcpu, entries[LEVEL_MEM].gfn);
+		if (!slot)
+			return PGM_ADDRESSING;
+		rc = gmap_link(vcpu->arch.mc, parent, entries + LEVEL_MEM, slot);
+		if (!rc)
+			goto retry;
+	}
 	if (!rc)
 		kvm_s390_release_faultin_array(vcpu->kvm, walk->raw_entries, false);
 	return rc;