[PATCH v1 5/7] KVM: s390: vsie: Fix unshadowing while shadowing

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

If shadowing causes the shadow gmap to get unshadowed, exit early to
prevent an attempt to dereference the parent pointer, which at this
point is NULL.

Opportunistically add some more checks to prevent NULL parents.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Fixes: a2c17f9270cc ("KVM: s390: New gmap code")
Fixes: e5f98a6899bd ("KVM: s390: Add some helper functions needed for vSIE")
Fixes: e38c884df921 ("KVM: s390: Switch to new gmap")
---
 arch/s390/kvm/gaccess.c |  2 ++
 arch/s390/kvm/gmap.c    | 11 ++++++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
index f5ffb11c8ef9..3bcf988d6faa 100644
--- a/arch/s390/kvm/gaccess.c
+++ b/arch/s390/kvm/gaccess.c
@@ -1520,6 +1520,8 @@ static int _gaccess_do_shadow(struct kvm_s390_mmu_cache *mc, struct gmap *sg,
 				       entries[i - 1].pfn, i, entries[i - 1].writable);
 		if (rc)
 			return rc;
+		if (!sg->parent)
+			return -EAGAIN;
 	}
 
 	rc = dat_entry_walk(NULL, entries[LEVEL_MEM].gfn, sg->parent->asce, DAT_WALK_LEAF,
diff --git a/arch/s390/kvm/gmap.c b/arch/s390/kvm/gmap.c
index 759a2ed17038..ba921da48019 100644
--- a/arch/s390/kvm/gmap.c
+++ b/arch/s390/kvm/gmap.c
@@ -1154,6 +1154,7 @@ struct gmap_protect_asce_top_level {
 static inline int __gmap_protect_asce_top_level(struct kvm_s390_mmu_cache *mc, struct gmap *sg,
 						struct gmap_protect_asce_top_level *context)
 {
+	struct gmap *parent;
 	int rc, i;
 
 	guard(write_lock)(&sg->kvm->mmu_lock);
@@ -1161,7 +1162,12 @@ static inline int __gmap_protect_asce_top_level(struct kvm_s390_mmu_cache *mc, s
 	if (kvm_s390_array_needs_retry_safe(sg->kvm, context->seq, context->f))
 		return -EAGAIN;
 
-	scoped_guard(spinlock, &sg->parent->children_lock) {
+	parent = READ_ONCE(sg->parent);
+	if (!parent)
+		return -EAGAIN;
+	scoped_guard(spinlock, &parent->children_lock) {
+		if (READ_ONCE(sg->parent) != parent)
+			return -EAGAIN;
 		for (i = 0; i < CRST_TABLE_PAGES; i++) {
 			if (!context->f[i].valid)
 				continue;
@@ -1244,6 +1250,9 @@ struct gmap *gmap_create_shadow(struct kvm_s390_mmu_cache *mc, struct gmap *pare
 	struct gmap *sg, *new;
 	int rc;
 
+	if (WARN_ON(!parent))
+		return ERR_PTR(-EINVAL);
+
 	scoped_guard(spinlock, &parent->children_lock) {
 		sg = gmap_find_shadow(parent, asce, edat_level);
 		if (sg) {
-- 
2.53.0