KVM: s390: vsie: Fix races with partial gmap invalidations

Introduce a new boolean flag, used for shadow gmaps, to keep track of
whether the gmap has been invalidated, either partially or totally.

Use the new flag to check whether shadow gmap invalidations happened
during shadowing. In such cases, abort whatever was going on, return
-EAGAIN and let the caller try again.

Fixes: 19d6c5b ("KVM: s390: vsie: Fix unshadowing while shadowing")
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-ID: <20260407161721.247044-1-imbrenda@linux.ibm.com>

Author: Claudio Imbrenda

arch/s390/kvm/gaccess.c

@@ -1449,7 +1449,7 @@ static int _do_shadow_pte(struct gmap *sg, gpa_t raddr, union pte *ptep_h, union
 	pgste_set_unlock(ptep_h, pgste);
 	if (rc)
 		return rc;
-	if (!sg->parent)
+	if (sg->invalidated)
 		return -EAGAIN;
 
 	newpte = _pte(f->pfn, 0, !p, 0);
@@ -1479,7 +1479,7 @@ static int _do_shadow_crste(struct gmap *sg, gpa_t raddr, union crste *host, uni
 
 	do {
 		/* _gmap_crstep_xchg_atomic() could have unshadowed this shadow gmap */
-		if (!sg->parent)
+		if (sg->invalidated)
 			return -EAGAIN;
 		oldcrste = READ_ONCE(*host);
 		newcrste = _crste_fc1(f->pfn, oldcrste.h.tt, f->writable, !p);
@@ -1492,7 +1492,7 @@ static int _do_shadow_crste(struct gmap *sg, gpa_t raddr, union crste *host, uni
 		if (!newcrste.h.p && !f->writable)
 			return -EOPNOTSUPP;
 	} while (!_gmap_crstep_xchg_atomic(sg->parent, host, oldcrste, newcrste, f->gfn, false));
-	if (!sg->parent)
+	if (sg->invalidated)
 		return -EAGAIN;
 
 	newcrste = _crste_fc1(f->pfn, oldcrste.h.tt, 0, !p);
@@ -1545,7 +1545,7 @@ static int _gaccess_do_shadow(struct kvm_s390_mmu_cache *mc, struct gmap *sg,
 				       entries[i].pfn, i + 1, entries[i].writable);
 		if (rc)
 			return rc;
-		if (!sg->parent)
+		if (sg->invalidated)
 			return -EAGAIN;
 	}
 
@@ -1601,6 +1601,7 @@ static inline int _gaccess_shadow_fault(struct kvm_vcpu *vcpu, struct gmap *sg,
 		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 == -ENOMEM)

arch/s390/kvm/gmap.c

@@ -181,6 +181,7 @@ void gmap_remove_child(struct gmap *child)
 
 	list_del(&child->list);
 	child->parent = NULL;
+	child->invalidated = true;
 }
 
 /**
@@ -1069,6 +1070,7 @@ static void gmap_unshadow_level(struct gmap *sg, gfn_t r_gfn, int level)
 	if (level > TABLE_TYPE_PAGE_TABLE)
 		align = 1UL << (11 * level + _SEGMENT_SHIFT);
 	kvm_s390_vsie_gmap_notifier(sg, ALIGN_DOWN(gaddr, align), ALIGN(gaddr + 1, align));
+	sg->invalidated = true;
 	if (dat_entry_walk(NULL, r_gfn, sg->asce, 0, level, &crstep, &ptep))
 		return;
 	if (ptep) {
@@ -1174,6 +1176,7 @@ static inline int __gmap_protect_asce_top_level(struct kvm_s390_mmu_cache *mc, s
 	scoped_guard(spinlock, &parent->children_lock) {
 		if (READ_ONCE(sg->parent) != parent)
 			return -EAGAIN;
+		sg->invalidated = false;
 		for (i = 0; i < CRST_TABLE_PAGES; i++) {
 			if (!context->f[i].valid)
 				continue;

arch/s390/kvm/gmap.h

@@ -60,6 +60,7 @@ enum gmap_flags {
 struct gmap {
 	unsigned long flags;
 	unsigned char edat_level;
+	bool invalidated;
 	struct kvm *kvm;
 	union asce asce;
 	struct list_head list;