[GIT PULL v1 17/23] KVM: s390: Fix race in __do_essa()

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

An unlikely race between __do_essa() and kvm_s390_vm_start_migration(),
kvm_s390_vm_stop_migration(), or dat_get_cmma() was possible.

Fix by locking kvm->slots_arch_lock. Since this is not a hot path, the
overhead of an additional mutex is negligible.

Fixes: e38c884df921 ("KVM: s390: Switch to new gmap")
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-ID: <20260803124040.126471-8-imbrenda@linux.ibm.com>
---
 arch/s390/kvm/kvm-s390.c | 20 ++++++++++----------
 arch/s390/kvm/priv.c     |  5 +++--
 2 files changed, 13 insertions(+), 12 deletions(-)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index e162efaa35b8..e5c5e9f61cb2 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -1219,8 +1219,8 @@ static void kvm_s390_sync_request_broadcast(struct kvm *kvm, int req)
 
 /*
  * Must be called with kvm->srcu held to avoid races on memslots, and with
- * kvm->slots_lock to avoid races with ourselves, kvm_s390_vm_stop_migration(),
- * and kvm_s390_get_cmma_bits().
+ * kvm->slots_arch_lock to avoid races with ourselves,
+ * kvm_s390_vm_stop_migration(), and kvm_s390_get_cmma_bits().
  */
 static int kvm_s390_vm_start_migration(struct kvm *kvm)
 {
@@ -1265,7 +1265,7 @@ static int kvm_s390_vm_start_migration(struct kvm *kvm)
 }
 
 /*
- * Must be called with kvm->slots_lock to avoid races with ourselves,
+ * Must be called with kvm->slots_arch_lock to avoid races with ourselves,
  * kvm_s390_vm_start_migration() and kvm_s390_get_cmma_bits().
  */
 static int kvm_s390_vm_stop_migration(struct kvm *kvm)
@@ -1300,7 +1300,9 @@ static int kvm_s390_vm_set_migration(struct kvm *kvm,
 {
 	int res = -ENXIO;
 
-	mutex_lock(&kvm->slots_lock);
+	guard(srcu)(&kvm->srcu);
+	guard(mutex)(&kvm->slots_arch_lock);
+
 	switch (attr->attr) {
 	case KVM_S390_VM_MIGRATION_START:
 		res = kvm_s390_vm_start_migration(kvm);
@@ -1311,7 +1313,6 @@ static int kvm_s390_vm_set_migration(struct kvm *kvm,
 	default:
 		break;
 	}
-	mutex_unlock(&kvm->slots_lock);
 
 	return res;
 }
@@ -3001,9 +3002,8 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
 		r = -EFAULT;
 		if (copy_from_user(&args, argp, sizeof(args)))
 			break;
-		mutex_lock(&kvm->slots_lock);
-		r = kvm_s390_get_cmma_bits(kvm, &args);
-		mutex_unlock(&kvm->slots_lock);
+		scoped_guard(mutex, &kvm->slots_arch_lock)
+			r = kvm_s390_get_cmma_bits(kvm, &args);
 		if (!r) {
 			r = copy_to_user(argp, &args, sizeof(args));
 			if (r)
@@ -3017,9 +3017,9 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
 		r = -EFAULT;
 		if (copy_from_user(&args, argp, sizeof(args)))
 			break;
-		mutex_lock(&kvm->slots_lock);
+		mutex_lock(&kvm->slots_arch_lock);
 		r = kvm_s390_set_cmma_bits(kvm, &args);
-		mutex_unlock(&kvm->slots_lock);
+		mutex_unlock(&kvm->slots_arch_lock);
 		break;
 	}
 	case KVM_S390_PV_COMMAND: {
diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
index ea5a99537346..b1ba24c346ef 100644
--- a/arch/s390/kvm/priv.c
+++ b/arch/s390/kvm/priv.c
@@ -1261,8 +1261,9 @@ static int handle_essa(struct kvm_vcpu *vcpu)
 		/* Retry the ESSA instruction */
 		kvm_s390_retry_instr(vcpu);
 	} else {
-		scoped_guard(read_lock, &vcpu->kvm->mmu_lock)
-			i = __do_essa(vcpu, orc);
+		scoped_guard(mutex, &vcpu->kvm->slots_arch_lock)
+			scoped_guard(read_lock, &vcpu->kvm->mmu_lock)
+				i = __do_essa(vcpu, orc);
 		if (i < 0)
 			return i;
 		/* Account for the possible extra cbrl entry */
-- 
2.55.0