KVM: s390: pv: simplify shutdown and fix race

Simplify the shutdown of non-protected VMs. There is no need to do complex manipulations of the counter if it was zero. This also fixes a very rare race which caused pages to be torn down from the address space with a non-zero counter even on older machines that don't support the UVC instruction, causing a crash. Reported-by: Marc Hartmayer <[email protected]> Fixes: fb491d5 ("KVM: s390: pv: asynchronous destroy for reboot") Reviewed-by: Nico Boehr <[email protected]> Signed-off-by: Claudio Imbrenda <[email protected]> Message-ID: <[email protected]>
niw · Jul 18, 2023 · 5ff9218 · 5ff9218
1 parent fdf0eaf
commit 5ff9218
Showing 1 changed file with 6 additions and 2 deletions.
diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
@@ -411,8 +411,12 @@ int kvm_s390_pv_deinit_cleanup_all(struct kvm *kvm, u16 *rc, u16 *rrc)
 	u16 _rc, _rrc;
 	int cc = 0;
 
-	/* Make sure the counter does not reach 0 before calling s390_uv_destroy_range */
-	atomic_inc(&kvm->mm->context.protected_count);
+	/*
+	 * Nothing to do if the counter was already 0. Otherwise make sure
+	 * the counter does not reach 0 before calling s390_uv_destroy_range.
+	 */
+	if (!atomic_inc_not_zero(&kvm->mm->context.protected_count))
+		return 0;
 
 	*rc = 1;
 	/* If the current VM is protected, destroy it */