timers/migration: Prevent out of bounds access on failure

When tmigr_setup_groups() fails the level 0 group allocation, then the
cleanup derefences index -1 of the local stack array.

Prevent this by checking the loop condition first.

Fixes: 7ee9887 ("timers: Implement the hierarchical pull model")
Signed-off-by: Levi Yun <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Anna-Maria Behnsen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: SupremacistLevi

kernel/time/timer_migration.c

@@ -1596,7 +1596,7 @@ static int tmigr_setup_groups(unsigned int cpu, unsigned int node)
 
 	} while (i < tmigr_hierarchy_levels);
 
-	do {
+	while (i > 0) {
 		group = stack[--i];
 
 		if (err < 0) {
@@ -1645,7 +1645,7 @@ static int tmigr_setup_groups(unsigned int cpu, unsigned int node)
 				tmigr_connect_child_parent(child, group);
 			}
 		}
-	} while (i > 0);
+	}
 
 	kfree(stack);