locking/lockdep: Fix buffer overrun problem in stack_trace[]

If the lockdep code is really running out of the stack_trace entries,
it is likely that buffer overrun can happen and the data immediately
after stack_trace[] will be corrupted.

If there is less than LOCK_TRACE_SIZE_IN_LONGS entries left before
the call to save_trace(), the max_entries computation will leave it
with a very large positive number because of its unsigned nature. The
subsequent call to stack_trace_save() will then corrupt the data after
stack_trace[]. Fix that by changing max_entries to a signed integer
and check for negative value before calling stack_trace_save().
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 12593b74 ("locking/lockdep: Reduce space occupied by stack traces")
Link: https://lkml.kernel.org/r/20191220135128.14876-1-longman@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

kernel/locking/lockdep.c

@@ -482,7 +482,7 @@ static struct lock_trace *save_trace(void)
 	struct lock_trace *trace, *t2;
 	struct hlist_head *hash_head;
 	u32 hash;
-	unsigned int max_entries;
+	int max_entries;
 
 	BUILD_BUG_ON_NOT_POWER_OF_2(STACK_TRACE_HASH_SIZE);
 	BUILD_BUG_ON(LOCK_TRACE_SIZE_IN_LONGS >= MAX_STACK_TRACE_ENTRIES);
@@ -490,10 +490,8 @@ static struct lock_trace *save_trace(void)
 	trace = (struct lock_trace *)(stack_trace + nr_stack_trace_entries);
 	max_entries = MAX_STACK_TRACE_ENTRIES - nr_stack_trace_entries -
 		LOCK_TRACE_SIZE_IN_LONGS;
-	trace->nr_entries = stack_trace_save(trace->entries, max_entries, 3);
 
-	if (nr_stack_trace_entries >= MAX_STACK_TRACE_ENTRIES -
-	    LOCK_TRACE_SIZE_IN_LONGS - 1) {
+	if (max_entries <= 0) {
 		if (!debug_locks_off_graph_unlock())
 			return NULL;
 
@@ -502,6 +500,7 @@ static struct lock_trace *save_trace(void)
 
 		return NULL;
 	}
+	trace->nr_entries = stack_trace_save(trace->entries, max_entries, 3);
 
 	hash = jhash(trace->entries, trace->nr_entries *
 		     sizeof(trace->entries[0]), 0);