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

[Waiman Long]

If the lockdep code is really running out of the stack_trace entries,
there is a possiblity that buffer overrun can happen and corrupt the
data immediately after stack_trace[].

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().

Fixes: 12593b7467f9 ("locking/lockdep: Reduce space occupied by stack traces")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 kernel/locking/lockdep.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 32282e7112d3..56e260a7582f 100644
--- a/kernel/locking/lockdep.c
+++ b/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);
-- 
2.18.1