tracing: ring-buffer: Fix to check event length before using

From: Masami Hiramatsu (Google) <[email protected]>

Check the event length before adding it for accessing next index in rb_read_data_buffer(). Since this function is used for validating possibly broken ring buffers, the length of the event could be broken. In that case, the new event (e + len) can point a wrong address. To avoid invalid memory access at boot, check whether the length of each event is in the possible range before using it.

Fixes 5f3b6e839f3c ("ring-buffer: Validate boot range memory events")

Cc [email protected]

Signed-off-by Masami Hiramatsu (Google) <[email protected]>

---
 kernel/trace/ring_buffer.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

kernel/trace/ring_buffer.c +5 -1

index 630221b00838..1ef17d6fd824 100644
@@ -1848,6 +1848,7 @@ static int rb_read_data_buffer(struct buffer_data_page *dpage, int tail, int cpu
 	struct ring_buffer_event *event;
 	u64 ts, delta;
 	int events = 0;
+	int len;
 	int e;
 
 	*delta_ptr = 0;

@@ -1855,9 +1856,12 @@ static int rb_read_data_buffer(struct buffer_data_page *dpage, int tail, int cpu
 
 	ts = dpage->time_stamp;
 
-	for (e = 0; e < tail; e += rb_event_length(event)) {
+	for (e = 0; e < tail; e += len) {
 
 		event = (struct ring_buffer_event *)(dpage->data + e);
+		len = rb_event_length(event);
+		if (len <= 0 || len > tail - e)
+			return -1;
 
 		switch (event->type_len) {