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

["Masami Hiramatsu (Google)" <mhiramat@kernel.org>]

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

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: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 kernel/trace/ring_buffer.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 630221b00838..1ef17d6fd824 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -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) {