From: "Steven Rostedt (VMware)" rostedt@goodmis.org

Two info bits were added to the "commit" part of the ring buffer data page when returned to be consumed. This was to inform the user space readers that events have been missed, and that the count may be stored at the end of the page.

What wasn't handled, was the splice code that actually called a function to return the length of the data in order to zero out the rest of the page before sending it up to user space. These data bits were returned with the length making the value negative, and that negative value was not checked. It was compared to PAGE_SIZE, and only used if the size was less than PAGE_SIZE. Luckily PAGE_SIZE is unsigned long which made the compare an unsigned compare, meaning the negative size value did not end up causing a large portion of memory to be randomly zeroed out.

Cc: stable@vger.kernel.org Fixes: 66a8cb95ed040 ("ring-buffer: Add place holder recording of dropped events") Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.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 c87766c1c204..e06cde093f76 100644 --- a/kernel/trace/ring_buffer.c +++ b/kernel/trace/ring_buffer.c @@ -280,6 +280,8 @@ EXPORT_SYMBOL_GPL(ring_buffer_event_data); /* Missed count stored at end */ #define RB_MISSED_STORED (1 << 30)

+#define RB_MISSED_FLAGS (RB_MISSED_EVENTS|RB_MISSED_STORED) + struct buffer_data_page { u64 time_stamp; /* page time stamp */ local_t commit; /* write committed index */ @@ -331,7 +333,9 @@ static void rb_init_page(struct buffer_data_page *bpage) */ size_t ring_buffer_page_len(void *page) { - return local_read(&((struct buffer_data_page *)page)->commit) + struct buffer_data_page *bpage = page; + + return (local_read(&bpage->commit) & ~RB_MISSED_FLAGS) + BUF_PAGE_HDR_SIZE; }