ring-buffer: Do not attempt to read past "commit"

In the Linux kernel, the following vulnerability has been resolved:

ring-buffer: Do not attempt to read past "commit"

When iterating over the ring buffer while the ring buffer is active, the

writer can corrupt the reader. There's barriers to help detect this and

handle it, but that code missed the case where the last event was at the

very end of the page and has only 4 bytes left.

The checks to detect the corruption by the writer to reads needs to see the

length of the event. If the length in the first 4 bytes is zero then the

length is stored in the second 4 bytes. But if the writer is in the process

of updating that code, there's a small window where the length in the first

4 bytes could be zero even though the length is only 4 bytes. That will

cause rb_event_length() to read the next 4 bytes which could happen to be off the

allocated page.

To protect against this, fail immediately if the next event pointer is

less than 8 bytes from the end of the commit (last byte of data), as all

events must be a minimum of 8 bytes anyway.