tracing: Fix trace_marker copy link list updates

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

tracing: Fix trace_marker copy link list updates

When the "copy_trace_marker" option is enabled for an instance, anything

written into /sys/kernel/tracing/trace_marker is also copied into that

instances buffer. When the option is set, that instance's trace_array

descriptor is added to the marker_copies link list. This list is protected

by RCU, as all iterations uses an RCU protected list traversal.

When the instance is deleted, all the flags that were enabled are cleared.

This also clears the copy_trace_marker flag and removes the trace_array

descriptor from the list.

The issue is after the flags are called, a direct call to

update_marker_trace() is performed to clear the flag. This function

returns true if the state of the flag changed and false otherwise. If it

returns true here, synchronize_rcu() is called to make sure all readers

see that its removed from the list.

But since the flag was already cleared, the state does not change and the

synchronization is never called, leaving a possible UAF bug.

Move the clearing of all flags below the updating of the copy_trace_marker

option which then makes sure the synchronization is performed.

Also use the flag for checking the state in update_marker_trace() instead

of looking at if the list is empty.