perf/x86/intel/bts: Fix exclusive event reference leak

Commit:

  d2878d642a4ed ("perf/x86/intel/bts: Disallow use by unprivileged users on paranoid systems")

... adds a privilege check in the exactly wrong place in the event init path:
after the 'LBR exclusive' reference has been taken, and doesn't release it
in the case of insufficient privileges. After this, nobody in the system
gets to use PT or LBR afterwards.

This patch moves the privilege check to where it should have been in the
first place.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: d2878d642a4ed ("perf/x86/intel/bts: Disallow use by unprivileged users on paranoid systems")
Link: http://lkml.kernel.org/r/20171023123533.16973-1-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/events/intel/bts.c

@@ -546,9 +546,6 @@ static int bts_event_init(struct perf_event *event)
 	if (event->attr.type != bts_pmu.type)
 		return -ENOENT;
 
-	if (x86_add_exclusive(x86_lbr_exclusive_bts))
-		return -EBUSY;
-
 	/*
 	 * BTS leaks kernel addresses even when CPL0 tracing is
 	 * disabled, so disallow intel_bts driver for unprivileged
@@ -562,6 +559,9 @@ static int bts_event_init(struct perf_event *event)
 	    !capable(CAP_SYS_ADMIN))
 		return -EACCES;
 
+	if (x86_add_exclusive(x86_lbr_exclusive_bts))
+		return -EBUSY;
+
 	ret = x86_reserve_hardware();
 	if (ret) {
 		x86_del_exclusive(x86_lbr_exclusive_bts);