Code: void perf_pmu_migrate_context(struct pmu *pmu, int src_cpu, int dst_cpu)
{
	struct perf_event_context *src_ctx;
	struct perf_event_context *dst_ctx;
	struct perf_event *event, *tmp;
	LIST_HEAD(events);

 	src_ctx = &per_cpu_ptr(pmu->pmu_cpu_context, src_cpu)->ctx;
 	dst_ctx = &per_cpu_ptr(pmu->pmu_cpu_context, dst_cpu)->ctx;
 
	mutex_lock(&src_ctx->mutex);
 	list_for_each_entry_safe(event, tmp, &src_ctx->event_list,
 				 event_entry) {
 		perf_remove_from_context(event, false);
 		unaccount_event_cpu(event, src_cpu);
 		put_ctx(src_ctx);
 		list_add(&event->migrate_entry, &events);
 	}
	mutex_unlock(&src_ctx->mutex);
 
 	synchronize_rcu();
 
	mutex_lock(&dst_ctx->mutex);
 	list_for_each_entry_safe(event, tmp, &events, migrate_entry) {
 		list_del(&event->migrate_entry);
 		if (event->state >= PERF_EVENT_STATE_OFF)
			event->state = PERF_EVENT_STATE_INACTIVE;
		account_event_cpu(event, dst_cpu);
		perf_install_in_context(dst_ctx, event, dst_cpu);
 		get_ctx(dst_ctx);
 	}
 	mutex_unlock(&dst_ctx->mutex);
 }

Vulnerability Type: +Priv 
CWE ID: CWE-264
Summary: kernel/events/core.c in the performance subsystem in the Linux kernel before 4.0 mismanages locks during certain migrations, which allows local users to gain privileges via a crafted application, aka Android internal bug 31095224.
Commit Message: perf: Fix event->ctx locking

There have been a few reported issues wrt. the lack of locking around
changing event->ctx. This patch tries to address those.

It avoids the whole rwsem thing; and while it appears to work, please
give it some thought in review.

What I did fail at is sensible runtime checks on the use of
event->ctx, the RCU use makes it very hard.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20150123125834.209535886@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>