perf: Fix race in perf_event_exit_task_context()

There is a race between perf_event_exit_task_context() and
orphans_remove_work() which results in a use-after-free.

We mark ctx->task with TASK_TOMBSTONE to indicate a context is
'dead', under ctx->lock. After which point event_function_call()
on any event of that context will NOP

A concurrent orphans_remove_work() will only hold ctx->mutex for
the list iteration and not serialize against this. Therefore its
possible that orphans_remove_work()'s perf_remove_from_context()
call will fail, but we'll continue to free the event, with the
result of free'd memory still being on lists and everything.

Once perf_event_exit_task_context() gets around to acquiring
ctx->mutex it too will iterate the event list, encounter the
already free'd event and proceed to free it _again_. This fails
with the WARN in free_event().

Plug the race by having perf_event_exit_task_context() hold
ctx::mutex over the whole tear-down, thereby 'naturally'
serializing against all other sites, including the orphan work.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: alexander.shishkin@linux.intel.com
Cc: dsahern@gmail.com
Cc: namhyung@kernel.org
Link: http://lkml.kernel.org/r/20160125130954.GY6357@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/core.c

@@ -8748,14 +8748,40 @@ static void perf_event_exit_task_context(struct task_struct *child, int ctxn)
 {
 	struct perf_event_context *child_ctx, *clone_ctx = NULL;
 	struct perf_event *child_event, *next;
-	unsigned long flags;
 
 	WARN_ON_ONCE(child != current);
 
-	child_ctx = perf_lock_task_context(child, ctxn, &flags);
+	child_ctx = perf_pin_task_context(child, ctxn);
 	if (!child_ctx)
 		return;
 
+	/*
+	 * In order to reduce the amount of tricky in ctx tear-down, we hold
+	 * ctx::mutex over the entire thing. This serializes against almost
+	 * everything that wants to access the ctx.
+	 *
+	 * The exception is sys_perf_event_open() /
+	 * perf_event_create_kernel_count() which does find_get_context()
+	 * without ctx::mutex (it cannot because of the move_group double mutex
+	 * lock thing). See the comments in perf_install_in_context().
+	 *
+	 * We can recurse on the same lock type through:
+	 *
+	 *   __perf_event_exit_task()
+	 *     sync_child_event()
+	 *       put_event()
+	 *         mutex_lock(&ctx->mutex)
+	 *
+	 * But since its the parent context it won't be the same instance.
+	 */
+	mutex_lock(&child_ctx->mutex);
+
+	/*
+	 * In a single ctx::lock section, de-schedule the events and detach the
+	 * context from the task such that we cannot ever get it scheduled back
+	 * in.
+	 */
+	raw_spin_lock_irq(&child_ctx->lock);
 	task_ctx_sched_out(__get_cpu_context(child_ctx), child_ctx);
 
 	/*
@@ -8767,14 +8793,8 @@ static void perf_event_exit_task_context(struct task_struct *child, int ctxn)
 	WRITE_ONCE(child_ctx->task, TASK_TOMBSTONE);
 	put_task_struct(current); /* cannot be last */
 
-	/*
-	 * If this context is a clone; unclone it so it can't get
-	 * swapped to another process while we're removing all
-	 * the events from it.
-	 */
 	clone_ctx = unclone_ctx(child_ctx);
-	update_context_time(child_ctx);
-	raw_spin_unlock_irqrestore(&child_ctx->lock, flags);
+	raw_spin_unlock_irq(&child_ctx->lock);
 
 	if (clone_ctx)
 		put_ctx(clone_ctx);
@@ -8786,18 +8806,6 @@ static void perf_event_exit_task_context(struct task_struct *child, int ctxn)
 	 */
 	perf_event_task(child, child_ctx, 0);
 
-	/*
-	 * We can recurse on the same lock type through:
-	 *
-	 *   __perf_event_exit_task()
-	 *     sync_child_event()
-	 *       put_event()
-	 *         mutex_lock(&ctx->mutex)
-	 *
-	 * But since its the parent context it won't be the same instance.
-	 */
-	mutex_lock(&child_ctx->mutex);
-
 	list_for_each_entry_safe(child_event, next, &child_ctx->event_list, event_entry)
 		__perf_event_exit_task(child_event, child_ctx, child);