[PATCH] perf: Fix race in perf_event_exit_task_context

[Peter Zijlstra]

Subject: perf: Fix race in perf_event_exit_task_context
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Mon Jan 25 13:03:18 CET 2016

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@xxxxxxxxxxxxx>
---
 kernel/events/core.c |   50 +++++++++++++++++++++++++++++---------------------
 1 file changed, 29 insertions(+), 21 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8748,14 +8748,40 @@ static void perf_event_exit_task_context
 {
 	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
 	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
 	 */
 	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);