The following scenario:

  CPU0					CPU1
  
  ctx = find_get_ctx();
  					perf_event_exit_task_context()
  mutex_lock(&ctx->mutex);
  perf_install_in_context(ctx, ...);
    /* NO-OP */
  mutex_unlock(&ctx->mutex);

  ...

  perf_release()
    WARN_ON_ONCE(event->state != STATE_EXIT);


Since the event doesn't pass through perf_remove_from_context()
because perf_install_in_context() NO-OPs because the ctx is dead, and
perf_event_exit_task_context() will not observe the event because its
not attached yet, the event->state will not be set.

Solve this by revalidating ctx->task after we acquire ctx->mutex and
failing the event creation as a whole.

Reviewed-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Tested-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/events/core.c |   35 ++++++++++++++++++++++++++---------
 1 file changed, 26 insertions(+), 9 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2158,13 +2158,15 @@ perf_install_in_context(struct perf_even
 	 */
 	raw_spin_lock_irq(&ctx->lock);
 	task = ctx->task;
+
 	/*
-	 * Worse, we cannot even rely on the ctx actually existing anymore. If
-	 * between find_get_context() and perf_install_in_context() the task
-	 * went through perf_event_exit_task() its dead and we should not be
-	 * adding new events.
+	 * If between ctx = find_get_context() and mutex_lock(&ctx->mutex) the
+	 * ctx gets destroyed, we must not install an event into it.
+	 *
+	 * This is normally tested for after we acquire the mutex, so this is
+	 * a sanity check.
 	 */
-	if (task == TASK_TOMBSTONE) {
+	if (WARN_ON_ONCE(task == TASK_TOMBSTONE)) {
 		raw_spin_unlock_irq(&ctx->lock);
 		return;
 	}
@@ -8389,10 +8391,19 @@ SYSCALL_DEFINE5(perf_event_open,
 	if (move_group) {
 		gctx = group_leader->ctx;
 		mutex_lock_double(&gctx->mutex, &ctx->mutex);
+		if (gctx->task == TASK_TOMBSTONE) {
+			err = -ESRCH;
+			goto err_locked;
+		}
 	} else {
 		mutex_lock(&ctx->mutex);
 	}
 
+	if (ctx->task == TASK_TOMBSTONE) {
+		err = -ESRCH;
+		goto err_locked;
+	}
+
 	if (!perf_event_validate_size(event)) {
 		err = -E2BIG;
 		goto err_locked;
@@ -8563,12 +8574,14 @@ perf_event_create_kernel_counter(struct
 
 	WARN_ON_ONCE(ctx->parent_ctx);
 	mutex_lock(&ctx->mutex);
+	if (ctx->task == TASK_TOMBSTONE) {
+		err = -ESRCH;
+		goto err_unlock;
+	}
+
 	if (!exclusive_event_installable(event, ctx)) {
-		mutex_unlock(&ctx->mutex);
-		perf_unpin_context(ctx);
-		put_ctx(ctx);
 		err = -EBUSY;
-		goto err_free;
+		goto err_unlock;
 	}
 
 	perf_install_in_context(ctx, event, cpu);
@@ -8577,6 +8590,10 @@ perf_event_create_kernel_counter(struct
 
 	return event;
 
+err_unlock:
+	mutex_unlock(&ctx->mutex);
+	perf_unpin_context(ctx);
+	put_ctx(ctx);
 err_free:
 	free_event(event);
 err: