Re: perf: use-after-free in perf_release

[Peter Zijlstra]

On Tue, Mar 14, 2017 at 04:26:25PM +0100, Peter Zijlstra wrote:
> A well.. I'll go do the 'trivial' patch then.

A little like so; completely untested.

---
 kernel/events/core.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 110b38a58493..6576449b6029 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -10346,6 +10346,17 @@ void perf_event_free_task(struct task_struct *task)
 			continue;
 
 		mutex_lock(&ctx->mutex);
+		raw_spin_lock_irq(&ctx->lock);
+		/*
+		 * Destroy the task <-> ctx relation and mark the context dead.
+		 *
+		 * This is important because even though the task hasn't been
+		 * exposed yet the context has been (through child_list).
+		 */
+		RCU_INIT_POINTER(task->perf_event_ctxp[ctxn], NULL);
+		WRITE_ONCE(ctx->task, TASK_TOMBSTONE);
+		put_task_struct(task); /* cannot be last */
+		raw_spin_unlock_irq(&ctx->lock);
 again:
 		list_for_each_entry_safe(event, tmp, &ctx->pinned_groups,
 				group_entry)