[PATCHv2] io_uring: consistently use rcu semantics with sqpoll thread

[Keith Busch <kbusch@meta.com>]

From: Keith Busch <kbusch@kernel.org>

The sqpoll thread is dereferenced with rcu read protection in one place,
so it needs to be annotated as an __rcu type, and should consistently
use rcu helpers for access and assignment to make sparse happy.

Since most of the accesses occur under the sqd->lock, we can use
rcu_dereference_protected() without declaring an rcu read section.
Provide a simple helper to get the thread from a locked context.

Fixes: ac0b8b327a5677d ("io_uring: fix use-after-free of sq->thread in __io_uring_show_fdinfo()")
Signed-off-by: Keith Busch <kbusch@kernel.org>
---
 io_uring/io_uring.c |  4 ++--
 io_uring/sqpoll.c   | 34 ++++++++++++++++++++++++----------
 io_uring/sqpoll.h   |  8 +++++++-
 3 files changed, 33 insertions(+), 13 deletions(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index cf759c172083c..4e32f808d07df 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2906,7 +2906,7 @@ static __cold void io_ring_exit_work(struct work_struct *work)
 			struct task_struct *tsk;
 
 			io_sq_thread_park(sqd);
-			tsk = sqd->thread;
+			tsk = sqpoll_task_locked(sqd);
 			if (tsk && tsk->io_uring && tsk->io_uring->io_wq)
 				io_wq_cancel_cb(tsk->io_uring->io_wq,
 						io_cancel_ctx_cb, ctx, true);
@@ -3142,7 +3142,7 @@ __cold void io_uring_cancel_generic(bool cancel_all, struct io_sq_data *sqd)
 	s64 inflight;
 	DEFINE_WAIT(wait);
 
-	WARN_ON_ONCE(sqd && sqd->thread != current);
+	WARN_ON_ONCE(sqd && sqpoll_task_locked(sqd) != current);
 
 	if (!current->io_uring)
 		return;
diff --git a/io_uring/sqpoll.c b/io_uring/sqpoll.c
index 0625a421626f4..268d2fbe6160c 100644
--- a/io_uring/sqpoll.c
+++ b/io_uring/sqpoll.c
@@ -30,7 +30,7 @@ enum {
 void io_sq_thread_unpark(struct io_sq_data *sqd)
 	__releases(&sqd->lock)
 {
-	WARN_ON_ONCE(sqd->thread == current);
+	WARN_ON_ONCE(sqpoll_task_locked(sqd) == current);
 
 	/*
 	 * Do the dance but not conditional clear_bit() because it'd race with
@@ -46,24 +46,32 @@ void io_sq_thread_unpark(struct io_sq_data *sqd)
 void io_sq_thread_park(struct io_sq_data *sqd)
 	__acquires(&sqd->lock)
 {
-	WARN_ON_ONCE(data_race(sqd->thread) == current);
+	struct task_struct *tsk;
 
 	atomic_inc(&sqd->park_pending);
 	set_bit(IO_SQ_THREAD_SHOULD_PARK, &sqd->state);
 	mutex_lock(&sqd->lock);
-	if (sqd->thread)
-		wake_up_process(sqd->thread);
+
+	tsk = sqpoll_task_locked(sqd);
+	if (tsk) {
+		WARN_ON_ONCE(tsk == current);
+		wake_up_process(tsk);
+	}
 }
 
 void io_sq_thread_stop(struct io_sq_data *sqd)
 {
-	WARN_ON_ONCE(sqd->thread == current);
+	struct task_struct *tsk;
+
 	WARN_ON_ONCE(test_bit(IO_SQ_THREAD_SHOULD_STOP, &sqd->state));
 
 	set_bit(IO_SQ_THREAD_SHOULD_STOP, &sqd->state);
 	mutex_lock(&sqd->lock);
-	if (sqd->thread)
-		wake_up_process(sqd->thread);
+	tsk = sqpoll_task_locked(sqd);
+	if (tsk) {
+		WARN_ON_ONCE(tsk == current);
+		wake_up_process(tsk);
+	}
 	mutex_unlock(&sqd->lock);
 	wait_for_completion(&sqd->exited);
 }
@@ -486,7 +494,10 @@ __cold int io_sq_offload_create(struct io_ring_ctx *ctx,
 			goto err_sqpoll;
 		}
 
-		sqd->thread = tsk;
+		mutex_lock(&sqd->lock);
+		rcu_assign_pointer(sqd->thread, tsk);
+		mutex_unlock(&sqd->lock);
+
 		task_to_put = get_task_struct(tsk);
 		ret = io_uring_alloc_task_context(tsk, ctx);
 		wake_up_new_task(tsk);
@@ -514,10 +525,13 @@ __cold int io_sqpoll_wq_cpu_affinity(struct io_ring_ctx *ctx,
 	int ret = -EINVAL;
 
 	if (sqd) {
+		struct task_struct *tsk;
+
 		io_sq_thread_park(sqd);
 		/* Don't set affinity for a dying thread */
-		if (sqd->thread)
-			ret = io_wq_cpu_affinity(sqd->thread->io_uring, mask);
+		tsk = sqpoll_task_locked(sqd);
+		if (tsk)
+			ret = io_wq_cpu_affinity(tsk->io_uring, mask);
 		io_sq_thread_unpark(sqd);
 	}
 
diff --git a/io_uring/sqpoll.h b/io_uring/sqpoll.h
index 4171666b1cf4c..b83dcdec9765f 100644
--- a/io_uring/sqpoll.h
+++ b/io_uring/sqpoll.h
@@ -8,7 +8,7 @@ struct io_sq_data {
 	/* ctx's that are using this sqd */
 	struct list_head	ctx_list;
 
-	struct task_struct	*thread;
+	struct task_struct __rcu *thread;
 	struct wait_queue_head	wait;
 
 	unsigned		sq_thread_idle;
@@ -29,3 +29,9 @@ void io_sq_thread_unpark(struct io_sq_data *sqd);
 void io_put_sq_data(struct io_sq_data *sqd);
 void io_sqpoll_wait_sq(struct io_ring_ctx *ctx);
 int io_sqpoll_wq_cpu_affinity(struct io_ring_ctx *ctx, cpumask_var_t mask);
+
+static inline struct task_struct *sqpoll_task_locked(struct io_sq_data *sqd)
+{
+	return rcu_dereference_protected(sqd->thread,
+					 lockdep_is_held(&sqd->lock));
+}
-- 
2.47.1