io_uring/sqpoll: don't put task_struct on tctx setup failure

JIRA: https://issues.redhat.com/browse/RHEL-105612

commit f2320f1
Author: Jens Axboe <axboe@kernel.dk>
Date:   Tue Jun 17 06:43:18 2025 -0600

    io_uring/sqpoll: don't put task_struct on tctx setup failure
    
    A recent commit moved the error handling of sqpoll thread and tctx
    failures into the thread itself, as part of fixing an issue. However, it
    missed that tctx allocation may also fail, and that
    io_sq_offload_create() does its own error handling for the task_struct
    in that case.
    
    Remove the manual task putting in io_sq_offload_create(), as
    io_sq_thread() will notice that the tctx did not get setup and hence it
    should put itself and exit.
    
    Reported-by: syzbot+763e12bbf004fb1062e4@syzkaller.appspotmail.com
    Fixes: ac0b8b3 ("io_uring: fix use-after-free of sq->thread in __io_uring_show_fdinfo()")
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>

Author: JeffMoyer

io_uring/sqpoll.c

@@ -427,7 +427,6 @@ void io_sqpoll_wait_sq(struct io_ring_ctx *ctx)
 __cold int io_sq_offload_create(struct io_ring_ctx *ctx,
 				struct io_uring_params *p)
 {
-	struct task_struct *task_to_put = NULL;
 	int ret;
 
 	/* Retain compatibility with failing for an invalid attach attempt */
@@ -511,7 +510,7 @@ __cold int io_sq_offload_create(struct io_ring_ctx *ctx,
 		rcu_assign_pointer(sqd->thread, tsk);
 		mutex_unlock(&sqd->lock);
 
-		task_to_put = get_task_struct(tsk);
+		get_task_struct(tsk);
 		ret = io_uring_alloc_task_context(tsk, ctx);
 		wake_up_new_task(tsk);
 		if (ret)
@@ -526,8 +525,6 @@ __cold int io_sq_offload_create(struct io_ring_ctx *ctx,
 	complete(&ctx->sq_data->exited);
 err:
 	io_sq_thread_finish(ctx);
-	if (task_to_put)
-		put_task_struct(task_to_put);
 	return ret;
 }