Re: [syzbot] [io-uring?] KASAN: slab-use-after-free Read in io_poll_remove_entries

[Hillf Danton <hdanton@sina.com>]

> Date: Sat, 19 Jul 2025 10:29:34 -0700	[thread overview]
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    4871b7cb27f4 Merge tag 'v6.16-rc6-smb3-client-fixes' of gi..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1288c38c580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=fa738a4418f051ee
> dashboard link: https://syzkaller.appspot.com/bug?extid=01523a0ae5600aef5895
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1688c38c580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=166ed7d4580000

What is difficult to understand is why rcu failed to prevent the uaf.

#syz test

--- x/include/linux/comedi/comedidev.h
+++ y/include/linux/comedi/comedidev.h
@@ -364,6 +364,7 @@ struct comedi_async {
 	struct comedi_cmd cmd;
 	wait_queue_head_t wait_head;
 	unsigned int cb_mask;
+	struct rcu_head rcu;
 	int (*inttrig)(struct comedi_device *dev, struct comedi_subdevice *s,
 		       unsigned int x);
 };
--- l/drivers/comedi/drivers.c
+++ y/drivers/comedi/drivers.c
@@ -168,7 +168,7 @@ static void comedi_device_detach_cleanup
 			comedi_free_subdevice_minor(s);
 			if (s->async) {
 				comedi_buf_alloc(dev, s, 0);
-				kfree(s->async);
+				kfree_rcu(s->async, rcu);
 			}
 			kfree(s->readback);
 		}
--