Re: [PATCH v2] io_uring: fix use-after-free of sq->thread in __io_uring_show_fdinfo()

[Keith Busch <kbusch@kernel.org>]

On Tue, Jun 10, 2025 at 12:56:31PM -0600, Jens Axboe wrote:
> On 6/10/25 12:44 PM, Keith Busch wrote:
> > On Tue, Jun 10, 2025 at 10:18:01AM -0700, Penglei Jiang wrote:
> >> @@ -379,7 +380,8 @@ static int io_sq_thread(void *data)
> >>  		io_sq_tw(&retry_list, UINT_MAX);
> >>  
> >>  	io_uring_cancel_generic(true, sqd);
> >> -	sqd->thread = NULL;
> >> +	rcu_assign_pointer(sqd->thread, NULL);
> > 
> > I believe this will fail a sparse check without adding the "__rcu" type
> > annotation on the struct's "thread" member.
> 
> I think that only happens the other way around, eg accessing them directly
> when marked with __rcu. I could be entirely wrong, though...

I was just looking at rcu_assign_pointer():

  #define rcu_assign_pointer(p, v)                                              \
  do {                                                                          \
          uintptr_t _r_a_p__v = (uintptr_t)(v);                                 \
          rcu_check_sparse(p, __rcu);                                           \

And rcu_check_sparse expands to this when __CHECKER__ is enabled:

  #define rcu_check_sparse(p, space) \
          ((void)(((typeof(*p) space *)p) == p))

So whatever "p" is, rcu_assign_pointer's checker appears to want it to
be of a type annotated with "__rcu".

But I don't know for sure, so let's just try it and see!

  # make C=1 io_uring/sqpoll.o
  io_uring/sqpoll.c:273:17: error: incompatible types in comparison expression (different address spaces):
  io_uring/sqpoll.c:273:17:    struct task_struct [noderef] __rcu *
  io_uring/sqpoll.c:273:17:    struct task_struct *
  io_uring/sqpoll.c:383:9: error: incompatible types in comparison expression (different address spaces):
  io_uring/sqpoll.c:383:9:    struct task_struct [noderef] __rcu *
  io_uring/sqpoll.c:383:9:    struct task_struct *