Re: [PATCH V1] block: Fix null pointer dereference issue on struct io_cq

[Yu Kuai]

Hi,

在 2023/05/17 16:58, Yu Kuai 写道:
> Hi,
>
> 在 2023/05/17 16:44, Pradeep P V K 写道:
> > There is a potential race between ioc_clear_fn() and
> > exit_io_context() as shown below, due to which below
> > crash is observed. It can also result into use-after-free
> > issue.
> >
> > context#1:                           context#2:
> > ioc_release_fn()                     do_exit();
> > ->spin_lock(&ioc->lock);             ->exit_io_context();
> > ->ioc_destroy_icq(icq);              ->ioc_exit_icqs();
> >   ->list_del_init(&icq->q_node);       ->spin_lock_irq(&ioc->lock);
> >   ->call_rcu(&icq->__rcu_head,
> >       icq_free_icq_rcu);
> > ->spin_unlock(&ioc->lock);

I think above concurrent scenario is not possible as well.

exit_io_context() doesn't release ioc refcount before ioc_exit_icqs() is
done, so that ioc_release_fn() can never concurrent with
ioc_exit_icqs().

do_exit
 exit_io_context
  ioc_exit_icqs
  put_io_context -> ioc_release_fn won't be called before this

> >                                        ->ioc_exit_icq(); gets the same 
> > icq
> I don't understand how is this possible, the list is protected by
> 'ioc->lock', both hlist_del_init and hlist_for_each_entry are called
> inside the lock.
>
> Thanks,
> Kuai
> >                        ->bfq_exit_icq();
> >                                    This results into below crash as bic
> >                   is NULL as it is derived from icq.
> >                   There is a chance that icq could be
> >                   free'd as well.
> >
> > [33.245722][ T8666] Unable to handle kernel NULL pointer dereference
> > at virtual address 0000000000000018.
> > ...
> > Call trace:
> > [33.325782][ T8666]  bfq_exit_icq+0x28/0xa8
> > [33.325785][ T8666]  exit_io_context+0xcc/0x100
> > [33.325786][ T8666]  do_exit+0x764/0xa58
> > [33.325791][ T8666]  do_group_exit+0x0/0xa0
> > [33.325793][ T8666]  invoke_syscall+0x48/0x114
> > [33.325802][ T8666]  el0_svc_common+0xcc/0x118
> > [33.325805][ T8666]  do_el0_svc+0x34/0xd0
> > [33.325807][ T8666]  el0_svc+0x38/0xd0
> > [33.325812][ T8666]  el0t_64_sync_handler+0x8c/0xfc
> > [33.325813][ T8666]  el0t_64_sync+0x1a0/0x1a4
> >
> > Fix this by checking with ICQ_DESTROYED flags in ioc_exit_icqs().
> > Also, ensure ioc_exit_icq() is accessing icq within rcu_read_lock/unlock
> > so that icq doesn't get free'd up while it is still using it.
> >
> > Signed-off-by: Pradeep P V K <quic_pragalla@xxxxxxxxxxx>
> > ---
> >   block/blk-ioc.c | 8 ++++++--
> >   1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/block/blk-ioc.c b/block/blk-ioc.c
> > index 63fc02042408..1aa34fd46ac8 100644
> > --- a/block/blk-ioc.c
> > +++ b/block/blk-ioc.c
> > @@ -60,10 +60,14 @@ static void ioc_exit_icqs(struct io_context *ioc)
> >   {
> >       struct io_cq *icq;
> > +    rcu_read_lock();
> >       spin_lock_irq(&ioc->lock);
> > -    hlist_for_each_entry(icq, &ioc->icq_list, ioc_node)
> > -        ioc_exit_icq(icq);
> > +    hlist_for_each_entry(icq, &ioc->icq_list, ioc_node) {
> > +        if (!(icq->flags & ICQ_DESTROYED))
By the way, above change doesn't make sense to me as well.
ioc_exit_icq() is called before setting ICQ_DESTROYED, hence if
ICQ_DESTROYED is set, then ICQ_EXITED is set as well, in this case
ioc_exit_icq() won't do anything.

> > +            ioc_exit_icq(icq);
> > +    }
> >       spin_unlock_irq(&ioc->lock);
> > +    rcu_read_unlock();
> >   }
> >   /*

I think I do found a problem, but I'm not sure it's the same in your
case, can you try the following patch?

diff --git a/block/blk-ioc.c b/block/blk-ioc.c
index 63fc02042408..37a56f2bb040 100644
--- a/block/blk-ioc.c
+++ b/block/blk-ioc.c
@@ -78,6 +78,9 @@ static void ioc_destroy_icq(struct io_cq *icq)

        lockdep_assert_held(&ioc->lock);

+       if (icq->flags & ICQ_DESTROYED)
+               return;
+
        radix_tree_delete(&ioc->icq_tree, icq->q->id);
        hlist_del_init(&icq->ioc_node);
        list_del_init(&icq->q_node);
@@ -128,12 +131,7 @@ static void ioc_release_fn(struct work_struct *work)
                        spin_lock(&q->queue_lock);
                        spin_lock(&ioc->lock);

-                       /*
-                        * The icq may have been destroyed when the ioc lock
-                        * was released.
-                        */
-                       if (!(icq->flags & ICQ_DESTROYED))
-                               ioc_destroy_icq(icq);
+                       ioc_destroy_icq(icq);

                        spin_unlock(&q->queue_lock);
                        rcu_read_unlock();
@@ -173,18 +171,17 @@ void ioc_clear_queue(struct request_queue *q)
 {
        LIST_HEAD(icq_list);

+       rcu_read_lock();
        spin_lock_irq(&q->queue_lock);
        list_splice_init(&q->icq_list, &icq_list);
        spin_unlock_irq(&q->queue_lock);

-       rcu_read_lock();
        while (!list_empty(&icq_list)) {
                struct io_cq *icq =
                        list_entry(icq_list.next, struct io_cq, q_node);

                spin_lock_irq(&icq->ioc->lock);
-               if (!(icq->flags & ICQ_DESTROYED))
-                       ioc_destroy_icq(icq);
+               ioc_destroy_icq(icq);
                spin_unlock_irq(&icq->ioc->lock);
        }
        rcu_read_unlock();