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From:   Pradeep P V K <quic_pragalla@quicinc.com>
To:     axboe@kernel.dk
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        Pradeep P V K <quic_pragalla@quicinc.com>
Subject: [PATCH V1] block: Fix null pointer dereference issue on struct io_cq
Date:   Wed, 17 May 2023 14:14:34 +0530
Message-Id: <20230517084434.18932-1-quic_pragalla@quicinc.com>
Precedence: bulk

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);
                                      ->ioc_exit_icq(); gets the same icq
				       ->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@quicinc.com>
---
 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))
+			ioc_exit_icq(icq);
+	}
 	spin_unlock_irq(&ioc->lock);
+	rcu_read_unlock();
 }
 
 /*
-- 
2.17.1