[PATCH 1/2] nbd: fix use-after-freed crash for nbd->recv_workq

[xiubli@redhat.com]

From: Xiubo Li <xiubli@redhat.com>

The crash call trace:

<6>[ 1012.319386] block nbd1: NBD_DISCONNECT
<1>[ 1012.319437] BUG: kernel NULL pointer dereference, address: 0000000000000020
<1>[ 1012.319439] #PF: supervisor write access in kernel mode
<1>[ 1012.319441] #PF: error_code(0x0002) - not-present page
<6>[ 1012.319442] PGD 0 P4D 0
<4>[ 1012.319448] Oops: 0002 [#1] SMP NOPTI
<4>[ 1012.319454] CPU: 9 PID: 25111 Comm: rbd-nbd Tainted: G            E     5.9.0+ #6
   [...]
<4>[ 1012.319505] PKRU: 55555554
<4>[ 1012.319506] Call Trace:
<4>[ 1012.319560]  flush_workqueue+0x81/0x440
<4>[ 1012.319598]  nbd_disconnect_and_put+0x50/0x70 [nbd]
<4>[ 1012.319607]  nbd_genl_disconnect+0xc7/0x170 [nbd]
<4>[ 1012.319627]  genl_rcv_msg+0x1dd/0x2f9
<4>[ 1012.319642]  ? genl_start+0x140/0x140
<4>[ 1012.319644]  netlink_rcv_skb+0x49/0x110
<4>[ 1012.319649]  genl_rcv+0x24/0x40
<4>[ 1012.319651]  netlink_unicast+0x1a5/0x280
<4>[ 1012.319653]  netlink_sendmsg+0x23d/0x470
<4>[ 1012.319667]  sock_sendmsg+0x5b/0x60
<4>[ 1012.319676]  ____sys_sendmsg+0x1ef/0x260
<4>[ 1012.319679]  ? copy_msghdr_from_user+0x5c/0x90
<4>[ 1012.319680]  ? ____sys_recvmsg+0xa5/0x180
<4>[ 1012.319682]  ___sys_sendmsg+0x7c/0xc0
<4>[ 1012.319683]  ? copy_msghdr_from_user+0x5c/0x90
<4>[ 1012.319685]  ? ___sys_recvmsg+0x89/0xc0
<4>[ 1012.319692]  ? __wake_up_common_lock+0x87/0xc0
<4>[ 1012.319715]  ? __check_object_size+0x46/0x173
<4>[ 1012.319727]  ? _copy_to_user+0x22/0x30
<4>[ 1012.319729]  ? move_addr_to_user+0xc3/0x100
<4>[ 1012.319731]  __sys_sendmsg+0x57/0xa0
<4>[ 1012.319744]  do_syscall_64+0x33/0x40
<4>[ 1012.319760]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
<4>[ 1012.319780] RIP: 0033:0x7f5baa8e3ad5

In case the reference of nbd->config reached zero and trying to
free the related resource, including the nbd->recv_workq, if another
process send one DISCONENCT cmd by using the netlink, it will be
potentially crash like this.

This will the nbd->recv_workq under the nbd->config_lock.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
---
 drivers/block/nbd.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index edf8b632e3d2..3d3f4255e495 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -1327,9 +1327,10 @@ static int nbd_start_device_ioctl(struct nbd_device *nbd, struct block_device *b
 					 atomic_read(&config->recv_threads) == 0);
 	if (ret)
 		sock_shutdown(nbd);
-	flush_workqueue(nbd->recv_workq);
 
 	mutex_lock(&nbd->config_lock);
+	flush_workqueue(nbd->recv_workq);
+
 	nbd_bdev_reset(bdev);
 	/* user requested, ignore socket errors */
 	if (test_bit(NBD_RT_DISCONNECT_REQUESTED, &config->runtime_flags))
@@ -1339,6 +1340,18 @@ static int nbd_start_device_ioctl(struct nbd_device *nbd, struct block_device *b
 	return ret;
 }
 
+static void nbd_config_clear_rt_ref_and_put(struct nbd_device *nbd)
+{
+	lockdep_assert_held(&nbd->config_lock);
+
+	if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF,
+			       &nbd->config->runtime_flags)) {
+		mutex_unlock(&nbd->config_lock);
+		nbd_config_put(nbd);
+		mutex_lock(&nbd->config_lock);
+	}
+}
+
 static void nbd_clear_sock_ioctl(struct nbd_device *nbd,
 				 struct block_device *bdev)
 {
@@ -2006,16 +2019,15 @@ static void nbd_disconnect_and_put(struct nbd_device *nbd)
 	mutex_lock(&nbd->config_lock);
 	nbd_disconnect(nbd);
 	nbd_clear_sock(nbd);
-	mutex_unlock(&nbd->config_lock);
+
 	/*
 	 * Make sure recv thread has finished, so it does not drop the last
 	 * config ref and try to destroy the workqueue from inside the work
 	 * queue.
 	 */
 	flush_workqueue(nbd->recv_workq);
-	if (test_and_clear_bit(NBD_RT_HAS_CONFIG_REF,
-			       &nbd->config->runtime_flags))
-		nbd_config_put(nbd);
+	nbd_config_clear_rt_ref_and_put(nbd);
+	mutex_unlock(&nbd->config_lock);
 }
 
 static int nbd_genl_disconnect(struct sk_buff *skb, struct genl_info *info)
-- 
2.18.4