Drivers: hv: util: Avoid accessing a ringbuffer not initialized yet

If the KVP (or VSS) daemon starts before the VMBus channel's ringbuffer is
fully initialized, we can hit the panic below:

hv_utils: Registering HyperV Utility Driver
hv_vmbus: registering driver hv_utils
...
BUG: kernel NULL pointer dereference, address: 0000000000000000
CPU: 44 UID: 0 PID: 2552 Comm: hv_kvp_daemon Tainted: G E 6.11.0-rc3+ #1
RIP: 0010:hv_pkt_iter_first+0x12/0xd0
Call Trace:
...
 vmbus_recvpacket
 hv_kvp_onchannelcallback
 vmbus_on_event
 tasklet_action_common
 tasklet_action
 handle_softirqs
 irq_exit_rcu
 sysvec_hyperv_stimer0
 </IRQ>
 <TASK>
 asm_sysvec_hyperv_stimer0
...
 kvp_register_done
 hvt_op_read
 vfs_read
 ksys_read
 __x64_sys_read

This can happen because the KVP/VSS channel callback can be invoked
even before the channel is fully opened:
1) as soon as hv_kvp_init() -> hvutil_transport_init() creates
/dev/vmbus/hv_kvp, the kvp daemon can open the device file immediately and
register itself to the driver by writing a message KVP_OP_REGISTER1 to the
file (which is handled by kvp_on_msg() ->kvp_handle_handshake()) and
reading the file for the driver's response, which is handled by
hvt_op_read(), which calls hvt->on_read(), i.e. kvp_register_done().

2) the problem with kvp_register_done() is that it can cause the
channel callback to be called even before the channel is fully opened,
and when the channel callback is starting to run, util_probe()->
vmbus_open() may have not initialized the ringbuffer yet, so the
callback can hit the panic of NULL pointer dereference.

To reproduce the panic consistently, we can add a "ssleep(10)" for KVP in
__vmbus_open(), just before the first hv_ringbuffer_init(), and then we
unload and reload the driver hv_utils, and run the daemon manually within
the 10 seconds.

Fix the panic by reordering the steps in util_probe() so the char dev
entry used by the KVP or VSS daemon is not created until after
vmbus_open() has completed. This reordering prevents the race condition
from happening.

Reported-by: Dexuan Cui <decui@microsoft.com>
Fixes: e0fa3e5 ("Drivers: hv: utils: fix a race on userspace daemons registration")
Cc: stable@vger.kernel.org
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Acked-by: Wei Liu <wei.liu@kernel.org>
Link: https://lore.kernel.org/r/20241106154247.2271-3-mhklinux@outlook.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20241106154247.2271-3-mhklinux@outlook.com>

drivers/hv/hv_kvp.c

@@ -767,6 +767,12 @@ hv_kvp_init(struct hv_util_service *srv)
 	 */
 	kvp_transaction.state = HVUTIL_DEVICE_INIT;
 
+	return 0;
+}
+
+int
+hv_kvp_init_transport(void)
+{
 	hvt = hvutil_transport_init(kvp_devname, CN_KVP_IDX, CN_KVP_VAL,
 				    kvp_on_msg, kvp_on_reset);
 	if (!hvt)

drivers/hv/hv_snapshot.c

@@ -389,6 +389,12 @@ hv_vss_init(struct hv_util_service *srv)
 	 */
 	vss_transaction.state = HVUTIL_DEVICE_INIT;
 
+	return 0;
+}
+
+int
+hv_vss_init_transport(void)
+{
 	hvt = hvutil_transport_init(vss_devname, CN_VSS_IDX, CN_VSS_VAL,
 				    vss_on_msg, vss_on_reset);
 	if (!hvt) {

drivers/hv/hv_util.c

@@ -141,6 +141,7 @@ static struct hv_util_service util_heartbeat = {
 static struct hv_util_service util_kvp = {
 	.util_cb = hv_kvp_onchannelcallback,
 	.util_init = hv_kvp_init,
+	.util_init_transport = hv_kvp_init_transport,
 	.util_pre_suspend = hv_kvp_pre_suspend,
 	.util_pre_resume = hv_kvp_pre_resume,
 	.util_deinit = hv_kvp_deinit,
@@ -149,6 +150,7 @@ static struct hv_util_service util_kvp = {
 static struct hv_util_service util_vss = {
 	.util_cb = hv_vss_onchannelcallback,
 	.util_init = hv_vss_init,
+	.util_init_transport = hv_vss_init_transport,
 	.util_pre_suspend = hv_vss_pre_suspend,
 	.util_pre_resume = hv_vss_pre_resume,
 	.util_deinit = hv_vss_deinit,
@@ -611,6 +613,13 @@ static int util_probe(struct hv_device *dev,
 	if (ret)
 		goto error;
 
+	if (srv->util_init_transport) {
+		ret = srv->util_init_transport();
+		if (ret) {
+			vmbus_close(dev->channel);
+			goto error;
+		}
+	}
 	return 0;
 
 error:

drivers/hv/hyperv_vmbus.h

@@ -370,12 +370,14 @@ void vmbus_on_event(unsigned long data);
 void vmbus_on_msg_dpc(unsigned long data);
 
 int hv_kvp_init(struct hv_util_service *srv);
+int hv_kvp_init_transport(void);
 void hv_kvp_deinit(void);
 int hv_kvp_pre_suspend(void);
 int hv_kvp_pre_resume(void);
 void hv_kvp_onchannelcallback(void *context);
 
 int hv_vss_init(struct hv_util_service *srv);
+int hv_vss_init_transport(void);
 void hv_vss_deinit(void);
 int hv_vss_pre_suspend(void);
 int hv_vss_pre_resume(void);

include/linux/hyperv.h

@@ -1559,6 +1559,7 @@ struct hv_util_service {
 	void *channel;
 	void (*util_cb)(void *);
 	int (*util_init)(struct hv_util_service *);
+	int (*util_init_transport)(void);
 	void (*util_deinit)(void);
 	int (*util_pre_suspend)(void);
 	int (*util_pre_resume)(void);