[PATCH RESEND 1/4] PM / devfreq: Fix possible null pointer issue in devfreq_add_governor()

[Yaxiong Tian]

When a user removes a governor using devfreq_remove_governor(), if
the current device is using this governor, devfreq->governor will
be set to NULL. When the user registers any governor
using devfreq_add_governor(), since devfreq->governor is NULL, a
null pointer error occurs in strncmp().

For example: A user loads the userspace gov through a module, then
a device selects userspace. When unloading the userspace module and
then loading it again, the null pointer error occurs:

Unable to handle kernel NULL pointer dereference at virtual address
0000000000000010
Mem abort info:
ESR = 0x0000000096000004
EC = 0x25: DABT (current EL), IL = 32 bits
*******************skip *********************
Call trace:
__pi_strncmp+0x20/0x1b8
devfreq_userspace_init+0x1c/0xff8 [governor_userspace]
do_one_initcall+0x4c/0x278
do_init_module+0x5c/0x218
load_module+0x1f1c/0x1fc8
init_module_from_file+0x8c/0xd0
__arm64_sys_finit_module+0x220/0x3d8
invoke_syscall+0x48/0x110
el0_svc_common.constprop.0+0xbc/0xe8
do_el0_svc+0x20/0x30
el0_svc+0x24/0xb8
el0t_64_sync_handler+0xb8/0xc0
el0t_64_sync+0x14c/0x150

To fix this issue, modify the relevant logic in devfreq_add_governor():
Only check whether the new governor matches the existing one when
devfreq->governor exists. When devfreq->governor is NULL, directly
select the new governor and perform the DEVFREQ_GOV_START operation.

Fixes: 1b5c1be2c88e ("PM / devfreq: map devfreq drivers to governor using name")
Signed-off-by: Yaxiong Tian <tianyaxiong@xxxxxxxxxx>
---
 drivers/devfreq/devfreq.c | 24 +++++++++++-------------
 1 file changed, 11 insertions(+), 13 deletions(-)

diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
index 54f0b18536db..63ce6e25abe2 100644
--- a/drivers/devfreq/devfreq.c
+++ b/drivers/devfreq/devfreq.c
@@ -1288,23 +1288,21 @@ int devfreq_add_governor(struct devfreq_governor *governor)
 		int ret = 0;
 		struct device *dev = devfreq->dev.parent;
 
-		if (!strncmp(devfreq->governor->name, governor->name,
+		if (devfreq->governor && !strncmp(devfreq->governor->name, governor->name,
 			     DEVFREQ_NAME_LEN)) {
 			/* The following should never occur */
-			if (devfreq->governor) {
+			dev_warn(dev,
+				 "%s: Governor %s already present\n",
+				 __func__, devfreq->governor->name);
+			ret = devfreq->governor->event_handler(devfreq,
+						DEVFREQ_GOV_STOP, NULL);
+			if (ret) {
 				dev_warn(dev,
-					 "%s: Governor %s already present\n",
-					 __func__, devfreq->governor->name);
-				ret = devfreq->governor->event_handler(devfreq,
-							DEVFREQ_GOV_STOP, NULL);
-				if (ret) {
-					dev_warn(dev,
-						 "%s: Governor %s stop = %d\n",
-						 __func__,
-						 devfreq->governor->name, ret);
-				}
-				/* Fall through */
+					 "%s: Governor %s stop = %d\n",
+					 __func__,
+					 devfreq->governor->name, ret);
 			}
+		} else if (!devfreq->governor) {
 			devfreq->governor = governor;
 			ret = devfreq->governor->event_handler(devfreq,
 						DEVFREQ_GOV_START, NULL);
-- 
2.25.1