[PATCH v2 1/2] kernel: param: handle NULL module_kset in lookup_or_create_module_kobject()

[Shashank Balaji <shashank.mahadasyam@sony.com>]

module_kset is initialized in a subsys_initcall. If a built-in driver tries to
register before subsys_initcall with its struct device_driver's mod_name set,
then a null module_kset is dereferenced via this call trace:

     [    0.095865] Call trace:
     [    0.095999]  _raw_spin_lock+0x4c/0x6c (P)
     [    0.096150]  kset_find_obj+0x24/0x104
     [    0.096209]  lookup_or_create_module_kobject+0x2c/0xd8
     [    0.096274]  module_add_driver+0xd4/0x138
     [    0.096328]  bus_add_driver+0x16c/0x268
     [    0.096380]  driver_register+0x68/0x100
     [    0.096428]  __platform_driver_register+0x24/0x30
     [    0.096486]  tegra194_cbb_init+0x24/0x30
     [    0.096540]  do_one_initcall+0xdc/0x250
     [    0.096608]  do_initcall_level+0x9c/0xd0
     [    0.096660]  do_initcalls+0x54/0x94
     [    0.096706]  do_basic_setup+0x20/0x2c
     [    0.096753]  kernel_init_freeable+0xc8/0x154
     [    0.096807]  kernel_init+0x20/0x1a0
     [    0.096851]  ret_from_fork+0x10/0x20

So, return null in lookup_or_create_module_kobject() if module_kset is null.
Existing callers handle null already.

Fixes: f30c53a873d0 ("MODULES: add the module name for built in kernel drivers")
Co-developed-by: Rahul Bukte <rahul.bukte@sony.com>
Signed-off-by: Rahul Bukte <rahul.bukte@sony.com>
Signed-off-by: Shashank Balaji <shashank.mahadasyam@sony.com>
---
This bug is triggered by the next patch on arm64 defconfig: tegra194-cbb tries
to register from a pure_initcall, and with the next patch adding mod_name, this
null deref is hit.
---
 kernel/params.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/params.c b/kernel/params.c
index 74d620bc2521..881c7328c059 100644
--- a/kernel/params.c
+++ b/kernel/params.c
@@ -752,6 +752,9 @@ lookup_or_create_module_kobject(const char *name)
 	struct kobject *kobj;
 	int err;
 
+	if (!module_kset)
+		return NULL;
+
 	kobj = kset_find_obj(module_kset, name);
 	if (kobj)
 		return to_module_kobject(kobj);

-- 
2.43.0