[PATCH] firmware: dmi-sysfs: Fix null-ptr-deref in dmi_sysfs_register_handle

[Chen Zhongjin]

KASAN reported a null-ptr-deref error:

KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
CPU: 0 PID: 1373 Comm: modprobe
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
RIP: 0010:dmi_sysfs_entry_release
...
Call Trace:
 <TASK>
 kobject_put
 dmi_sysfs_register_handle (drivers/firmware/dmi-sysfs.c:540) dmi_sysfs
 dmi_decode_table (drivers/firmware/dmi_scan.c:133)
 dmi_walk (drivers/firmware/dmi_scan.c:1115)
 dmi_sysfs_init (drivers/firmware/dmi-sysfs.c:149) dmi_sysfs
 do_one_initcall (init/main.c:1296)
 ...
Kernel panic - not syncing: Fatal exception
Kernel Offset: 0x4000000 from 0xffffffff81000000
---[ end Kernel panic - not syncing: Fatal exception ]---

It is because previous patch added kobject_put() to release the memory
which will call  dmi_sysfs_entry_release() and list_del().

However, list_add_tail(entry->list) is called after the error block,
so the list_head is uninitialized and cannot be deleted.

Because entry is allocated by kzalloc() so the list.prev is NULL in
the error path. Check it in dmi_sysfs_entry_release() to avoid
deleting uninitialized list_head.

Fixes: 660ba678f999 ("firmware: dmi-sysfs: Fix memory leak in dmi_sysfs_register_handle")

Signed-off-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>
---
 drivers/firmware/dmi-sysfs.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/firmware/dmi-sysfs.c b/drivers/firmware/dmi-sysfs.c
index 66727ad3361b..f8815eeed00c 100644
--- a/drivers/firmware/dmi-sysfs.c
+++ b/drivers/firmware/dmi-sysfs.c
@@ -557,9 +557,12 @@ static void dmi_sysfs_entry_release(struct kobject *kobj)
 {
 	struct dmi_sysfs_entry *entry = to_entry(kobj);
 
-	spin_lock(&entry_list_lock);
-	list_del(&entry->list);
-	spin_unlock(&entry_list_lock);
+	if (entry->list.prev != NULL) {
+		spin_lock(&entry_list_lock);
+		list_del(&entry->list);
+		spin_unlock(&entry_list_lock);
+	}
+
 	kfree(entry);
 }
 
-- 
2.17.1