[PATCH -next] ubi: fix race between ctrl_cdev_ioctl and ubi_cdev_ioctl

[Baokun Li]

Hulk Robot reported a KASAN report about use-after-free:
 ==================================================================
 BUG: KASAN: use-after-free in __list_del_entry_valid+0x13d/0x160
 Read of size 8 at addr ffff888035e37d98 by task ubiattach/1385
 [...]
 Call Trace:
  klist_dec_and_del+0xa7/0x4a0
  klist_put+0xc7/0x1a0
  device_del+0x4d4/0xed0
  cdev_device_del+0x1a/0x80
  ubi_attach_mtd_dev+0x2951/0x34b0 [ubi]
  ctrl_cdev_ioctl+0x286/0x2f0 [ubi]

 Allocated by task 1414:
  device_add+0x60a/0x18b0
  cdev_device_add+0x103/0x170
  ubi_create_volume+0x1118/0x1a10 [ubi]
  ubi_cdev_ioctl+0xb7f/0x1ba0 [ubi]

 Freed by task 1385:
  cdev_device_del+0x1a/0x80
  ubi_remove_volume+0x438/0x6c0 [ubi]
  ubi_cdev_ioctl+0xbf4/0x1ba0 [ubi]
 [...]
 ==================================================================

The following race could cause the use-after-free problem:
           cpu1                   cpu2                  cpu3
_______________________|________________________|______________________
ctrl_cdev_ioctl
 ubi_attach_mtd_dev
  uif_init
                           ubi_cdev_ioctl
                            ubi_create_volume
                             cdev_device_add
   ubi_add_volume
   // sysfs exist
   kill_volumes
                                                    ubi_cdev_ioctl
                                                     ubi_remove_volume
                                                      cdev_device_del
                                                       // first free
    ubi_free_volume
     cdev_del
     // double free
   cdev_device_del

The lock held by ctrl_cdev_ioctl is ubi_devices_mutex, but the lock held by
ubi cdev_ioctl is ubi->device_mutex. Therefore, the two locks can be
concurrent. However, volume addition and deletion operations are performed
in both ctrl_cdev_ioctl and ubi_cdev_ioctl. When these operations are
performed concurrently in both ioctl, a race condition may be triggered.The
UAF stack above is one such case.

To solve this problem, add spin_lock(&ubi->volumes_lock) to ctrl_cdev_ioctl
where volumes are added and deleted.

Reported-by: Hulk Robot <hulkci@xxxxxxxxxx>
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
---
 drivers/mtd/ubi/build.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/mtd/ubi/build.c b/drivers/mtd/ubi/build.c
index a7e3eb9befb6..708b1b96de01 100644
--- a/drivers/mtd/ubi/build.c
+++ b/drivers/mtd/ubi/build.c
@@ -467,6 +467,7 @@ static int uif_init(struct ubi_device *ubi)
 	if (err)
 		goto out_unreg;
 
+	spin_lock(&ubi->volumes_lock);
 	for (i = 0; i < ubi->vtbl_slots; i++)
 		if (ubi->volumes[i]) {
 			err = ubi_add_volume(ubi, ubi->volumes[i]);
@@ -475,11 +476,13 @@ static int uif_init(struct ubi_device *ubi)
 				goto out_volumes;
 			}
 		}
+	spin_unlock(&ubi->volumes_lock);
 
 	return 0;
 
 out_volumes:
 	kill_volumes(ubi);
+	spin_unlock(&ubi->volumes_lock);
 	cdev_device_del(&ubi->cdev, &ubi->dev);
 out_unreg:
 	unregister_chrdev_region(ubi->cdev.dev, ubi->vtbl_slots + 1);
-- 
2.31.1