CVE-2021-47634: In the Linux kernel, the following vulnerability has been resolved:  ubi: Fix race condition between ctrl_cdev_ioctl and ubi_cdev_ioctl  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 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.  ctrl_cdev_ioctl contains two operations: ubi_attach and ubi_detach. ubi_detach is bug-free because it uses reference counting to prevent concurrency. However, uif_init and uif_close in ubi_attach may race with ubi_cdev_ioctl.  uif_init will race with ubi_cdev_ioctl as in the following stack.            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  And uif_close will race with ubi_cdev_ioctl as in the following stack.            cpu1                   cpu2                  cpu3 _______________________|________________________|______________________ ctrl_cdev_ioctl  ubi_attach_mtd_dev   uif_init                            ubi_cdev_ioctl                             ubi_create_volume                              cdev_device_add   ubi_debugfs_init_dev   //error goto out_uif;   uif_close    kill_volumes                                                     ubi_cdev_ioctl                                                      ubi_remove_volume                                                       cdev_device_del                                                        // first free     ubi_free_volume     // double free  The cause of this problem is that commit 714fb87e8bc0 make device "available" before it becomes accessible via sysfs. Therefore, we roll back the modification. We will fix the race condition between ubi device creation and udev by removing ubi_get_device in vol_attribute_show and dev_attribute_show.This avoids accessing uninitialized ubi_devices[ubi_num].  ubi_get_device is used to prevent devices from being deleted during sysfs execution. However, now kernfs ensures that devices will not be deleted before all reference counting are released. The key process is shown in the following stack.  device_del   device_remove_attrs     device_remove_groups       sysfs_remove_groups         sysfs_remove_group           remove_files             kernfs_remove_by_name               kernfs_remove_by_name_ns                 __kernfs_remove                   kernfs_drain