In the Linux kernel, the following vulnerability has been resolved: nfs/blocklayout: Don't attempt unregister for invalid block device Since commit d869da91cccb ("nfs/blocklayout: Fix premature PR key unregistration") an unmount of a pNFS SCSI layout-enabled NFS may dereference a NULL block_device in: bl_unregister_scsi+0x16/0xe0 [blocklayoutdriver] bl_free_device+0x70/0x80 [blocklayoutdriver] bl_free_deviceid_node+0x12/0x30 [blocklayoutdriver] nfs4_put_deviceid_node+0x60/0xc0 [nfsv4] nfs4_deviceid_purge_client+0x132/0x190 [nfsv4] unset_pnfs_layoutdriver+0x59/0x60 [nfsv4] nfs4_destroy_server+0x36/0x70 [nfsv4] nfs_free_server+0x23/0xe0 [nfs] deactivate_locked_super+0x30/0xb0 cleanup_mnt+0xba/0x150 task_work_run+0x59/0x90 syscall_exit_to_user_mode+0x217/0x220 do_syscall_64+0x8e/0x160 This happens because even though we were able to create the nfs4_deviceid_node, the lookup for the device was unable to attach the block device to the pnfs_block_dev. If we never found a block device to register, we can avoid this case with the PNFS_BDEV_REGISTERED flag. Move the deref behind the test for the flag.