inode->i_private is assigned by a Node pointer only after registering a
new binary format, so it could be NULL if inode was created by
bm_fill_super() (or iput() was called by the error path in
bm_register_write()), and this could result in NULL pointer dereference
when evicting such an inode.  e.g.  mount binfmt_misc filesystem then
umount it immediately:

  mount -t binfmt_misc binfmt_misc /proc/sys/fs/binfmt_misc
  umount /proc/sys/fs/binfmt_misc

will result in

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000013
  IP: bm_evict_inode+0x16/0x40 [binfmt_misc]
  ...
  Call Trace:
   evict+0xd3/0x1a0
   iput+0x17d/0x1d0
   dentry_unlink_inode+0xb9/0xf0
   __dentry_kill+0xc7/0x170
   shrink_dentry_list+0x122/0x280
   shrink_dcache_parent+0x39/0x90
   do_one_tree+0x12/0x40
   shrink_dcache_for_umount+0x2d/0x90
   generic_shutdown_super+0x1f/0x120
   kill_litter_super+0x29/0x40
   deactivate_locked_super+0x43/0x70
   deactivate_super+0x45/0x60
   cleanup_mnt+0x3f/0x70
   __cleanup_mnt+0x12/0x20
   task_work_run+0x86/0xa0
   exit_to_usermode_loop+0x6d/0x99
   syscall_return_slowpath+0xba/0xf0
   entry_SYSCALL_64_fastpath+0xa3/0xa

Fix it by making sure Node (e) is not NULL.

Link: http://lkml.kernel.org/r/20171010100642.31786-1-eguan@redhat.com
Fixes: 83f91827 ("exec: binfmt_misc: shift filp_close(interp_file) from kill_node() to bm_evict_inode()")
Signed-off-by: Eryu Guan <eguan@redhat.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>