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[Steve Langasek <1856949@xxxxxxxxxxxxxxxxxx>]

** Changed in: linux (Ubuntu Disco)
       Status: Fix Committed => Won't Fix

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https://bugs.launchpad.net/bugs/1856949

Title:
  cifs: kernel NULL pointer dereference, address: 0000000000000038

Status in linux package in Ubuntu:
  Fix Committed
Status in linux source package in Xenial:
  Fix Released
Status in linux source package in Bionic:
  Fix Committed
Status in linux source package in Disco:
  Won't Fix
Status in linux source package in Eoan:
  Fix Committed

Bug description:
  [Impact]

  Currently when the client creates a cifsFileInfo structure for
  a newly opened file, it allocates a list of byte-range locks
  with a pointer to the new cfile and attaches this list to the
  inode's lock list. The latter happens before initializing all
  other fields, e.g. cfile->tlink. Thus a partially initialized
  cifsFileInfo structure becomes available to other threads that
  walk through the inode's lock list. One example of such a thread
  may be an oplock break worker thread that tries to push all
  cached byte-range locks. This causes NULL-pointer dereference
  in smb2_push_mandatory_locks() when accessing cfile->tlink:

  [598428.945633] BUG: kernel NULL pointer dereference, address: 0000000000000038
  ...
  [598428.945749] Workqueue: cifsoplockd cifs_oplock_break [cifs]
  [598428.945793] RIP: 0010:smb2_push_mandatory_locks+0xd6/0x5a0 [cifs]
  ...
  [598428.945834] Call Trace:
  [598428.945870]  ? cifs_revalidate_mapping+0x45/0x90 [cifs]
  [598428.945901]  cifs_oplock_break+0x13d/0x450 [cifs]
  [598428.945909]  process_one_work+0x1db/0x380
  [598428.945914]  worker_thread+0x4d/0x400
  [598428.945921]  kthread+0x104/0x140
  [598428.945925]  ? process_one_work+0x380/0x380
  [598428.945931]  ? kthread_park+0x80/0x80
  [598428.945937]  ret_from_fork+0x35/0x40

  
  [Test Case]

  TBD.

  
  [Fix]

  Backport commit 6f582b273ec23332074d970a7fb25bef835df71f ("CIFS: Fix
  NULL-pointer dereference in smb2_push_mandatory_locks")

  [Regression Potential]

  Low. The patch is fairly simple and it's tagged for stable kernels. In
  fact it is already in some of the released upstream stable kernels.

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