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Subject[PATCH 3.16 133/148] CIFS: Fix NULL-pointer dereference in smb2_push_mandatory_locks
3.16.82-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Pavel Shilovsky <pshilov@microsoft.com>

commit 6f582b273ec23332074d970a7fb25bef835df71f upstream.

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

Fix this by reordering initialization steps of the cifsFileInfo
structure: initialize all the fields first and then add the new
byte-range lock list to the inode's lock list.

Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/cifs/file.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -313,9 +313,6 @@ cifs_new_fileinfo(struct cifs_fid *fid,
INIT_LIST_HEAD(&fdlocks->locks);
fdlocks->cfile = cfile;
cfile->llist = fdlocks;
- cifs_down_write(&cinode->lock_sem);
- list_add(&fdlocks->llist, &cinode->llist);
- up_write(&cinode->lock_sem);

cfile->count = 1;
cfile->pid = current->tgid;
@@ -339,6 +336,10 @@ cifs_new_fileinfo(struct cifs_fid *fid,
oplock = 0;
}

+ cifs_down_write(&cinode->lock_sem);
+ list_add(&fdlocks->llist, &cinode->llist);
+ up_write(&cinode->lock_sem);
+
spin_lock(&tcon->open_file_lock);
if (fid->pending_open->oplock != CIFS_OPLOCK_NO_CHANGE && oplock)
oplock = fid->pending_open->oplock;
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