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    Subject[PATCH 5.4 46/92] CIFS: Fix NULL-pointer dereference in smb2_push_mandatory_locks
    Date
    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.

    Cc: Stable <stable@vger.kernel.org>
    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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    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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