Ack: [xenial:linux-azure PATCH] CIFS: Fix NULL-pointer dereference in smb2_push_mandatory_locks

[Sultan Alsawaf]

On Thu, Jan 23, 2020 at 01:47:47PM -0300, Marcelo Henrique Cerri wrote:
> From: Pavel Shilovsky <pshilov at microsoft.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1860603
> 
> 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 at vger.kernel.org>
> Signed-off-by: Pavel Shilovsky <pshilov at microsoft.com>
> Reviewed-by: Aurelien Aptel <aaptel at suse.com>
> Signed-off-by: Steve French <stfrench at microsoft.com>
> (cherry picked from commit 6f582b273ec23332074d970a7fb25bef835df71f)
> Signed-off-by: Marcelo Henrique Cerri <marcelo.cerri at canonical.com>
> ---
>  fs/cifs/file.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/cifs/file.c b/fs/cifs/file.c
> index efad56aeb161..9febc609cf2c 100644
> --- a/fs/cifs/file.c
> +++ b/fs/cifs/file.c
> @@ -312,9 +312,6 @@ cifs_new_fileinfo(struct cifs_fid *fid, struct file *file,
>  	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;
> @@ -338,6 +335,10 @@ cifs_new_fileinfo(struct cifs_fid *fid, struct file *file,
>  		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;
> -- 
> 2.20.1
> 
> 
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Acked-by: Sultan Alsawaf <sultan.alsawaf at canonical.com>