Re: [PATCH] locks: close potential race between setlease and open

[Bruce Fields]

On Mon, Jul 08, 2013 at 09:30:55AM -0400, Jeff Layton wrote:
> As Al Viro points out, there is an unlikely, but possible race between
> opening a file and setting a lease on it. generic_add_lease is done with
> the i_lock held, but the inode->i_flock check in break_lease is
> lockless. It's possible for another task doing an open to do the entire
> pathwalk and call break_lease between the point where generic_add_lease
> checks for a conflicting open and adds the lease to the list. If this
> occurs, we can end up with a lease set on the file with a conflicting
> open.
> 
> To guard against that, check again for a conflicting open after adding
> the lease to the i_flock list. If the above race occurs, then we can
> simply unwind the lease setting and return -EAGAIN.

Maybe it's an entirely theoretical question at this point, but in the
absence of any lock or memory barrier on the lease-setter's side I still
don't understand what guarantees that the opener calling break_lease
will see the new value of i_flock.

--b.


> 
> Cc: Bruce Fields <bfields@xxxxxxxxxxxx>
> Reported-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
>  fs/locks.c | 31 ++++++++++++++++++++++++-------
>  1 file changed, 24 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/locks.c b/fs/locks.c
> index b27a300..9f7f647 100644
> --- a/fs/locks.c
> +++ b/fs/locks.c
> @@ -1455,6 +1455,19 @@ int fcntl_getlease(struct file *filp)
>  	return type;
>  }
>  
> +static int
> +check_conflicting_open(struct dentry *dentry, long arg)
> +{
> +	struct inode *inode = dentry->d_inode;
> +
> +	if ((arg == F_RDLCK) && (atomic_read(&inode->i_writecount) > 0))
> +		return -EAGAIN;
> +	if ((arg == F_WRLCK) && ((d_count(dentry) > 1) ||
> +	    (atomic_read(&inode->i_count) > 1)))
> +		return -EAGAIN;
> +	return 0;
> +}
> +
>  static int generic_add_lease(struct file *filp, long arg, struct file_lock **flp)
>  {
>  	struct file_lock *fl, **before, **my_before = NULL, *lease;
> @@ -1464,12 +1477,8 @@ static int generic_add_lease(struct file *filp, long arg, struct file_lock **flp
>  
>  	lease = *flp;
>  
> -	error = -EAGAIN;
> -	if ((arg == F_RDLCK) && (atomic_read(&inode->i_writecount) > 0))
> -		goto out;
> -	if ((arg == F_WRLCK)
> -	    && ((d_count(dentry) > 1)
> -		|| (atomic_read(&inode->i_count) > 1)))
> +	error = check_conflicting_open(dentry, arg);
> +	if (error)
>  		goto out;
>  
>  	/*
> @@ -1514,8 +1523,16 @@ static int generic_add_lease(struct file *filp, long arg, struct file_lock **flp
>  		goto out;
>  
>  	locks_insert_lock(before, lease);
> -	return 0;
>  
> +	/*
> +	 * The check in break_lease() is lockless. It's possible for another
> +	 * open to race in after we did the earlier check for a conflicting
> +	 * open but before the lease was inserted. Check again for a
> +	 * conflicting open and cancel the lease if there is one.
> +	 */
> +	error = check_conflicting_open(dentry, arg);
> +	if (error)
> +		locks_delete_lock(flp);
>  out:
>  	return error;
>  }
> -- 
> 1.8.1.4
> 
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