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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