Btrfs: fix checks in BTRFS_IOC_CLONE_RANGE

1.  The BTRFS_IOC_CLONE and BTRFS_IOC_CLONE_RANGE ioctls should check
whether the donor file is append-only before writing to it.

2.  The BTRFS_IOC_CLONE_RANGE ioctl appears to have an integer
overflow that allows a user to specify an out-of-bounds range to copy
from the source file (if off + len wraps around).  I haven't been able
to successfully exploit this, but I'd imagine that a clever attacker
could use this to read things he shouldn't.  Even if it's not
exploitable, it couldn't hurt to be safe.
Signed-off-by: Dan Rosenberg <dan.j.rosenberg@gmail.com>
cc: stable@kernel.org
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ioctl.c

@@ -1458,7 +1458,7 @@ static noinline long btrfs_ioctl_clone(struct file *file, unsigned long srcfd,
 	 */
 
 	/* the destination must be opened for writing */
-	if (!(file->f_mode & FMODE_WRITE))
+	if (!(file->f_mode & FMODE_WRITE) || (file->f_flags & O_APPEND))
 		return -EINVAL;
 
 	ret = mnt_want_write(file->f_path.mnt);
@@ -1511,7 +1511,7 @@ static noinline long btrfs_ioctl_clone(struct file *file, unsigned long srcfd,
 
 	/* determine range to clone */
 	ret = -EINVAL;
-	if (off >= src->i_size || off + len > src->i_size)
+	if (off + len > src->i_size || off + len < off)
 		goto out_unlock;
 	if (len == 0)
 		olen = len = src->i_size - off;