dax: explain how read(2)/write(2) addresses are validated

dax: explain how read(2)/write(2) addresses are validated

Add a comment explaining how the user addresses provided to read(2) and
write(2) are validated in the DAX I/O path.

We call dax_copy_from_iter() or copy_to_iter() on these without calling
access_ok() first in the DAX code, and there was a concern that the user
might be able to read/write to arbitrary kernel addresses with this
path.

Link: http://lkml.kernel.org/r/20170816173615.10098-1-ross.zwisler@linux.intel.com
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>