From 23b4df5d08ff3dfb57a07929f776ae1ce30c5786 Mon Sep 17 00:00:00 2001
From: Ross Zwisler <ross.zwisler@linux.intel.com>
Date: Wed, 6 Sep 2017 16:18:54 -0700
Subject: [PATCH] dax: explain how read(2)/write(2) addresses are validated
Git-commit: a2e050f5a9a9bd2b632d67bd06d87088e6a02dae
Patch-Mainline: v4.14-rc1
References: FATE#323721
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>
Acked-by: Hannes Reinecke <hare@suse.com>
---
fs/dax.c | 5 +++++
1 file changed, 5 insertions(+)
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -966,6 +966,11 @@ dax_iomap_actor(struct inode *inode, lof
if (map_len > end - pos)
map_len = end - pos;
+ /*
+ * The userspace address for the memory copy has already been
+ * validated via access_ok() in either vfs_read() or
+ * vfs_write(), depending on which operation we are doing.
+ */
if (iov_iter_rw(iter) == WRITE)
map_len = dax_copy_from_iter(dax_dev, pgoff, kaddr,
map_len, iter);