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SubjectRe: [PATCH] dax: explain how read(2)/write(2) addresses are validated
On Wed 16-08-17 11:36:15, Ross Zwisler wrote:
> 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.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>

Looks OK to me so feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Just I'd note that standard buffered read / write path is no different so I
don't see a big point in adding this comment when it is not in any other
path either...

Honza

> ---
>
> Adding a comment instead of adding redundant access_ok() calls in the DAX
> code. If this is the wrong path to take, please let me know.
>
> fs/dax.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 8c67517..2d50f32 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -1060,6 +1060,11 @@ dax_iomap_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> 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);
> --
> 2.9.5
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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