Re: [PATCH] dax: explain how read(2)/write(2) addresses are validated

From: Jan Kara <jack@suse.cz>
To: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>, Jan Kara <jack@suse.cz>,
	Kees Cook <keescook@chromium.org>,
	Matthew Wilcox <mawilcox@microsoft.com>,
	Will Deacon <will.deacon@arm.com>,
	linux-nvdimm@lists.01.org, Dave Chinner <david@fromorbit.com>,
	linux-kernel@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Catalin Marinas <catalin.marinas@arm.com>,
	linux-fsdevel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH] dax: explain how read(2)/write(2) addresses are validated
Date: Thu, 17 Aug 2017 10:53:32 +0200	[thread overview]
Message-ID: <20170817085332.GA7644@quack2.suse.cz> (raw)
In-Reply-To: <20170816173615.10098-1-ross.zwisler@linux.intel.com>

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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