From: Kent Overstreet <kent.overstreet@gmail.com>
Subject: Re: [PATCH 1/2] fscrypto: don't use on-stack buffer for
 filename encryption
Date: Sat, 5 Nov 2016 07:13:49 -0800
Message-ID: <20161105151349.e5ap547uno3hfit7@kmo-pixel>
References: <1478210582-86338-1-git-send-email-ebiggers@google.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Cc: tytso@mit.edu, richard@nod.at, linux-f2fs-devel@lists.sourceforge.net,
	linux-crypto@vger.kernel.org, luto@kernel.org,
	linux-fsdevel@vger.kernel.org, jaegeuk@kernel.org,
	linux-ext4@vger.kernel.org
To: Eric Biggers <ebiggers@google.com>
Content-Disposition: inline
In-Reply-To: <1478210582-86338-1-git-send-email-ebiggers@google.com>
Errors-To: linux-f2fs-devel-bounces@lists.sourceforge.net

On Thu, Nov 03, 2016 at 03:03:01PM -0700, Eric Biggers wrote:
> With the new (in 4.9) option to use a virtually-mapped stack
> (CONFIG_VMAP_STACK), stack buffers cannot be used as input/output for
> the scatterlist crypto API because they may not be directly mappable to
> struct page.  For short filenames, fname_encrypt() was encrypting a
> stack buffer holding the padded filename.  Fix it by encrypting the
> filename in-place in the output buffer, thereby making the temporary
> buffer unnecessary.
> 
> This bug could most easily be observed in a CONFIG_DEBUG_SG kernel
> because this allowed the BUG in sg_set_buf() to be triggered.
> 
> Signed-off-by: Eric Biggers <ebiggers@google.com>

> -		alloc_buf = kmalloc(ciphertext_len, GFP_NOFS);
> -		if (!alloc_buf)
> -			return -ENOMEM;
> -		workbuf = alloc_buf;

Vmalloc memory does have struct pages - you just need to use vmalloc_to_page()
instead of virt_to_page. Look at drivers/md/bcache/util.c bch_bio_map() if you
want an example.

It would be better to just fix the sg code to handle vmalloc memory, instead of
adding a kmalloc() that can fail (and an error path that inevitably won't be
tested).

------------------------------------------------------------------------------
Developer Access Program for Intel Xeon Phi Processors
Access to Intel Xeon Phi processor-based developer platforms.
With one year of Intel Parallel Studio XE.
Training and support from Colfax.
Order your platform today. http://sdm.link/xeonphi