linux-ext4 - Re: [PATCH 1/2] fscrypto: don't use on-stack buffer for filename encryption

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Date:   Sat, 5 Nov 2016 07:13:49 -0800
From:   Kent Overstreet <kent.overstreet@...il.com>
To:     Eric Biggers <ebiggers@...gle.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-crypto@...r.kernel.org, tytso@....edu, jaegeuk@...nel.org,
        richard@....at, luto@...nel.org
Subject: Re: [PATCH 1/2] fscrypto: don't use on-stack buffer for filename
 encryption

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@...gle.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).
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