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

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On Nov 5, 2016 8:13 AM, "Kent Overstreet" <kent.overstreet@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx>
>
> > -             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).

Probably not, because (a) vmalloc_to_page is slow and (b) stack
buffers can span physically noncontiguous pages.

I think it's best to either avoid stack buffers or to teach crypto about kiov.
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