Re: [PATCH 10/13] iomap: use a function pointer for dio submits

[Gao Xiang <hsiangkao@aol.com>]

Hi Willy,

On Fri, Aug 09, 2019 at 01:45:17PM -0700, Matthew Wilcox wrote:
> On Wed, Aug 07, 2019 at 10:49:36PM -0700, Eric Biggers wrote:
> > On Thu, Aug 08, 2019 at 12:26:42PM +0800, Gao Xiang wrote:
> > >     1. decrypt->verity->decompress
> > > 
> > >     2. verity->decompress->decrypt
> > > 
> > >     3. decompress->decrypt->verity
> > > 
> > >    1. and 2. could cause less computation since it processes
> > >    compressed data, and the security is good enough since
> > >    the behavior of decompression algorithm is deterministic.
> > >    3 could cause more computation.
> > > 
> > > All I want to say is the post process is so complicated since we have
> > > many selection if encryption, decompression, verification are all involved.
> > > 
> > > Maybe introduce a core subset to IOMAP is better for long-term
> > > maintainment and better performance. And we should consider it
> > > more carefully.
> > > 
> > 
> > FWIW, the only order that actually makes sense is decrypt->decompress->verity.
> 
> That used to be true, but a paper in 2004 suggested it's not true.
> Further work in this space in 2009 based on block ciphers:
> https://arxiv.org/pdf/1009.1759
> 
> It looks like it'd be computationally expensive to do, but feasible.

Yes, maybe someone cares where encrypt is at due to their system design.

and I thought over these days, I have to repeat my thought of verity
again :( the meaningful order ought to be "decrypt->verity->decompress"
rather than "decrypt->decompress->verity" if compression is involved.

since most (de)compress algorithms are complex enough (allocate memory and
do a lot of unsafe stuffes such as wildcopy) and even maybe unsafe by its
design, we cannot do verity in the end for security consideration thus
the whole system can be vulnerable by this order from malformed on-disk
data. In other words, we need to verify on compressed data.

Fsverity is fine for me since most decrypt algorithms is stable and reliable
and no compression by its design, but if some decrypt software algorithms is
complicated enough, I'd suggest "verity->decrypt" as well to some extent.

Considering transformation "A->B->C->D->....->verity", if any of "A->B->C
->D->..." is attacked by the malformed on-disk data... It would crash or
even root the whole operating system.

All in all, we have to verify data earlier in order to get trusted data
for later complex transformation chains.

The performance benefit I described in my previous email, it seems no need
to say again... please take them into consideration and I think it's no
easy to get a unique generic post-read order for all real systems.

Thanks,
Gao Xiang

> 
> > Decrypt before decompress, i.e. encrypt after compress, because only the
> > plaintext can be compressible; the ciphertext isn't.