Re: [PATCH 0/3] add support for metadata encryption to F2FS

[Chao Yu <yuchao0@huawei.com>]

On 2020/12/24 18:13, Satya Tangirala wrote:
> On Tue, Dec 22, 2020 at 07:47:45PM +0800, Chao Yu wrote:
>> On 2020/12/18 19:53, Satya Tangirala wrote:
>>> On Fri, Dec 18, 2020 at 05:02:23PM +0800, Chao Yu wrote:
>>>> But, what's the plan about supporting software encryption for metadata? Current
>>>> f2fs write flow will handle all operations which may encounter failure before
>>>> allocating block address for node, if we do allocation first, and then use pba
>>>> as IV to encrypt node block, it will be a little complicated to revert allocation
>>>> if we fail to encrypt node block.
>>>>
>>> Software encryption for metadata is supported through the blk-crypto
>>
>> blk-crypto will encrypt all data in filesystem, if FBE is enabled, data may
>> be encrypted twice?
> blk-crypto will only encrypt bios as directed to do so by the encryption
> context set on the bio. That encryption context is constructed by the
> submitter of the bio - in our case, the submitter is the filesystem.
> So the filesystem decides which bio gets encrypted with
> which key/algorithm/etc (and in the current implementation, each bio
> only supports a single bi_crypt_context, so *only one* layer of
> encryption is possible with blk-crypto anyway). So no, data won't be
> encrypted twice, because F2FS/fscrypt ensure that it does not (and the
> filesystem knows exactly which blocks need metadata encryption, and
> which blocks need FBE).

Oh, sorry, I misunderstand blk-crypto as dm-crypt...

So once hardware encryption is absent, blk-crypto will use blk-crypto-fallback
to encrypt bio data with software crypto, I see.

>>
>> And why not supporting hardware encryption for metadata in blk-crypto? then
>> both f2fs and ext4 can use inline-encryption based blk-crypto?
>>
> I may be misunderstanding what you're asking, but I think you're asking
> why not make blk-crypto do metadata encryption (without explicit
> involvement from filesystems)? Or more generally, why not do metadata
> encryption below the filesystem layer?

Yes.

> 
> As mentioned above, the filesystem is what knows which blocks need to be
> metadata encrypted and which blocks need to be FBE encrypted (or even
> just read without any encryption at all) - the block layer doesn't have
> this information, and so can't effectively decide which blocks to use
> the metadata encryption key on. Fwiw, Android does take a somewhat
> similar approach to what you're suggesting here (I explain more in
> detail in the cover letter for v2 of this patch series at
> https://lore.kernel.org/linux-fscrypt/20201217150435.1505269-1-satyat@google.com/

Ah, thanks for all your detailed explanation, now, I can see the
context.

> ). In Android, we have a new DM target (called dm-default-key) that adds
> an encryption context to any bio that doesn't already have an encryption
> context - so the assumption in general is that if the filesystem wants to
> use an FBE key, it would have already set the encryption context on the
> bio, so if a bio reaches dm-default-key without an encryption context,
> it must mean that it needs metadata encryption. However, that assumption
> doesn't always hold because F2FS sometimes needs to read the ciphertext
> of FBE files without having the file's FBE key available - in those
> situations, F2FS will send a bio without any encryption context, but
> will also tell dm-default-key to *not* add the metadata encryption
> context. That's a layering violation, which is why I'm not using that
> approach here.
> 
> Does that answer your question? Or am I misunderstanding what you're
> asking?

Yup, thank you!

Thanks,

>> Thanks,
>>
>>> framework - so encryption will happen in the block layer, not the
>>> filesystem layer. So there's nothing extra/special we need to do if
>>> there's an encryption failure - an encryption failure is no different
>>> from a read/write failure in a lower layer from f2fs' perspective.
>>> .
>>>
> .
>