Re: [PATCH v3 04/22] fscrypt: add extent-based encryption

[Sweet Tea Dorminy <sweettea-kernel@dorminy.me>]

On 10/20/22 19:56, Eric Biggers wrote:
> On Thu, Oct 20, 2022 at 06:55:04PM -0400, Sweet Tea Dorminy wrote:
>>
>>
>> On 10/20/22 17:45, Eric Biggers wrote:
>>> On Thu, Oct 20, 2022 at 12:58:23PM -0400, Sweet Tea Dorminy wrote:
>>>> Some filesystems need to encrypt data based on extents, rather than on
>>>> inodes, due to features incompatible with inode-based encryption. For
>>>> instance, btrfs can have multiple inodes referencing a single block of
>>>> data, and moves logical data blocks to different physical locations on
>>>> disk in the background; these two features mean traditional inode-based
>>>> file contents encryption will not work for btrfs.
>>>>
>>>> This change introduces fscrypt_extent_context objects, in analogy to
>>>> existing context objects based on inodes. For a filesystem which opts to
>>>> use extent-based encryption, a new hook provides a new
>>>> fscrypt_extent_context, generated in close analogy to the IVs generated
>>>> with existing policies. During file content encryption/decryption, the
>>>> existing fscrypt_context object provides key information, while the new
>>>> fscrypt_extent_context provides IV information. For filename encryption,
>>>> the existing IV generation methods are still used, since filenames are
>>>> not stored in extents.
>>>>
>>>> Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
>>>> ---
>>>>    fs/crypto/crypto.c          | 20 ++++++++--
>>>>    fs/crypto/fscrypt_private.h | 25 +++++++++++-
>>>>    fs/crypto/inline_crypt.c    | 28 ++++++++++---
>>>>    fs/crypto/policy.c          | 79 +++++++++++++++++++++++++++++++++++++
>>>>    include/linux/fscrypt.h     | 47 ++++++++++++++++++++++
>>>>    5 files changed, 189 insertions(+), 10 deletions(-)
>>>>
>>>> diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c
>>>> index 7fe5979fbea2..08b495dc5c0c 100644
>>>> --- a/fs/crypto/crypto.c
>>>> +++ b/fs/crypto/crypto.c
>>>> @@ -81,8 +81,22 @@ void fscrypt_generate_iv(union fscrypt_iv *iv, u64 lblk_num,
>>>>    			 const struct fscrypt_info *ci)
>>>>    {
>>>>    	u8 flags = fscrypt_policy_flags(&ci->ci_policy);
>>>> +	struct inode *inode = ci->ci_inode;
>>>> +	const struct fscrypt_operations *s_cop = inode->i_sb->s_cop;
>>>> -	memset(iv, 0, ci->ci_mode->ivsize);
>>>> +	memset(iv, 0, sizeof(*iv));
>>>> +	if (s_cop->get_extent_context && lblk_num != U64_MAX) {
>>>> +		size_t extent_offset;
>>>> +		union fscrypt_extent_context ctx;
>>>> +		int ret;
>>>> +
>>>> +		ret = fscrypt_get_extent_context(inode, lblk_num, &ctx,
>>>> +						 &extent_offset, NULL);
>>>> +		WARN_ON_ONCE(ret);
>>>> +		memcpy(iv->raw, ctx.v1.iv.raw, sizeof(*iv));
>>>> +		iv->lblk_num += cpu_to_le64(extent_offset);
>>>> +		return;
>>>> +	}
>>>
>>> Please read through my review comment
>>> https://lore.kernel.org/linux-fscrypt/Yx6MnaUqUTdjCmX+@quark/ again, as it
>>> doesn't seem that you've addressed it.
>>>
>>> - Eric
>>
>> I probably didn't understand it correctly. I think there were three points
>> in it:
>>
>> 1) reconsider per-extent keys
>> 2) make IV generation work for non-directkey policies as similarly as
>> possible to how they work in inode-based filesystems
>> 3) never use 'file-based' except in contrast to dm-crypt and other
>> block-layer encryption.
>>
>> For point 2, I changed the initial extent context generation to match up
>> with fscrypt_generate_iv() (and probably didn't call that out enough in the
>> description). (Looking at it again, I could literally call
>> fscrypt_generate_iv() to generate the initial extent context; I didn't
>> realize that before).
>>
>> Then adding lblk_num to the existing lblk_num in the iv from the start of
>> the extent should be the same as the iv->lblk_num setting in the inode-based
>> case: for lblk 12, for instance, the same IV should result from inode-based
>> with lblk 12, as with extent-based with an initial lblk_num of 9 and an
>> extent_offset of 3. For shared extents, they'll be different, but for
>> singly-referenced extents, the IVs should be exactly the same in theory.
>>
>> I'm not sure whether I misunderstood the points or didn't address them
>> fully, I apologize. Would you be up for elaborating where I missed, either
>> by email or by videochat whenever works for you?
> 
> It seems you misunderstood point (2).  See what I said below:
> 
> 	So if you do want to implement the DIRECT_KEY method, the natural thing
> 	to do would be to store a 16-byte nonce along with each extent, and use
> 	the DIRECT_KEY IV generation method as-is.  It seems that you've done it
> 	a bit differently; you store a 32-byte nonce and generate the IV as
> 	'nonce + lblk_num', instead of 'nonce || lblk_num'.  I think that's a
> 	mistake -- it should be exactly the same.
> 
> 	If the issue is that the 'nonce || lblk_num' method doesn't allow for
> 	AES-XTS support, we could extend DIRECT_KEY to do 'nonce + lblk_num'
> 	*if* the algorithm has a 16-byte IV size and thus has to tolerate some
> 	chance of IV reuse.  Note that this change would be unrelated to
> 	extent-based encryption, and could be applied regardless of it.
> 
> So:
> 
> 1.) Provided that you've decided against per-extent keys, and are not trying to
>      support UFS and eMMC inline encryption hardware, then you should *only*
>      support DIRECT_KEY -- not other settings that don't make sense.
> 
> 2.) There should be a preparatory patch that makes DIRECT_KEY be allowed when
>      the IV length is 16 bytes, using the method 'nonce + lblk_num' -- assuming
>      that you need AES-XTS support and aren't planning on supporting Adiantum or
>      AES-HCTR2 only.  (The small chance for IV reuse that it results in is not
>      ideal, but it's probably tolerable.  Maybe the nonce should also be hashed
>      with a secret key, like what IV_INO_LBLK_32 does with the inode number; I'll
>      have to think about it.)  If you plan to just support AES-HCTR2 instead of
>      AES-XTS, then you'd need a patch to allow AES-HCTR2 for contents encryption,
>      as currently it is only allowed for filenames.
> 
> 3.) Each extent context should contain a 16-byte random nonce, that is newly
>      generated just for that extent -- not copied from anywhere.
> 
> 4.) IVs should be generated using the DIRECT_KEY method.  That is,
>      'nonce || lblk_num' if the IV length allows it, otherwise 'nonce + lblk_num'
>      as mentioned in (2).  For inode-based encryption, nonce means the inode's
>      nonce, and lblk_num means the index of the block in the inode.  For
>      extent-based encryption, nonce will mean the extent's nonce, and lblk_num
>      will mean the index of the block in the extent.
> 
> - Eric

Awesome, thank you for the elaboration. I'll give it a shot tonight and 
will send out v4 as soon as it's ready.

-Sweet Tea