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

Sweet Tea Dorminy
2022-10-20T22:55:17Z

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?

Thanks.