On 7/3/23 14:17, Eric Riggers wrote:
On Mon, Jul 03, 2023 at 01:06:17PM -0400, Sweet Tea Dorminy wrote:I think that would avoid many of the problems that it seems you've had to work around or have had to change user-visible semantics for. For example the problems involving master keys being added and removed. It would also avoid having to overload 'fscrypt_info' to be either a per-inode or a per-extent key. And it would save space on disk and in memory.I might be misunderstanding what you're referencing, but I think you're talking about the change where with extent fscrypt, IO has to be forced down before removing a key, otherwise it is lost. I think that's a fundamental problem given the filesystem has no way to know that there are new, dirty pages in the pagecache until those pages are issued for write, so it can't create a new extent or few until that point, potentially after the relevant key has been evicted. But maybe I'm missing a hook that would let us make extents earlier. I suppose we could give each leaf inode a proper nonce/prepared key instead of borrowing its parent dir's: if a write came in after the key was removed but the inode is still open, the new extent(s) could grab the key material out of the inode's info. I don't like this very much since it could result in multiple extents grabbing the same key material, but I suppose it could work if it's important to maintain that behavior.Right, if extent keys are derived directly from the master key, and FS_IOC_REMOVE_ENCRYPTION_KEY is executed which causes the master key secret to be wiped, then no more extent keys can be derived. So I see why you implemented the behavior you did. It does seem a bit dangerous, though. If I understand correctly, under your proposal, if FS_IOC_REMOVE_ENCRYPTION_KEY is executed while a process is doing buffered writes to an encrypted file protected by that key, then past writes will get synced out just before the key removal, but future writes will just be silently thrown away. (Remember, writes in Linux are asynchronous; processes don't get informed of write errors unless they call fsync() or are doing direct I/O.) I wonder if we should just keep the master key around, for per-extent key derivation only, until all files using that master key have been closed. That would minimize the changes from the current fscrypt semantics.
That would work, if you're good with that. It felt like a larger deviation, with the possibility of unbounded continued use of a soft-removed master key; should fscrypt restrict use of the master key only to extents that are part of inodes that are already open?
Can you elaborate on why you went with a more "heavyweight" extents design?Being able to rekey a directory is the reason for having full contexts: suppose I take a snapshot of an encrypted dir and want to change the key for new data going forward, to avoid using a single key on too much data. It's too expensive to reencrypt every extent with the new key, since the whole point of a snapshot is to make a lightweight copy that gets COWed on write. Then each extent needs to know what its own master key identifier/policy/flags are, since different extents in the same file could have different master keys. We could say the mode and flags have to match, but it doesn't seem to me worth saving seven bytes to add a new structure to just store the master key identifier and nonce. For a non-Meta usecase, from what I've heard from Fedora-land, it's possibly interesting to them to be able to ship an encrypted image, and then be able to change the key after encrypted install to something user-controlled. Without rekeying, my understanding is that we may write too much data with one key for safety; notes in the updated design doc https://docs.google.com/document/d/1janjxewlewtVPqctkWOjSa7OhCgB8Gdx7iDaCDQQNZA/edit?usp=sharing are that writing more than 1P per key raises cryptographic concerns, and since btrfs is COW and could have volumes up to the full 16E size that btrfs supports, we don't want to have just one immutable key per subvol. To me the lightweight-on-disk vision sounds a lot like the original design: https://lore.kernel.org/linux-btrfs/YXGyq+buM79A1S0L@relinquished.localdomain and the Nov '22 version of the patchset: https://lore.kernel.org/linux-btrfs/cover.1667389115.git.sweettea-kernel@xxxxxxxxxx/ (which didn't have rekeying). I think rekeying is worth the higher disk usage; but I'm probably missing something about how your vision differs from the original. Could you please take a look at it again?Your original design didn't use per-extent keys, but rather had a single contents encryption key per master key. We had discussed that that approach had multiple disadvantages, one of which is that on btrfs it can run uncomfortably close to the cryptographic limits for the contents encryption modes such as AES-XTS. So we decided to go with per-extent keys instead.
So you're proposing just storing a nonce per extent, then setting up a prepared key only, taking the inode's master key and doing the hkdf expand with the nonce into the prepared key? A terse approach, a lot closer to the original design than I thought.I don't think we discussed cryptographic limits on the master key itself. That is actually much less of a concern, as the master key is just used for HKDF-SHA512. I don't think there is any real need to ever change the master key. (Well, if it is compromised, it could be needed, but that's not really relevant here. If that happens you'd need to re-encrypt everything anyway.)
I do recall some discussion of making it possible to set an encryption policy on an *unencrypted* directory, causing new files in that directory to be encrypted. However, I don't recall any discussion of making it possible to add another encryption policy to an *already-encrypted* directory. I think this is the first time this has been brought up.
I think I referenced it in the updated design (fifth paragraph of "Extent encryption" https://docs.google.com/document/d/1janjxewlewtVPqctkWOjSa7OhCgB8Gdx7iDaCDQQNZA/edit?usp=sharing) but I didn't talk about it enough -- 'rekeying' is a substitute for adding a policy to a directory full of unencrypted data. Ya'll's points about the badness of having mixed unencrypted and encrypted data in a single dir were compelling. (As I recall it, the issue with having mixed enc/unenc data is that a bug or attacker could point an encrypted file autostarted in a container, say /container/my-service, at a unencrypted extent under their control, say /bin/bash, and thereby acquire a backdoor.)
I'd planned to use add_enckey to add all the necessary keys, set_encpolicy on an encrypted directory under the proper conditions (flags interpreted by ioctl? check if filesystem has hook?) recursively calls a filesystem-provided hook on each inode within to change the fscrypt_context. Either dir items could be reencrypted with the new key or dirs could keep around the oldest key for encryption and the newest key for new leaf inodes to inherit.I think that allowing directories to have multiple encryption policies would bring in a lot of complexity. How would it be configured, and what would the semantics for accessing it be? Where would the encryption policies be stored? What if you have added some of the keys but not all of them? What if some of the keys get removed but not all of them?
For leaf inodes, 2 options for the remaining questions:1) leaf inodes just stores the policy to be used for writing new data or inherits it from a parent directory. If you're reading and you hit an extent whose key isn't loaded, you get an IO error. If you're writing and you try to write to an inode who needs a key that isn't loaded, you get an IO error eventually.
This is what I'd prefer: Josef and Chris and I discussed this a while back and thought this was acceptable semantics for our uses, I think, and it's simple; you can already get an IO error if you're reading a file and suddenly hit an invalid extent, or if you are writing a file and run out of space.
2) all inodes store a fscrypt_context for all policies configured for the inode. ->get_context() gets a index parameter to get the index'th context. directory inodes' info is loaded off the one needed for dir entry encryption, leaf inodes' off the newest context. When any inode is opened, all the contexts are read and checked that their key is loaded, and then discarded, except the one in use for writes to that inode.
If a key is removed, you get an IO error when you do IO that needs that key, with the usual async conditions. Or, some lightweight structure is added to each master key's open inode list to point at the inode using it, and the inode keeps the master key around while it's open, thereby preventing IO errors after open.
Can you elaborate more on why you want this? I was already a bit concerned about the plan for making it possible to set an encryption policy on an unencrypted directory, as that already diverges from the existing fscrypt semantics. But now it sounds like the scope has grown even more. Keep in mind that in general, the closer we are able to stick to the existing fscrypt semantics and design, the easier it is going to be to get the initial btrfs fscrypt support merged. Planning ahead for new features is good, but we should also be careful not to overdesign.
Here's a shot at elaboration of usecase more:On various machines, we currently have a btrfs filesystem containing various volumes/snapshots containing starting states for containers. The snapshots are generated by common snapshot images built centrally. The machines, as the scheduler requests, start and stop containers on those volumes.
We want to be able to start a container on a snapshot/volume such that every write to the relevant snapshot/volume is using a per-container key, but we don't care about reads of the starting snapshot image being encrypted since the starting snapshot image is widely shared. When the container is stopped, no future different container (or human or host program) knows its key. This limits the data which could be lost to a malicious service/human on the host to only the volumes containing currently running containers.
Some other folks envision having a base image encrypted with some per-vendor key. Then the machine is rekeyed with a per-machine key in perhaps the TPM to use for updates and logfiles. When a user is created, a snapshot of a base homedir forms the base of their user subvolume/directory, which is then rekeyed with a per-user key. When the user logs in, systemd-homedir or the like could load their per-user key for their user subvolume/directory.
Since we don't care about encrypting the common image, we initially envisioned unencrypted snapshot images where we then turn on encryption and have mixed unenc/enc data. The other usecase, though, really needs key change so that everything's encrypted. And the argument that mixed unenc/enc data is not safe was compelling.
Hope that helps? Sweet Tea
|