Ranting about OCF / crypto(9)
jhb at freebsd.org
Thu Jan 11 00:19:04 UTC 2018
While working on hooking the ccr(4) driver into our in-kernel crypto
framework (along with some out-of-tree patches to extend OpenSSL's
/dev/crypto engine to support AES-CTR/XTS/GCM and some further changes to
do zero-copy), I've run into several bumps / oddities in OCF. I'm probably
going to miss several of them, but here's at least a start of a list of
things. In some cases I have some suggestions on improvements.
I will try to start with more broad / higher-level items first before
diving into minutiae:
- OCF is over flexible and overly broad. Rather than supporting
arbitrary stacking of transforms (and arbitrary depths), I think we
should probably aim to support more specific workloads and support
them well. To my mind the classes of things we should support are
- Simple block cipher requests.
- Simple "hash a buffer" requests. (Both HMAC and non-HMAC)
- IPSec-style requests (combined auth and encryption using
"encrypt-then-mac" with an optional AAD region before the
ciphertext). Note that geli requests fall into this type.
- TLS-style requests (using TLS's different methods of
combining auth and encryption methods when those are
- Simple compression / decompression requests. While this isn't
"crypto", per se, I do think it is probably still simpler to
manage this via OCF than a completely separate interface.
In terms of algorithms, I suspect there are some older algorithms
we could drop. Modern hardware doesn't offload DES for example.
Both ccr(4) and aesni(4) only support AES for encryption. We
do need to keep algorithms required for IPSec in the kernel, but
we could probably drop some others?
- To better support OpenSSL's engine, the /dev/crypto hash interface
should not require monotonic buffers, but support requests for
large buffers that span multiple requests (so you can do something
akin to the 'Init' / 'Update' (N times) / 'Final' model existing
software hashing APIs use). In particular, the bigger win for
hashing in hardware is when you can offload the hashing of a large
thing rather than small requests.
- To better support OpenSSL's engine, the /dev/crypto hash interface
should support "plain" hash algorithms such as SHA* without an
HMAC. By default OpenSSL's engine interface does the HMAC-specific
bits (generating pads, etc.) in software and only defers to the
engine for the raw hash (e.g. if you use the HMAC() function from
libcrypto it will only ask the engine interface for a raw hash,
not for an HMAC hash).
- To better support OpenSSL's engine, the /dev/crypto cipher
interface should also support non-monolithic buffers. The existing
engine does this now by copying the last block of the output data
out as a saved IV to use for a subsequent request, but it might be
nicer to be more formal here and return the IV to userland for
non-"final" cipher requests.
- The interface between the crypto layer and backend drivers should
_not_ use integer session IDs. This is rediculously dumb and
inefficient. All the drivers have silly algorithms to try to manage
growable arrays that can be indexed by the returned session ID.
Instead, drivers should be able to return a 'void *' cookie when
creating a session and get that cookie pointer as an argument to
the 'process' and 'freesession' callbacks. Imagine if vnodes used
an i-node number rather than 'v_data' and you'd have the model OCF
uses. I don't mind if we have a kind of generic 'session' structure
that we export to drivers and pass in the callbacks and the drivers
get to use a 'foo_data' member of.
- The interface to describe crypto requests needs to move away from
arbitrary linked lists of descriptors. We should just have a
single "session" structure that assumes you have one cipher and
one auth with a "mode" member to indicate the particular direction
/ combination. Likewise, the description of a request needs to
have a similar assumption. The structures used by the /dev/crypto
ioctl's are a bit closer to what I think we should use compared to
the linked-list thing we have now. Related is that we should be
able to get rid of having the three separate "algorithms" for GCM
hashes. For AES-GCM one would just say they are using AES-GCM
and both the hash/tag and ciphertext would be valid inputs / outputs
with a single key.
- To support non-monolithic buffers from the OpenSSL engine, crypto
requests to drivers also have to support non-monolithic buffers.
This means having a notion of a buffer that may be at the start,
middle, or end of a larger transformation (e.g. for hash only the
start gets the IPAD, only the end gets the OPAD and returns a
valid hash, etc., whereas for ciphers any non-end requests would
return the IV to use for the next request).
For drivers that have buffer size limits, it would be nice to expose
those limits in the driver capabilities and depend on the upper layer
to "split" requests such as happens now for disk drivers.
- For hashing algorithms we should support a "verify" mode in addition
to the current "compute" mode. The verify mode would accept a block
of data to hash along with an expected mac and return a success
/ failure rather than an computed hash value. AES-GCM already works
this way for decryption, but this would extend that mode for other
hash algorithms (e.g. AES-CBC+SHA2-256-HMAC). Existing crypto
co-processors (e.g. ccr(4)) already support these types of requests.
Related is that we need to fix IPSec to treat EBADMSG errors from
descryption as auth failure rather than encryption failure (right
now AES-GCM auth failures are reported incorrectly in netstat -s
due to this).
- Sessions for a combined cipher + hash should also be tied to a
specific way of combining the algorithms. Right now you can
create a session for AES-CBC with a SHA hash and the driver has no
way to know if you are going to do encrypt-then-mac or one of the
other variants. We should include this in the session (so a given
session can only be used for one type which is normally true anyway),
and drivers can then only claim to support combinations they
- The CRD_F_IV_PRESENT flag should be removed and replaced with
a CRD_F_IV_INJECT flag which means "inject the IV". Right now
the _lack_ of CRD_F_IV_PRESENT for encryption (but not decryption!)
requests means "inject the IV". It would be clearer to just have
a flag that is only set when you want the driver to take the
- Speaking of IV handling, drivers have to do some extra handling for
IVs including possibly generating them. I think the idea is that
some co-processors might support generating IVs, but most of the
drivers I've looked at just end up duplicating the same block of
code to call arc4rand() for encryption requests without
CRD_F_IV_EXPLICIT. I don't believe Linux tries to support this and
instead always supplies an IV to the driver. I'd rather we do that
and only depend on a flag to indicate where the IV is (crd_iv vs
in the buffer).
- The API for copying data to/from crypto buffers is a bit obtuse and
limiting. Rather than accepting the crypto operation ('crp') as
a parameter to describe the crypto buffer, the crypto_copyback()
and crypto_copydata() functions accept various members of that
function explicitly (e.g. crp_flags and crp_buf). However, in my
experiments with zero-copy AES-GCM via /dev/crypto and OpenSSL it
was convenient to store the AAD in a KVA buffer in the 'crp' and
the payload to transform in an array of VM pages. However, for
this model 'crp_buf' is useless. I ended up adding a wrapper API
'crypto_copyto' and 'crypto_copyfrom' which accept a 'crp' directly.
Linux's API actually passes something akin to sglist as the
description of the buffers in a crypto request.
- We need to not treat accelerated software (e.g. AES-NI) as a
hardware interface. Right now OCF's model of priorities when
trying to choose a backend driver for a session only has two
"levels" software vs hardware and aesni(4) (and the ARMv8 variant)
are lumped into the hardware bucket so that they have precedence
over the "dumb" software implementation. However, the accelerated
software algorithms do need some of the same support features of
the "dumb" software implementation (such as being scheduled on a
thread pool to use CPU cycles) that are not needed by other "hardware"
engines. OCF needs to understand this distinction.
- Somewhat related, we should try to use accelerated software when
possible (e.g. AES-CBC with SHA) doesn't use AES-NI unless the
CPU supports accelerated SHA. Ideally for this case we'd still
use AES-NI for the AES portion along with the software SHA
implementation (and we'd do it one pass over the data rather than
two when possible).
- Sometimes a crypto driver might need to defer certain requests to
software (e.g. ccr(4) has to do this for some GCM requests). In
addition, there are some other cases when we might want requests
from a single session to be sent to different backends (e.g. you
may want to use accelerated software for requests below a certain
size, and a crypto engine for larger requests. You might also want
to take NUMA into account when choosing which backend crypto engine
to dispatch a request to.) To that end, I think we want to have the
ability for a single OCF session to support multiple backend
One use case is that if I as a driver can't handle a request I'd like
to be able to fail it with a special error code and have the crypto
later fall back to software for me (and to use accelerated software if
possible). Right now ccr(4) duplicates the "dumb" software for GCM
requests it can't handle explicitly.
Another use case might be failover if a hardware engine experiences
a hardware failure. In theory it should be possible to fail over
to a different driver at that point including resubmitting pending
requests that weren't completed, and it should be possible (I think)
to manage this in the crypto framework rather than in consumers like
IPSec and GELI.
Load distribution among backends might be another case to consider
(e.g. GELI or ZFS encryption once that lands) if you have long-
running sessions that spawn lots of self-contained requests.
Note that if we want to spawn additional backend sessions on the fly
(e.g. only create a software fallback session on demand if a driver
fails a request with the "use software" magic error code), we will
have to keep per-session state such as keys around. We probably
already do that now, but this would definitely require doing that.
One concern with some of these changes is that there are several drivers
in the tree for older hardware that I'm not sure is really used anymore.
That is an impediment to making changes to the crypto <-> driver interface
if we can't find folks willing to at least test changes to those drivers
if not maintain them.
This is all I could think of today. What do other folks think?
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