Message-ID: <CAA8EJppXsbpFCeGJOMGKOQddy0fF4uW3rt4RUuDTQq6mPunBkg@mail.gmail.com>
Date: Fri, 21 Jun 2024 18:16:37 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Eric Biggers <ebiggers@...nel.org>
Cc: "Gaurav Kashyap (QUIC)" <quic_gaurkash@...cinc.com>, 
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	Sonal Gupta <sonalg@...cinc.com>
Subject: Re: [PATCH v5 04/15] soc: qcom: ice: add hwkm support in ice

On Fri, 21 Jun 2024 at 07:47, Eric Biggers <ebiggers@...nel.org> wrote:
>
> On Thu, Jun 20, 2024 at 02:57:40PM +0300, Dmitry Baryshkov wrote:
> > > > >
> > > > > > Is it possible to use both kind of keys when working on standard mode?
> > > > > > If not, it should be the user who selects what type of keys to be used.
> > > > > > Enforcing this via DT is not a way to go.
> > > > > >
> > > > >
> > > > > Unfortunately, that support is not there yet. When you say user, do
> > > > > you mean to have it as a filesystem mount option?
> > > >
> > > > During cryptsetup time. When running e.g. cryptsetup I, as a user, would like
> > > > to be able to use either a hardware-wrapped key or a standard key.
> > > >
> > >
> > > What we are looking for with these patches is for per-file/folder encryption using fscrypt policies.
> > > Cryptsetup to my understanding supports only full-disk , and does not support FBE (File-Based)
> >
> > I must admit, I mostly used dm-crypt beforehand, so I had to look at
> > fscrypt now. Some of my previous comments might not be fully
> > applicable.
> >
> > > Hence the idea here is that we mount an unencrypted device (with the inlinecrypt option that indicates inline encryption is supported)
> > > And specify policies (links to keys) for different folders.
> > >
> > > > > The way the UFS/EMMC crypto layer is designed currently is that, this
> > > > > information is needed when the modules are loaded.
> > > > >
> > > > > https://lore.kernel.org/all/20231104211259.17448-2-ebiggers@kernel.org
> > > > > /#Z31drivers:ufs:core:ufshcd-crypto.c
> > > >
> > > > I see that the driver lists capabilities here. E.g. that it supports HW-wrapped
> > > > keys. But the line doesn't specify that standard keys are not supported.
> > > >
> > >
> > > Those are capabilities that are read from the storage controller. However, wrapped keys
> > > Are not a standard in the ICE JEDEC specification, and in most cases, is a value add coming
> > > from the SoC.
> > >
> > > QCOM SOC and firmware currently does not support both kinds of keys in the HWKM mode.
> > > That is something we are internally working on, but not available yet.
> >
> > I'd say this is a significant obstacle, at least from my point of
> > view. I understand that the default might be to use hw-wrapped keys,
> > but it should be possible for the user to select non-HW keys if the
> > ability to recover the data is considered to be important. Note, I'm
> > really pointing to the user here, not to the system integrator. So
> > using DT property or specifying kernel arguments to switch between
> > these modes is not really an option.
> >
> > But I'd really love to hear some feedback from linux-security and/or
> > linux-fscrypt here.
> >
> > In my humble opinion the user should be able to specify that the key
> > is wrapped using the hardware KMK. Then if the hardware has already
> > started using the other kind of keys, it should be able to respond
> > with -EINVAL / whatever else. Then the user can evict previously
> > programmed key and program a desired one.
> >
> > > > Also, I'd have expected that hw-wrapped keys are handled using trusted
> > > > keys mechanism (see security/keys/trusted-keys/). Could you please point
> > > > out why that's not the case?
> > > >
> > >
> > > I will evaluate this.
> > > But my initial response is that we currently cannot communicate to our TPM directly from HLOS, but
> > > goes through QTEE, and I don't think our qtee currently interfaces with the open source tee
> > > driver. The interface is through QCOM SCM driver.
> >
> > Note, this is just an API interface, see how it is implemented for the
> > CAAM hardware.
> >
>
> The problem is that this patchset was sent out without the patches that add the
> block and filesystem-level framework for hardware-wrapped inline encryption
> keys, which it depends on.  So it's lacking context.  The proposed framework can
> be found at
> https://lore.kernel.org/linux-block/20231104211259.17448-1-ebiggers@kernel.org/T/#u

Thank you. I have quickly skimmed through the patches, but I didn't
review them thoroughly. Maybe the patchset already implements the
interfaces that I'm thinking about. In such a case please excuse me. I
will give it a more thorough look later today.

> As for why "trusted keys" aren't used, they just aren't helpful here.  "Trusted
> keys" are based around a model where the kernel can request that keys be sealed
> and unsealed using a trust source, and the kernel gets access to the raw
> unsealed keys.  Hardware-wrapped inline encryption keys use a different model
> where the kernel never gets access to the raw keys.  They also have the concept
> of ephemeral wrapping which does not exist in "trusted keys".  And they need to
> be properly integrated with the inline encryption framework in the block layer.

Then what exactly does qcom_scm_derive_sw_secret() do? Does it rewrap
the key under some other key?
I had the feeling that there are two separate pieces of functionality
being stuffed into a single patchset and into a single solution.

First one is handling the keys. I keep on thinking that there should
be a separate software interface to unseal the key and rewrap it under
an ephemeral key. Some hardware might permit importing raw keys. Other
hardware might insist on generating the keys on-chip so that raw keys
can never be used. Anyway, the net result is the binary blob + cookie
for the ephemeral key.

Second part is the actual block interface. Gaurav wrote about
targeting fscrypt, but there should be no actual difference between
crypto targets. FDE or having a single partition encrypted should
probably work in the same way. Convert the key into blk_crypto_key
(including the cookie for the ephemeral key), program the key into the
slot, use the slot to en/decrypt hardware blocks.

My main point is that the decision on the key type should be coming
from the user. I can easily imagine a user, which wants to use
password / raw key for documents storage so that it is possible to
recover the data, hw-wrapped long-term key for app & data storage and
generated one-time random key for the swap, so that memory contents
can never be recovered after reboot / device capture.

-- 
With best wishes
Dmitry

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