Date: Fri, 21 Jun 2024 08:39:39 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: "Gaurav Kashyap (QUIC)" <quic_gaurkash@...cinc.com>,
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Subject: Re: [PATCH v5 04/15] soc: qcom: ice: add hwkm support in ice

On Fri, Jun 21, 2024 at 06:16:37PM +0300, Dmitry Baryshkov wrote:
> 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?

It derives a secret for functionality such as filenames encryption that can't
use inline encryption.

> 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.

There is.  That's what the BLKCRYPTOPREPAREKEY ioctl is for.

> Some hardware might permit importing raw keys.

That's what BLKCRYPTOIMPORTKEY is for.

> Other hardware might insist on generating the keys on-chip so that raw keys
> can never be used.

And that's what BLKCRYPTOGENERATEKEY is for.

> 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.

That's exactly how it works.  There is a block interface for specifying an
inline encryption key along with each bio.  The submitter of the bio can specify
either a standard key or a HW-wrapped key.

Again, take a look at the patchset
https://lore.kernel.org/linux-block/20231104211259.17448-1-ebiggers@kernel.org/T/#u.
That's where all this stuff is.

Thanks,

- Eric

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