configfs-tsm: Introduce a shared ABI for attestation reports

One of the common operations of a TSM (Trusted Security Module) is to
provide a way for a TVM (confidential computing guest execution
environment) to take a measurement of its launch state, sign it and
submit it to a verifying party. Upon successful attestation that
verifies the integrity of the TVM additional secrets may be deployed.
The concept is common across TSMs, but the implementations are
unfortunately vendor specific. While the industry grapples with a common
definition of this attestation format [1], Linux need not make this
problem worse by defining a new ABI per TSM that wants to perform a
similar operation. The current momentum has been to invent new ioctl-ABI
per TSM per function which at best is an abdication of the kernel's
responsibility to make common infrastructure concepts share common ABI.

The proposal, targeted to conceptually work with TDX, SEV-SNP, COVE if
not more, is to define a configfs interface to retrieve the TSM-specific
blob.

report=/sys/kernel/config/tsm/report/report0
mkdir $report
dd if=binary_userdata_plus_nonce > $report/inblob
hexdump $report/outblob

This approach later allows for the standardization of the attestation
blob format without needing to invent a new ABI. Once standardization
happens the standard format can be emitted by $report/outblob and
indicated by $report/provider, or a new attribute like
"$report/tcg_coco_report" can emit the standard format alongside the
vendor format.

Review of previous iterations of this interface identified that there is
a need to scale report generation for multiple container environments
[2]. Configfs enables a model where each container can bind mount one or
more report generation item instances. Still, within a container only a
single thread can be manipulating a given configuration instance at a
time. A 'generation' count is provided to detect conflicts between
multiple threads racing to configure a report instance.

The SEV-SNP concepts of "extended reports" and "privilege levels" are
optionally enabled by selecting 'tsm_report_ext_type' at register_tsm()
time. The expectation is that those concepts are generic enough that
they may be adopted by other TSM implementations. In other words,
configfs-tsm aims to address a superset of TSM specific functionality
with a common ABI where attributes may appear, or not appear, based on
the set of concepts the implementation supports.

Link: http://lore.kernel.org/r/64961c3baf8ce_142af829436@dwillia2-xfh.jf.intel.com.notmuch [1]
Link: http://lore.kernel.org/r/57f3a05e-8fcd-4656-beea-56bb8365ae64@linux.microsoft.com [2]
Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc: Dionna Amalie Glaze <dionnaglaze@google.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Peter Gonda <pgonda@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Samuel Ortiz <sameo@rivosinc.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Tested-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>