[PATCH v3 0/4] tdx-guest: Make Quote buffer size dynamic

[Peter Fang]

Hi,

A new TDX module ABI provides the maximum attestation report size. This
series changes the TDX guest driver's report buffer size from a fixed
constant to that queried value. So effectively:

  s/FIXED_BUF_SIZE/queried_buf_size/g

...in the TDX guest driver.

Terminology
===========

A "TD Quote" is an attestation structure signed with a platform key. It
contains information about a TDX guest and the platform it's running on.

The "Quote buffer" in the TDX guest driver is a memory buffer shared
between the TDX guest and the host VMM to retrieve TD Quotes. It has a
header defined in the GHCI spec [1].

Device Identifier Composition Engine ("DICE") provides a framework for
layering attestation evidence. This replaces the SGX model of contacting
an Intel server to obtain a certificate.

Problem
=======

The fixed-size Quote buffer approach is not sustainable. As
cryptographic algorithms evolve, TD Quote sizes also grow. A previous
commit [2] increased the guest driver's fixed-size Quote buffer to 128
KB to accommodate DICE Quotes, but it may still be insufficient when
those Quotes use post-quantum cryptography (PQC). PQC certificate chains
are roughly 10x-15x larger than conventional ones, which can increase
Quote sizes significantly.

What's in this series
=====================

To avoid changing the driver whenever the Quote buffer becomes too
small, newer TDX modules report their maximum Quote size via a metadata
field. The guest driver uses this value for its Quote buffer when
available. Older TDX modules continue to use the 128 KB buffer.

Patches 2 and 3 refactor the existing fixed buffer handling. Patch 4
then makes the buffer size dynamic.

The "outblob" file in configfs-tsm no longer has a fixed maximum size.
The limit can now come from this new TDX module ABI.

Patch 1/4: Add a helper to read the QUOTE_MAX_SIZE metadata field.
Patch 2/4: Calculate the Quote buffer size with struct_size_t().
Patch 3/4: Store the Quote buffer size in a variable instead of a
           constant.
Patch 4/4: Allocate the Quote buffer using the queried size, when
           available.

AI use
======

I used Claude:claude-opus-4-8 to help edit this cover letter and the
changelogs, and to collect the review feedback on lore. The series also
underwent AI code review (Claude:claude-opus-4-7), but its comments were
limited to style suggestions.

v2: https://lore.kernel.org/all/20260717214349.4075994-1-peter.fang@xxxxxxxxx/

Changes in v3:
 - Split the v2 "Allocate Quote buffer dynamically" patch to do the
   refactoring first, then make the buffer size dynamic. [Dave]
 - Improve patterns for readability. [Dave]
 - Drop __GFP_NOWARN so an allocation failure warns. [Dave, Rick, Kiryl]
 - Add Binbin's Reviewed-by to patch 1.
 - Drop the Reviewed-by tags (Kiryl, Binbin) as the patch was reworked.

v1: https://lore.kernel.org/all/20260612110853.3188196-1-peter.fang@xxxxxxxxx/

Changes in v2:
 - Collect Reviewed-by tags. [Kiryl, Xiaoyao, Binbin, Sathya]
 - Keep the explicit (u32) cast in tdx_get_max_quote_size(). [Binbin]
 - Calculate the Quote buffer size with struct_size_t(). [Kiryl, Binbin]
 - Add __GFP_NOWARN to the allocation since its size comes from the
   host. [sashiko]
 - Rename quote_data_size to quote_data_len. [Sathya]
 - Drop the Assisted-by tags, as AI was not used to write the code.

[1] Guest Hypervisor Communication Interface (GHCI) Specification,
    Version 1.5, Section "TDG.VP.VMCALL<GetQuote>"
[2] 43185067c6fd ("configfs-tsm-report: tdx_guest: Increase Quote buffer
    size to 128KB")

Kuppuswamy Sathyanarayanan (1):
  virt: tdx-guest: Allocate Quote buffer dynamically

Peter Fang (3):
  x86/tdx: Add helper to query maximum TD Quote size
  virt: tdx-guest: Calculate the Quote buffer size safely
  virt: tdx-guest: Use a variable to store the Quote buffer size

 arch/x86/coco/tdx/tdx.c                 | 19 ++++++++++
 arch/x86/include/asm/shared/tdx.h       |  1 +
 arch/x86/include/asm/tdx.h              |  2 ++
 drivers/virt/coco/tdx-guest/tdx-guest.c | 46 ++++++++++++++++++-------
 4 files changed, 55 insertions(+), 13 deletions(-)


base-commit: f5098b6bae761e346ebcd9da7f95622c04733cff
-- 
2.53.0