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Jorijn van der Graaf 1d26852a6b Initial commit: the gpfile wire format, pinned by two real containers

fingerprintd will own the FP6's fingerprint sensor: the rail, the QTEE session,
the storage callbacks QTEE makes back into the normal world, and
net.reactivated.Fprint so pam_fprintd and the desktop need no changes. None of
that runs yet. What is here is the first core module and the machinery around
it.

Fingerprintd:Sfs is the gpfile listener's frame -- the callback that carries
47 of 66 storage requests during an enrolment. It is parse, reply and root
mapping only: no file I/O, no TEE, no allocation of the shared buffer. The
daemon shell supplies those, which is what lets every byte-level decision be
tested on a dev box with no phone.

The module exists mainly to hold one fact. READ answers at req+0x00c and WRITE
reads its payload from req+0x110, because the frame is a union: a WRITE still
needs its path while the payload is copied out, so it sits past the 256-byte
path field, while a READ has consumed the path and packs its reply over it.
Conflating them is wrong in both directions with the same symptom -- the
container does not round-trip, QTEE's HMAC check fails, and the file is
unlinked as tampered on the next session.

So the tests do not assert the constants against themselves. They load two real
containers off the phone -- one written correctly, one written with the offsets
conflated -- and re-derive the bug: the broken one opens with ASCII path text
rather than a binary HMAC, that text is the group name from character 8 because
the read offset is 8 bytes into the path field, and the real container sits
exactly 0x104 further in. Then a write-store-read round trip must be the
identity, and the same round trip through a single offset must not be.

O_TRUNC gets a static_assert of its own. QTEE writes a container as
write(0,4096), write(4096,N), write(0,4096), so truncating on open leaves 4096
bytes where a 258850-byte template belongs; it unlinks a file it means to
shorten rather than relying on the opener.

Verified by mutation: conflating the offsets, making DataOffset return the read
offset for writes, and setting O_TRUNC each fail the suite.

2026-09-02 16:02:46 +02:00

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fingerprintd

Fingerprint daemon for the Fairphone 6 (milos, SM7635) on mainline Linux.

Why a daemon

The sensor is a FocalTech FT9391 on a TrustZone-owned SPI bus. spi@a88000 is disabled in both the mainline and the stock Android device tree, and the pads are XPU-protected — touching them from the normal world is an instant SError reboot. Every pixel the sensor produces stays inside the TEE: capture, preprocessing, the classifier, enrolment and matching all run in the focal64 trustlet, which reports a matched finger id and nothing else. A libfprint-style driver cannot exist on this device.

So the normal world's job is narrower than usual, and none of it is per-request work:

Power the sensor. Rail on gpio29, reset on gpio74, interrupt on gpio75 — the same division of labour the downstream driver uses. One sensor reset buys exactly one trustlet init, so whatever powers the sensor must also hold the session open.
Be QTEE's filesystem. QTEE cannot reach storage. When the trustlet saves or loads a template it calls back into the normal world through the gpfile (0x7000) and RPMB (0x2000) listeners, and expects them served. QTEE does the crypto and the anti-rollback; this side moves opaque bytes and performs the authenticated RPMB transactions against the UFS device.
Speak a biometrics API. The daemon owns net.reactivated.Fprint, so pam_fprintd, the Plasma fingerprint KCM and fprintd-enroll(1) work against it unmodified.

A listener registration is held for as long as the process lives and QTEE's listener table is global to the boot, so this has to be one long-lived process rather than a tool spawned per request.

Layout

interfaces/     Fingerprintd{,-Sfs}.cppm   the core: pure C++ modules
implementations/main.cpp                   the daemon shell
tests/                                     one suite per core module

fingerprintd-core is a static library with no GLib, no libqcomtee and no system headers. Everything in it is a wire format or a state machine that was recovered by reverse-engineering, so all of it is pinned by tests that run on a dev box with no phone, no TEE and no sensor. The daemon shell holds everything that touches hardware.

Build

crafter-build              # bin/fingerprintd-<target>-<march>/fingerprintd
crafter-build test         # the unit suites

Cross-compiling for the phone:

crafter-build -- --target=aarch64-alpine-linux-musl \
  --sysroot=<alpine-aarch64-sysroot> --march=armv8-a --mtune=generic
crafter-build test --target=aarch64-alpine-linux-musl

Status

Early. The core is being ported one wire format at a time out of the research harness that first made the sensor work (utilities/fpta.c in the fp6 repo), each piece landing with tests before the next starts. Fingerprintd:Sfs — the gpfile frame — is done. The daemon itself does not run yet.

The working reference enrols a finger, keeps it across a reboot, and matches it with zero false accepts; the port exists to turn that into a service rather than to rediscover it.

Runtime dependencies, not carried here

The focal64 trustlet is proprietary and is not in this repo. It is extracted from the device's own stock Android partition on first boot by the fp6-vendor-blobs mechanism, the same way the audio firmware is.

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