A signed FIT configuration can delegate the integrity of a (potentially
large) root filesystem image to the kernel's dm-verity instead of having
U-Boot hash the whole payload at boot: the FIT carries a "dm-verity"
subnode with the roothash, salt and block parameters, U-Boot passes the
roothash to Linux through the dm-mod.create bootargs, and dm-verity then
validates the filesystem block by block against it.
For that to be safe the roothash has to be trusted, and in a signed
configuration the only thing that establishes trust is the configuration
signature. The roothash was not covered by it. fit_config_add_hash()
collected the image node, its hash subnodes and its cipher subnode into
the signed region, but not the dm-verity subnode, so the roothash, the
sole integrity anchor for the filesystem, was left unsigned.
The result is a verified-boot bypass for the root filesystem: an
attacker who can rewrite the boot medium can replace the filesystem,
recompute a matching dm-verity tree, write the new roothash into the
unsigned dm-verity subnode, and the configuration signature still
verifies. dm-verity then faithfully validates the malicious filesystem
against the attacker's roothash.
This series closes the gap.
v4: fix the CI failure Tom Rini reported
* test_fit_verity_roothash_signed() built a FIT with a dm-verity
subnode, which makes mkimage shell out to veritysetup internally to
compute the Merkle tree/roothash, but the test only declared dtc,
fdtget, fdtput and openssl as required tools. On the CI image, which
doesn't have veritysetup installed, mkimage's internal exec of it
failed silently (the diagnostic goes to a private pipe mkimage uses
to parse veritysetup's output, not to stderr) and the test hard
failed with "Can't add verification data ... (Input/output error)"
instead of skipping, the way test_fit_verity.py's veritysetup-using
tests already do via the same marker.
* add @pytest.mark.requiredtool('veritysetup') to
test_fit_verity_roothash_signed(), matching test_fit_verity.py
* collect Reviewed-by tags from Simon Glass on all three patches.
Simon also asked, on this cover letter, whether the series should
spell out the compatibility break explicitly (an image with a
dm-verity subnode signed by an older mkimage stops verifying, and
vice versa); resolved in-thread since there are no real-world
deployments of this mechanism yet, only the upcoming OpenWrt bootstd
use case -- no documentation change needed for this revision
v3: address comments by Simon Glass
* factor tools/image-host.c's node-path collection into patch 1 too,
mirroring the boot-side helper, so both the sign side and the verify
side share the same shape and stay easy to compare
* use present tense for the pre-patch code description in patch 2;
document the dm-verity subnode in the rebuilt node list in
doc/usage/fit/signature.rst and note the signature coverage in
doc/usage/fit/dm-verity.rst
* in the unit test, bound-check fdt_find_regions()'s returned count
the same way fit_config_check_sig() does, tamper the digest through
the buffer instead of casting away const, and note in a comment that
the digest check stands in for the whole dm-verity node
* fix test_fit_verity_roothash_signed(), which never actually ran: its
ITS baked the tmpdir prefix into the /incbin/() paths while dtc also
resolves incbin paths relative to the .its file's own directory, so
the path was searched doubled and mkimage always failed
* kept Reviewed-by tags on patches 1 and 2: neither change invalidates
what was reviewed, the diffs are mechanical/cosmetic
v2: address comments by Tom Rini
* drop the VISIBLE_IF_UT visibility macro; fit_config_get_signed_nodes()
is now simply non-static (previously the function would end up being
inlined, so there *is* a real cost to this)
* document test_fit_verity_sign.py with pydoc docstrings including an
ITS example, and add a page under doc/develop/pytest/ so the module
is rendered in the generated documentation
* collect Reviewed-by tags on patches 1 and 2
Daniel Golle (3):
boot: fit: factor out node-path collection in fit_config_add_hash()
boot: fit: cover the dm-verity roothash with the config signature
test: fit: verify dm-verity roothash is covered by the config
signature
boot/image-fit-sig.c | 108 +++++++----
doc/develop/pytest/test_fit_verity_sign.rst | 10 +
doc/usage/fit/dm-verity.rst | 5 +
doc/usage/fit/signature.rst | 2 +-
include/image.h | 23 +++
test/boot/fit_verity.c | 200 +++++++++++++++++++
test/py/tests/test_fit_verity_sign.py | 203 ++++++++++++++++++++
tools/image-host.c | 94 ++++++---
8 files changed, 579 insertions(+), 66 deletions(-)
create mode 100644 doc/develop/pytest/test_fit_verity_sign.rst
create mode 100644 test/py/tests/test_fit_verity_sign.py
base-commit: b635d43bca429500cb8ef20aa151cb5773b9a8a5
--
2.55.0
