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fix: analyze standalone UEFI variable stores that have no FV header #31

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Merged
nmatt0 merged 1 commit into from
Sep 18, 2026
Merged

fix: analyze standalone UEFI variable stores that have no FV header #31

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11 changes: 10 additions & 1 deletion src/boot.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -775,7 +775,16 @@ std::vector<Finding> scan_boot(const std::string& path, std::span<const uint8_t>
{
static const uint8_t kFvh[4] = {'_', 'F', 'V', 'H'};
std::span<const uint8_t> head = data.subspan(0, std::min<size_t>(data.size(), 64u << 20));
if (find_bytes(head, kFvh, 4, 0) != std::string::npos) {
// A full BIOS image carries the FV header signature "_FVH" somewhere. A
// variable store extracted on its own (the usual chipsec/UEFITool artifact)
// has no FV wrapper: an EDK2 authenticated/variable store begins with its
// store GUID, an AMI store begins with an "NVAR" entry. Trigger on either
// so an extracted store is analyzed too; analyze_uefi self-filters (emits
// nothing without a real recovered variable), so the loose trigger is safe.
bool is_varstore = guid_at(data, 0, kAuthVarStore) || guid_at(data, 0, kVarStore) ||
(data.size() >= 4 && data[0] == 'N' && data[1] == 'V' &&
data[2] == 'A' && data[3] == 'R');
if (is_varstore || find_bytes(head, kFvh, 4, 0) != std::string::npos) {
analyze_uefi(data, out);
if (!out.empty()) return out;
}
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18 changes: 18 additions & 0 deletions tests/test_boot.py
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Expand Up @@ -457,6 +457,24 @@ def check(cond, msg):
check("uefi-platform-key" in types(ph) and "uefi-secureboot-on" in types(ph),
"nvar: FV at a nonzero offset (full-flash dump) still analyzed")

# A variable store extracted on its own (a chipsec/UEFITool artifact) has no FV
# wrapper, so no "_FVH" signature. It must still be analyzed -- the raw AMI store
# begins with an "NVAR" entry. A large defaults-container entry ("StdDefaults")
# sits up front, as on a real BIOS; the top-level PK/dbx after it are still
# recovered. Regression: without the store-start trigger this was silently
# skipped, so an extracted NVAR store reported nothing.
raw = b"".join(_nvar_entry(n, d) for (n, d) in
[("StdDefaults", b"\x00" * 200), ("PK", b"PKCERT" * 40),
("KEK", b"KEK" * 30), ("db", b"DB" * 300), ("dbx", sha256_siglist(5)),
("SecureBoot", b"\x01")])
check(raw[:4] == b"NVAR", "nvar: standalone store begins with an NVAR entry (no FV)")
rh = run("varstore.bin", raw)
check("uefi-platform-key" in types(rh) and "uefi-secureboot-on" in types(rh),
"nvar: standalone extracted store (no FV wrapper) is analyzed")
rdbx = [x for x in rh if x["type"] == "uefi-dbx"]
check(bool(rdbx) and "5 revocation" in rdbx[0]["label"],
"nvar: standalone store dbx enumerated past a leading container entry")

# PKFAIL through the NVAR store: PK is the AMI "DO NOT TRUST" test key.
nvpk = nvar_store([("PK", efi_sig_list(ami_cert)), ("SecureBoot", b"\x01")])
check("uefi-test-platform-key" in types(run("bios.bin", nvpk)),
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