[PATCH 1/2] efi_loader: fix dual signed image certification

Ilias Apalodimas
2022-02-11T07:38:00Z

The EFI spec allows for images to carry multiple signatures. Currently
we don't adhere to the verification process for such images.

The spec says:
"Multiple signatures are allowed to exist in the binary's certificate
table (as per PE/COFF Section "Attribute Certificate Table"). Only one
hash or signature is required to be present in db in order to pass
validation, so long as neither the SHA-256 hash of the binary nor any
present signature is reflected in dbx."

With our current implementation signing the image with two certificates
and inserting both of them in db and one of them dbx doesn't always reject
the image. The rejection depends on the order that the image was signed
and the order the certificates are read (and checked) in db.

While at it move the sha256 hash verification outside the signature
checking loop, since it only needs to run once per image and get simplify
the logic for authenticating an unsigned imahe using sha256 hashes.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
---
changes since RFC:
- none
lib/efi_loader/efi_image_loader.c | 88 +++++++------------------------
1 file changed, 18 insertions(+), 70 deletions(-)

diff --git a/lib/efi_loader/efi_image_loader.c b/lib/efi_loader/efi_image_loader.c
index f41cfa4fccd5..5df35939f702 100644
--- a/lib/efi_loader/efi_image_loader.c
+++ b/lib/efi_loader/efi_image_loader.c
@@ -516,53 +516,6 @@ err:
}

#ifdef CONFIG_EFI_SECURE_BOOT
-/**
- * efi_image_unsigned_authenticate() - authenticate unsigned image with
- * SHA256 hash
- * @regs: List of regions to be verified
- *
- * If an image is not signed, it doesn't have a signature. In this case,
- * its message digest is calculated and it will be compared with one of
- * hash values stored in signature databases.
- *
- * Return: true if authenticated, false if not
- */
-static bool efi_image_unsigned_authenticate(struct efi_image_regions *regs)
-{
- struct efi_signature_store *db = NULL, *dbx = NULL;
- bool ret = false;
-
- dbx = efi_sigstore_parse_sigdb(u"dbx");
- if (!dbx) {
- EFI_PRINT("Getting signature database(dbx) failed\n");
- goto out;
- }
-
- db = efi_sigstore_parse_sigdb(u"db");
- if (!db) {
- EFI_PRINT("Getting signature database(db) failed\n");
- goto out;
- }
-
- /* try black-list first */
- if (efi_signature_lookup_digest(regs, dbx, true)) {
- EFI_PRINT("Image is not signed and its digest found in \"dbx\"\n");
- goto out;
- }
-
- /* try white-list */
- if (efi_signature_lookup_digest(regs, db, false))
- ret = true;
- else
- EFI_PRINT("Image is not signed and its digest not found in \"db\" or \"dbx\"\n");
-
-out:
- efi_sigstore_free(db);
- efi_sigstore_free(dbx);
-
- return ret;
-}
-
/**
* efi_image_authenticate() - verify a signature of signed image
* @efi: Pointer to image
@@ -608,14 +561,7 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
if (!efi_image_parse(new_efi, efi_size, &regs, &wincerts,
&wincerts_len)) {
EFI_PRINT("Parsing PE executable image failed\n");
- goto err;
- }
-
- if (!wincerts) {
- /* The image is not signed */
- ret = efi_image_unsigned_authenticate(regs);
-
- goto err;
+ goto out;
}

/*
@@ -624,18 +570,18 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
db = efi_sigstore_parse_sigdb(u"db");
if (!db) {
EFI_PRINT("Getting signature database(db) failed\n");
- goto err;
+ goto out;
}

dbx = efi_sigstore_parse_sigdb(u"dbx");
if (!dbx) {
EFI_PRINT("Getting signature database(dbx) failed\n");
- goto err;
+ goto out;
}

if (efi_signature_lookup_digest(regs, dbx, true)) {
EFI_PRINT("Image's digest was found in \"dbx\"\n");
- goto err;
+ goto out;
}

/*
@@ -678,7 +624,8 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
if (guidcmp(auth, &efi_guid_cert_type_pkcs7)) {
EFI_PRINT("Certificate type not supported: %pUs\n",
auth);
- continue;
+ ret = false;
+ goto out;
}

auth += sizeof(efi_guid_t);
@@ -686,7 +633,8 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
} else if (wincert->wCertificateType
!= WIN_CERT_TYPE_PKCS_SIGNED_DATA) {
EFI_PRINT("Certificate type not supported\n");
- continue;
+ ret = false;
+ goto out;
}

msg = pkcs7_parse_message(auth, auth_size);
@@ -717,32 +665,32 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
*/
/* try black-list first */
if (efi_signature_verify_one(regs, msg, dbx)) {
+ ret = false;
EFI_PRINT("Signature was rejected by \"dbx\"\n");
- continue;
+ goto out;
}

if (!efi_signature_check_signers(msg, dbx)) {
+ ret = false;
EFI_PRINT("Signer(s) in \"dbx\"\n");
- continue;
+ goto out;
}

/* try white-list */
if (efi_signature_verify(regs, msg, db, dbx)) {
ret = true;
- break;
+ continue;
}

EFI_PRINT("Signature was not verified by \"db\"\n");
+ }

- if (efi_signature_lookup_digest(regs, db, false)) {
- ret = true;
- break;
- }

- EFI_PRINT("Image's digest was not found in \"db\" or \"dbx\"\n");
- }
+ /* last resort try the image sha256 hash in db */
+ if (!ret && efi_signature_lookup_digest(regs, db, false))
+ ret = true;

-err:
+out:
efi_sigstore_free(db);
efi_sigstore_free(dbx);
pkcs7_free_message(msg);
--
2.32.0