efi_loader: image_loader: add digest-based verification for signed image
In case that a type of certificate in "db" or "dbx" is
EFI_CERT_X509_SHA256_GUID, it is actually not a certificate which contains
a public key for RSA decryption, but a digest of image to be loaded.
If the value matches to a value calculated from a given binary image, it is
granted for loading.
With this patch, common digest check code, which used to be used for
unsigned image verification, will be extracted from
efi_signature_verify_with_sigdb() into efi_signature_lookup_digest(), and
extra step for digest check will be added to efi_image_authenticate().
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
diff --git a/lib/efi_loader/efi_image_loader.c b/lib/efi_loader/efi_image_loader.c
index 058359f..b7cf260 100644
--- a/lib/efi_loader/efi_image_loader.c
+++ b/lib/efi_loader/efi_image_loader.c
@@ -448,16 +448,16 @@
}
/* try black-list first */
- if (efi_signature_verify_one(regs, NULL, dbx)) {
- EFI_PRINT("Image is not signed and rejected by \"dbx\"\n");
+ if (efi_signature_lookup_digest(regs, dbx)) {
+ EFI_PRINT("Image is not signed and its digest found in \"dbx\"\n");
goto out;
}
/* try white-list */
- if (efi_signature_verify_one(regs, NULL, db))
+ if (efi_signature_lookup_digest(regs, db))
ret = true;
else
- EFI_PRINT("Image is not signed and not found in \"db\" or \"dbx\"\n");
+ EFI_PRINT("Image is not signed and its digest not found in \"db\" or \"dbx\"\n");
out:
efi_sigstore_free(db);
@@ -605,6 +605,25 @@
continue;
}
+ /*
+ * NOTE:
+ * UEFI specification defines two signature types possible
+ * in signature database:
+ * a. x509 certificate, where a signature in image is
+ * a message digest encrypted by RSA public key
+ * (EFI_CERT_X509_GUID)
+ * b. bare hash value of message digest
+ * (EFI_CERT_SHAxxx_GUID)
+ *
+ * efi_signature_verify() handles case (a), while
+ * efi_signature_lookup_digest() handles case (b).
+ *
+ * There is a third type:
+ * c. message digest of a certificate
+ * (EFI_CERT_X509_SHAAxxx_GUID)
+ * This type of signature is used only in revocation list
+ * (dbx) and handled as part of efi_signatgure_verify().
+ */
/* try black-list first */
if (efi_signature_verify_one(regs, msg, dbx)) {
EFI_PRINT("Signature was rejected by \"dbx\"\n");
@@ -616,11 +635,22 @@
goto err;
}
- /* try white-list */
- if (!efi_signature_verify_with_sigdb(regs, msg, db, dbx)) {
- EFI_PRINT("Signature was not verified by \"db\"\n");
+ if (efi_signature_lookup_digest(regs, dbx)) {
+ EFI_PRINT("Image's digest was found in \"dbx\"\n");
goto err;
}
+
+ /* try white-list */
+ if (efi_signature_verify_with_sigdb(regs, msg, db, dbx))
+ continue;
+
+ debug("Signature was not verified by \"db\"\n");
+
+ if (efi_signature_lookup_digest(regs, db))
+ continue;
+
+ debug("Image's digest was not found in \"db\" or \"dbx\"\n");
+ goto err;
}
ret = true;