efi_loader: correctly handle mixed hashes and signatures in db

A mix of signatures and hashes in db doesn't always work as intended.
Currently if the digest algorithm is not explicitly set to sha256 we
stop walking the security database and reject the image.

That's problematic in case we find and try to check a signature before
inspecting the sha256 hash.  If the image is unsigned we will reject it
even if the digest matches.

Since we no longer reject the image on unknown algorithms add an explicit
check and reject the image if any other hash algorithm apart from sha256
is detected on dbx.

Suggested-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

2 files changed, 46 insertions, 11 deletions

diff --git a/lib/efi_loader/efi_image_loader.c b/lib/efi_loader/efi_image_loader.c
index 255613eb72..f43dfb3d57 100644
--- a/lib/efi_loader/efi_image_loader.c
+++ b/lib/efi_loader/efi_image_loader.c
@@ -545,13 +545,13 @@ static bool efi_image_unsigned_authenticate(struct efi_image_regions *regs)
 	}
 
 	/* try black-list first */
-	if (efi_signature_lookup_digest(regs, dbx)) {
+	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))
+	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");
@@ -633,7 +633,7 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
 		goto err;
 	}
 
-	if (efi_signature_lookup_digest(regs, dbx)) {
+	if (efi_signature_lookup_digest(regs, dbx, true)) {
 		EFI_PRINT("Image's digest was found in \"dbx\"\n");
 		goto err;
 	}
@@ -734,7 +734,7 @@ static bool efi_image_authenticate(void *efi, size_t efi_size)
 
 		EFI_PRINT("Signature was not verified by \"db\"\n");
 
-		if (efi_signature_lookup_digest(regs, db)) {
+		if (efi_signature_lookup_digest(regs, db, false)) {
 			ret = true;
 			break;
 		}
diff --git a/lib/efi_loader/efi_signature.c b/lib/efi_loader/efi_signature.c
index 3243e2c60d..eb6886cdcc 100644
--- a/lib/efi_loader/efi_signature.c
+++ b/lib/efi_loader/efi_signature.c
@@ -147,9 +147,34 @@ static bool efi_hash_regions(struct image_region *regs, int count,
 }
 
 /**
+ * hash_algo_supported - check if the requested hash algorithm is supported
+ * @guid: guid of the algorithm
+ *
+ * Return: true if supported false otherwise
+ */
+static bool hash_algo_supported(const efi_guid_t guid)
+{
+	int i;
+	const efi_guid_t unsupported_hashes[] = {
+		 EFI_CERT_SHA1_GUID,
+		 EFI_CERT_SHA224_GUID,
+		 EFI_CERT_SHA384_GUID,
+		 EFI_CERT_SHA512_GUID,
+	};
+
+	for (i = 0; i < ARRAY_SIZE(unsupported_hashes); i++) {
+		if (!guidcmp(&unsupported_hashes[i], &guid))
+			return false;
+	}
+
+	return true;
+}
+
+/**
  * efi_signature_lookup_digest - search for an image's digest in sigdb
  * @regs:	List of regions to be authenticated
  * @db:		Signature database for trusted certificates
+ * @dbx		Caller needs to set this to true if he is searching dbx
  *
  * A message digest of image pointed to by @regs is calculated and
  * its hash value is compared to entries in signature database pointed
@@ -158,7 +183,9 @@ static bool efi_hash_regions(struct image_region *regs, int count,
  * Return:	true if found, false if not
  */
 bool efi_signature_lookup_digest(struct efi_image_regions *regs,
-				 struct efi_signature_store *db)
+				 struct efi_signature_store *db,
+				 bool dbx)
+
 {
 	struct efi_signature_store *siglist;
 	struct efi_sig_data *sig_data;
@@ -172,12 +199,20 @@ bool efi_signature_lookup_digest(struct efi_image_regions *regs,
 		goto out;
 
 	for (siglist = db; siglist; siglist = siglist->next) {
-		/* TODO: support other hash algorithms */
-		if (guidcmp(&siglist->sig_type, &efi_guid_sha256)) {
-			EFI_PRINT("Digest algorithm is not supported: %pUs\n",
-				  &siglist->sig_type);
-			break;
-		}
+		/*
+		 * if the hash algorithm is unsupported and we get an entry in
+		 * dbx reject the image
+		 */
+		if (dbx && !hash_algo_supported(siglist->sig_type)) {
+			found = true;
+			continue;
+		};
+		/*
+		 * Only support sha256 for now, that's what
+		 * hash-to-efi-sig-list produces
+		 */
+		if (guidcmp(&siglist->sig_type, &efi_guid_sha256))
+			continue;
 
 		if (!efi_hash_regions(regs->reg, regs->num, &hash, &size)) {
 			EFI_PRINT("Digesting an image failed\n");