Load concatenated EFI_SIGNATURE_LISTs from shim_certificate.efi

For multiple reasons, it may be useful for different keys to be used to
sign different parts of the boot chain (e.g. a different key for GRUB
and the Linux kernel). Allow this by loading concatenated
EFI_SIGNATURE_LISTs from shim_certificate.efi rather than only the
first.

At the same time, be a bit more robust by checking for allocation
failures and overflows due to invalid data in the binary.
Use VirtualSize rather than SizeOfRawData since the latter is rounded up
to the section alignment and therefore contains non-certificate data.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>

shim.c

@@ -1518,6 +1518,7 @@ load_cert_file(EFI_HANDLE image_handle, CHAR16 *filename, CHAR16 *PathName)
 	EFI_SIGNATURE_LIST *certlist;
 	void *pointer;
 	UINT32 original;
+	UINT32 offset;
 	int datasize = 0;
 	void *data = NULL;
 	int i;
@@ -1535,22 +1536,36 @@ load_cert_file(EFI_HANDLE image_handle, CHAR16 *filename, CHAR16 *PathName)
 
 	Section = context.FirstSection;
 	for (i = 0; i < context.NumberOfSections; i++, Section++) {
-		if (CompareMem(Section->Name, ".db\0\0\0\0\0", 8) == 0) {
-			original = user_cert_size;
-			if (Section->SizeOfRawData < sizeof(EFI_SIGNATURE_LIST)) {
-				continue;
-			}
-			pointer = ImageAddress(data, datasize,
-					       Section->PointerToRawData);
-			if (!pointer) {
-				continue;
+		if (CompareMem(Section->Name, ".db\0\0\0\0\0", 8) == 0 &&
+                    Section->Misc.VirtualSize <= Section->SizeOfRawData) {
+			offset = 0;
+			while ((Section->Misc.VirtualSize - offset) >= sizeof(EFI_SIGNATURE_LIST)) {
+				original = user_cert_size;
+				pointer = ImageAddress(data, datasize,
+						   Section->PointerToRawData + offset);
+				if (!pointer) {
+				    break;
+				}
+				certlist = pointer;
+
+				if (certlist->SignatureListSize < sizeof(EFI_SIGNATURE_LIST) ||
+					checked_add(offset, certlist->SignatureListSize, &offset) ||
+					offset > Section->Misc.VirtualSize ||
+					checked_add(user_cert_size, certlist->SignatureListSize,
+						    &user_cert_size)) {
+					break;
+				}
+
+				user_cert = ReallocatePool(user_cert, original,
+						       user_cert_size);
+				if (!user_cert) {
+					FreePool(data);
+					return EFI_OUT_OF_RESOURCES;
+				}
+
+				CopyMem(user_cert + original, pointer,
+				    certlist->SignatureListSize);
 			}
-			certlist = pointer;
-			user_cert_size += certlist->SignatureListSize;;
-			user_cert = ReallocatePool(user_cert, original,
-						   user_cert_size);
-			CopyMem(user_cert + original, pointer,
-			        certlist->SignatureListSize);
 		}
 	}
 	FreePool(data);