LoadPin: Require file with verity root digests to have a header

LoadPin expects the file with trusted verity root digests to be
an ASCII file with one digest (hex value) per line. A pinned
root could contain files that meet these format requirements,
even though the hex values don't represent trusted root
digests.

Add a new requirement to the file format which consists in
the first line containing a fixed string. This prevents
attackers from feeding files with an otherwise valid format
to LoadPin.

Suggested-by: Sarthak Kukreti <sarthakkukreti@chromium.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220906181725.1.I3f51d1bb0014e5a5951be4ad3c5ad7c7ca1dfc32@changeid

security/loadpin/Kconfig

@@ -33,4 +33,9 @@ config SECURITY_LOADPIN_VERITY
 	  on the LoadPin securityfs entry 'dm-verity'. The ioctl
 	  expects a file descriptor of a file with verity digests as
 	  parameter. The file must be located on the pinned root and
-	  contain one digest per line.
+	  start with the line:
+
+	  # LOADPIN_TRUSTED_VERITY_ROOT_DIGESTS
+
+	  This is followed by the verity digests, with one digest per
+	  line.

security/loadpin/loadpin.c

@@ -21,6 +21,8 @@
 #include <linux/dm-verity-loadpin.h>
 #include <uapi/linux/loadpin.h>
 
+#define VERITY_DIGEST_FILE_HEADER "# LOADPIN_TRUSTED_VERITY_ROOT_DIGESTS"
+
 static void report_load(const char *origin, struct file *file, char *operation)
 {
 	char *cmdline, *pathname;
@@ -292,9 +294,21 @@ static int read_trusted_verity_root_digests(unsigned int fd)
 
 	p = strim(data);
 	while ((d = strsep(&p, "\n")) != NULL) {
-		int len = strlen(d);
+		int len;
 		struct dm_verity_loadpin_trusted_root_digest *trd;
 
+		if (d == data) {
+			/* first line, validate header */
+			if (strcmp(d, VERITY_DIGEST_FILE_HEADER)) {
+				rc = -EPROTO;
+				goto err;
+			}
+
+			continue;
+		}
+
+		len = strlen(d);
+
 		if (len % 2) {
 			rc = -EPROTO;
 			goto err;