ima: fail file signature verification on non-init mounted filesystems

FUSE can be mounted by unprivileged users either today with fusermount
installed with setuid, or soon with the upcoming patches to allow FUSE
mounts in a non-init user namespace.

This patch addresses the new unprivileged non-init mounted filesystems,
which are untrusted, by failing the signature verification.

This patch defines two new flags SB_I_IMA_UNVERIFIABLE_SIGNATURE and
SB_I_UNTRUSTED_MOUNTER.

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Seth Forshee <seth.forshee@canonical.com>
Cc: Dongsu Park <dongsu@kinvolk.io>
Cc: Alban Crequy <alban@kinvolk.io>
Acked-by: Serge Hallyn <serge@hallyn.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>

security/integrity/ima/ima_appraise.c

@@ -302,7 +302,19 @@ int ima_appraise_measurement(enum ima_hooks func,
 	}
 
 out:
-	if (status != INTEGRITY_PASS) {
+	/*
+	 * File signatures on some filesystems can not be properly verified.
+	 * On these filesytems, that are mounted by an untrusted mounter,
+	 * fail the file signature verification.
+	 */
+	if ((inode->i_sb->s_iflags &
+	    (SB_I_IMA_UNVERIFIABLE_SIGNATURE | SB_I_UNTRUSTED_MOUNTER)) ==
+	    (SB_I_IMA_UNVERIFIABLE_SIGNATURE | SB_I_UNTRUSTED_MOUNTER)) {
+		status = INTEGRITY_FAIL;
+		cause = "unverifiable-signature";
+		integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, filename,
+				    op, cause, rc, 0);
+	} else if (status != INTEGRITY_PASS) {
 		if ((ima_appraise & IMA_APPRAISE_FIX) &&
 		    (!xattr_value ||
 		     xattr_value->type != EVM_IMA_XATTR_DIGSIG)) {
@@ -319,6 +331,7 @@ out:
 	} else {
 		ima_cache_flags(iint, func);
 	}
+
 	ima_set_cache_status(iint, func, status);
 	return status;
 }