1 files changed, 122 insertions, 0 deletions
diff --git a/fs/verity/signature.c b/fs/verity/signature.c
new file mode 100644
index 0000000000..90c07573dd
--- /dev/null
+++ b/fs/verity/signature.c
@@ -0,0 +1,122 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Verification of builtin signatures
+ *
+ * Copyright 2019 Google LLC
+ */
+
+/*
+ * This file implements verification of fs-verity builtin signatures.  Please
+ * take great care before using this feature.  It is not the only way to do
+ * signatures with fs-verity, and the alternatives (such as userspace signature
+ * verification, and IMA appraisal) can be much better.  For details about the
+ * limitations of this feature, see Documentation/filesystems/fsverity.rst.
+ */
+
+#include "fsverity_private.h"
+
+#include <linux/cred.h>
+#include <linux/key.h>
+#include <linux/slab.h>
+#include <linux/verification.h>
+
+/*
+ * /proc/sys/fs/verity/require_signatures
+ * If 1, all verity files must have a valid builtin signature.
+ */
+int fsverity_require_signatures;
+
+/*
+ * Keyring that contains the trusted X.509 certificates.
+ *
+ * Only root (kuid=0) can modify this.  Also, root may use
+ * keyctl_restrict_keyring() to prevent any more additions.
+ */
+static struct key *fsverity_keyring;
+
+/**
+ * fsverity_verify_signature() - check a verity file's signature
+ * @vi: the file's fsverity_info
+ * @signature: the file's built-in signature
+ * @sig_size: size of signature in bytes, or 0 if no signature
+ *
+ * If the file includes a signature of its fs-verity file digest, verify it
+ * against the certificates in the fs-verity keyring.
+ *
+ * Return: 0 on success (signature valid or not required); -errno on failure
+ */
+int fsverity_verify_signature(const struct fsverity_info *vi,
+			      const u8 *signature, size_t sig_size)
+{
+	const struct inode *inode = vi->inode;
+	const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
+	struct fsverity_formatted_digest *d;
+	int err;
+
+	if (sig_size == 0) {
+		if (fsverity_require_signatures) {
+			fsverity_err(inode,
+				     "require_signatures=1, rejecting unsigned file!");
+			return -EPERM;
+		}
+		return 0;
+	}
+
+	if (fsverity_keyring->keys.nr_leaves_on_tree == 0) {
+		/*
+		 * The ".fs-verity" keyring is empty, due to builtin signatures
+		 * being supported by the kernel but not actually being used.
+		 * In this case, verify_pkcs7_signature() would always return an
+		 * error, usually ENOKEY.  It could also be EBADMSG if the
+		 * PKCS#7 is malformed, but that isn't very important to
+		 * distinguish.  So, just skip to ENOKEY to avoid the attack
+		 * surface of the PKCS#7 parser, which would otherwise be
+		 * reachable by any task able to execute FS_IOC_ENABLE_VERITY.
+		 */
+		fsverity_err(inode,
+			     "fs-verity keyring is empty, rejecting signed file!");
+		return -ENOKEY;
+	}
+
+	d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
+	if (!d)
+		return -ENOMEM;
+	memcpy(d->magic, "FSVerity", 8);
+	d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
+	d->digest_size = cpu_to_le16(hash_alg->digest_size);
+	memcpy(d->digest, vi->file_digest, hash_alg->digest_size);
+
+	err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
+				     signature, sig_size, fsverity_keyring,
+				     VERIFYING_UNSPECIFIED_SIGNATURE,
+				     NULL, NULL);
+	kfree(d);
+
+	if (err) {
+		if (err == -ENOKEY)
+			fsverity_err(inode,
+				     "File's signing cert isn't in the fs-verity keyring");
+		else if (err == -EKEYREJECTED)
+			fsverity_err(inode, "Incorrect file signature");
+		else if (err == -EBADMSG)
+			fsverity_err(inode, "Malformed file signature");
+		else
+			fsverity_err(inode, "Error %d verifying file signature",
+				     err);
+		return err;
+	}
+
+	return 0;
+}
+
+void __init fsverity_init_signature(void)
+{
+	fsverity_keyring =
+		keyring_alloc(".fs-verity", KUIDT_INIT(0), KGIDT_INIT(0),
+			      current_cred(), KEY_POS_SEARCH |
+				KEY_USR_VIEW | KEY_USR_READ | KEY_USR_WRITE |
+				KEY_USR_SEARCH | KEY_USR_SETATTR,
+			      KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
+	if (IS_ERR(fsverity_keyring))
+		panic("failed to allocate \".fs-verity\" keyring");
+}