[PATCH v3 1/3] dm: Add verity helpers for LoadPin

[Matthias Kaehlcke <mka@chromium.org>]

LoadPin limits loading of kernel modules, firmware and certain
other files to a 'pinned' file system (typically a read-only
rootfs). To provide more flexibility LoadPin is being extended
to also allow loading these files from trusted dm-verity
devices. For that purpose LoadPin can be provided with a list
of verity root digests that it should consider as trusted.

Add a bunch of helpers to allow LoadPin to check whether a DM
device is a trusted verity device. The new functions broadly
fall in two categories: those that need access to verity
internals (like the root digest), and the 'glue' between
LoadPin and verity. The new file dm-verity-loadpin.c contains
the glue functions.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
---

Changes in v3:
- none

Changes in v2:
- none

 drivers/md/Makefile               |  6 +++
 drivers/md/dm-verity-loadpin.c    | 80 +++++++++++++++++++++++++++++++
 drivers/md/dm-verity-target.c     | 33 +++++++++++++
 drivers/md/dm-verity.h            |  4 ++
 include/linux/dm-verity-loadpin.h | 27 +++++++++++
 5 files changed, 150 insertions(+)
 create mode 100644 drivers/md/dm-verity-loadpin.c
 create mode 100644 include/linux/dm-verity-loadpin.h

diff --git a/drivers/md/Makefile b/drivers/md/Makefile
index 0454b0885b01..e12cd004d375 100644
--- a/drivers/md/Makefile
+++ b/drivers/md/Makefile
@@ -100,6 +100,12 @@ ifeq ($(CONFIG_IMA),y)
 dm-mod-objs			+= dm-ima.o
 endif
 
+ifeq ($(CONFIG_DM_VERITY),y)
+ifeq ($(CONFIG_SECURITY_LOADPIN),y)
+dm-mod-objs			+= dm-verity-loadpin.o
+endif
+endif
+
 ifeq ($(CONFIG_DM_VERITY_FEC),y)
 dm-verity-objs			+= dm-verity-fec.o
 endif
diff --git a/drivers/md/dm-verity-loadpin.c b/drivers/md/dm-verity-loadpin.c
new file mode 100644
index 000000000000..972ca93a2231
--- /dev/null
+++ b/drivers/md/dm-verity-loadpin.c
@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <linux/list.h>
+#include <linux/kernel.h>
+#include <linux/dm-verity-loadpin.h>
+
+#include "dm.h"
+#include "dm-verity.h"
+
+static struct list_head *trusted_root_digests;
+
+/*
+ * Sets the root digests of verity devices which LoadPin considers as trusted.
+ *
+ * This function must only be called once.
+ */
+void dm_verity_loadpin_set_trusted_root_digests(struct list_head *digests)
+{
+	if (!trusted_root_digests)
+		trusted_root_digests = digests;
+	else
+		pr_warn("verity root digests trusted by LoadPin are already set!!!\n");
+}
+
+static bool is_trusted_verity_target(struct dm_target *ti)
+{
+	u8 *root_digest;
+	unsigned int digest_size;
+	struct trusted_root_digest *trd;
+	bool trusted = false;
+
+	if (!dm_is_verity_target(ti))
+		return false;
+
+	if (dm_verity_get_root_digest(ti, &root_digest, &digest_size))
+		return false;
+
+	list_for_each_entry(trd, trusted_root_digests, node) {
+		if ((trd->len == digest_size) &&
+		    !memcmp(trd->data, root_digest, digest_size)) {
+			trusted = true;
+			break;
+		}
+	}
+
+	kfree(root_digest);
+
+	return trusted;
+}
+
+/*
+ * Determines whether a mapped device is a verity device that is trusted
+ * by LoadPin.
+ */
+bool dm_verity_loadpin_is_md_trusted(struct mapped_device *md)
+{
+	int srcu_idx;
+	struct dm_table *table;
+	unsigned int num_targets;
+	bool trusted = false;
+	int i;
+
+	if (!trusted_root_digests || list_empty(trusted_root_digests))
+		return false;
+
+	table = dm_get_live_table(md, &srcu_idx);
+	num_targets = dm_table_get_num_targets(table);
+	for (i = 0; i < num_targets; i++) {
+		struct dm_target *ti = dm_table_get_target(table, i);
+
+		if (is_trusted_verity_target(ti)) {
+			trusted = true;
+			break;
+		}
+	}
+
+	dm_put_live_table(md, srcu_idx);
+
+	return trusted;
+}
diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
index 80133aae0db3..6f07b849fcb2 100644
--- a/drivers/md/dm-verity-target.c
+++ b/drivers/md/dm-verity-target.c
@@ -19,6 +19,7 @@
 #include <linux/module.h>
 #include <linux/reboot.h>
 #include <linux/scatterlist.h>
+#include <linux/string.h>
 
 #define DM_MSG_PREFIX			"verity"
 
@@ -1310,6 +1311,38 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
 	return r;
 }
 
+/*
+ * Check whether a DM target is a verity target.
+ */
+bool dm_is_verity_target(struct dm_target *ti)
+{
+	return ti->type->module == THIS_MODULE;
+}
+EXPORT_SYMBOL_GPL(dm_is_verity_target);
+
+/*
+ * Get the root digest of a verity target.
+ *
+ * Returns a copy of the root digest, the caller is responsible for
+ * freeing the memory of the digest.
+ */
+int dm_verity_get_root_digest(struct dm_target *ti, u8 **root_digest, unsigned int *digest_size)
+{
+	struct dm_verity *v = ti->private;
+
+	if (!dm_is_verity_target(ti))
+		return -EINVAL;
+
+	*root_digest = kmemdup(v->root_digest, v->digest_size, GFP_KERNEL);
+	if (*root_digest == NULL)
+		return -ENOMEM;
+
+	*digest_size = v->digest_size;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(dm_verity_get_root_digest);
+
 static struct target_type verity_target = {
 	.name		= "verity",
 	.version	= {1, 8, 0},
diff --git a/drivers/md/dm-verity.h b/drivers/md/dm-verity.h
index 4e769d13473a..c832cc3e3d24 100644
--- a/drivers/md/dm-verity.h
+++ b/drivers/md/dm-verity.h
@@ -129,4 +129,8 @@ extern int verity_hash(struct dm_verity *v, struct ahash_request *req,
 extern int verity_hash_for_block(struct dm_verity *v, struct dm_verity_io *io,
 				 sector_t block, u8 *digest, bool *is_zero);
 
+extern bool dm_is_verity_target(struct dm_target *ti);
+extern int dm_verity_get_root_digest(struct dm_target *ti, u8 **root_digest,
+				     unsigned int *digest_size);
+
 #endif /* DM_VERITY_H */
diff --git a/include/linux/dm-verity-loadpin.h b/include/linux/dm-verity-loadpin.h
new file mode 100644
index 000000000000..12a86911d05a
--- /dev/null
+++ b/include/linux/dm-verity-loadpin.h
@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef __LINUX_DM_VERITY_LOADPIN_H
+#define __LINUX_DM_VERITY_LOADPIN_H
+
+#include <linux/list.h>
+
+struct mapped_device;
+
+struct trusted_root_digest {
+	u8 *data;
+	unsigned int len;
+	struct list_head node;
+};
+
+#if IS_ENABLED(CONFIG_SECURITY_LOADPIN) && IS_BUILTIN(CONFIG_DM_VERITY)
+void dm_verity_loadpin_set_trusted_root_digests(struct list_head *digests);
+bool dm_verity_loadpin_is_md_trusted(struct mapped_device *md);
+#else
+static inline void dm_verity_loadpin_set_trusted_root_digests(struct list_head *digests) {}
+static inline bool dm_verity_loadpin_is_md_trusted(struct mapped_device *md)
+{
+	return false;
+}
+#endif
+
+#endif /* __LINUX_DM_LOADPIN_H */
-- 
2.36.0.464.gb9c8b46e94-goog