LoadPin: Enable loading from trusted dm-verity devices

Extend LoadPin to allow loading of kernel files from trusted dm-verity [1]
devices.

This change adds the concept of trusted verity devices to LoadPin. LoadPin
maintains a list of root digests of verity devices it considers trusted.
Userspace can populate this list through an ioctl on the new LoadPin
securityfs entry 'dm-verity'. The ioctl receives a file descriptor of
a file with verity digests as parameter. Verity reads the digests from
this file after confirming that the file is located on the pinned root.
The digest file must contain one digest per line. The list of trusted
digests can only be set up once, which is typically done at boot time.

When a kernel file is read LoadPin first checks (as usual) whether the file
is located on the pinned root, if so the file can be loaded. Otherwise, if
the verity extension is enabled, LoadPin determines whether the file is
located on a verity backed device and whether the root digest of that
device is in the list of trusted digests. The file can be loaded if the
verity device has a trusted root digest.

Background:

As of now LoadPin restricts loading of kernel files to a single pinned
filesystem, typically the rootfs. This works for many systems, however it
can result in a bloated rootfs (and OTA updates) on platforms where
multiple boards with different hardware configurations use the same rootfs
image. Especially when 'optional' files are large it may be preferable to
download/install them only when they are actually needed by a given board.
Chrome OS uses Downloadable Content (DLC) [2] to deploy certain 'packages'
at runtime. As an example a DLC package could contain firmware for a
peripheral that is not present on all boards. DLCs use dm-verity to verify
the integrity of the DLC content.

[1] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/verity.html
[2] https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/dlcservice/docs/developer.mdSigned-off-by: Matthias Kaehlcke <mka@chromium.org>
Acked-by: Mike Snitzer <snitzer@kernel.org>
Link: https://lore.kernel.org/lkml/20220627083512.v7.2.I01c67af41d2f6525c6d023101671d7339a9bc8b5@changeidSigned-off-by: Kees Cook <keescook@chromium.org>

include/uapi/linux/loadpin.h

+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/*
+ * Copyright (c) 2022, Google LLC
+ */
+
+#ifndef _UAPI_LINUX_LOOP_LOADPIN_H
+#define _UAPI_LINUX_LOOP_LOADPIN_H
+
+#define LOADPIN_IOC_MAGIC	'L'
+
+/**
+ * LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS - Set up the root digests of verity devices
+ *                                          that loadpin should trust.
+ *
+ * Takes a file descriptor from which to read the root digests of trusted verity devices. The file
+ * is expected to contain a list of digests in ASCII format, with one line per digest. The ioctl
+ * must be issued on the securityfs attribute 'loadpin/dm-verity' (which can be typically found
+ * under /sys/kernel/security/loadpin/dm-verity).
+ */
+#define LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS _IOW(LOADPIN_IOC_MAGIC, 0x00, unsigned int)
+
+#endif /* _UAPI_LINUX_LOOP_LOADPIN_H */

security/loadpin/Kconfig

@@ -18,3 +18,19 @@ config SECURITY_LOADPIN_ENFORCE
 	  If selected, LoadPin will enforce pinning at boot. If not
 	  selected, it can be enabled at boot with the kernel parameter
 	  "loadpin.enforce=1".
+
+config SECURITY_LOADPIN_VERITY
+	bool "Allow reading files from certain other filesystems that use dm-verity"
+	depends on SECURITY_LOADPIN && DM_VERITY=y && SECURITYFS
+	help
+	  If selected LoadPin can allow reading files from filesystems
+	  that use dm-verity. LoadPin maintains a list of verity root
+	  digests it considers trusted. A verity backed filesystem is
+	  considered trusted if its root digest is found in the list
+	  of trusted digests.
+
+	  The list of trusted verity can be populated through an ioctl
+	  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 a comma separated list of digests.

security/loadpin/loadpin.c

@@ -18,6 +18,8 @@
 #include <linux/path.h>
 #include <linux/sched.h>	/* current */
 #include <linux/string_helpers.h>
+#include <linux/dm-verity-loadpin.h>
+#include <uapi/linux/loadpin.h>
 
 static void report_load(const char *origin, struct file *file, char *operation)
 {
@@ -43,6 +45,9 @@ static char *exclude_read_files[READING_MAX_ID];
 static int ignore_read_file_id[READING_MAX_ID] __ro_after_init;
 static struct super_block *pinned_root;
 static DEFINE_SPINLOCK(pinned_root_spinlock);
+#ifdef CONFIG_SECURITY_LOADPIN_VERITY
+static bool deny_reading_verity_digests;
+#endif
 
 #ifdef CONFIG_SYSCTL
 
@@ -171,7 +176,8 @@ static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
 		spin_unlock(&pinned_root_spinlock);
 	}
 
-	if (IS_ERR_OR_NULL(pinned_root) || load_root != pinned_root) {
+	if (IS_ERR_OR_NULL(pinned_root) ||
+	    ((load_root != pinned_root) && !dm_verity_loadpin_is_bdev_trusted(load_root->s_bdev))) {
 		if (unlikely(!enforce)) {
 			report_load(origin, file, "pinning-ignored");
 			return 0;
@@ -237,6 +243,7 @@ static int __init loadpin_init(void)
 		enforce ? "" : "not ");
 	parse_exclude();
 	security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
+
 	return 0;
 }
 
@@ -245,6 +252,164 @@ DEFINE_LSM(loadpin) = {
 	.init = loadpin_init,
 };
 
+#ifdef CONFIG_SECURITY_LOADPIN_VERITY
+
+enum loadpin_securityfs_interface_index {
+	LOADPIN_DM_VERITY,
+};
+
+static int read_trusted_verity_root_digests(unsigned int fd)
+{
+	struct fd f;
+	void *data;
+	int rc;
+	char *p, *d;
+
+	if (deny_reading_verity_digests)
+		return -EPERM;
+
+	/* The list of trusted root digests can only be set up once */
+	if (!list_empty(&dm_verity_loadpin_trusted_root_digests))
+		return -EPERM;
+
+	f = fdget(fd);
+	if (!f.file)
+		return -EINVAL;
+
+	data = kzalloc(SZ_4K, GFP_KERNEL);
+	if (!data) {
+		rc = -ENOMEM;
+		goto err;
+	}
+
+	rc = kernel_read_file(f.file, 0, (void **)&data, SZ_4K - 1, NULL, READING_POLICY);
+	if (rc < 0)
+		goto err;
+
+	p = data;
+	p[rc] = '\0';
+	p = strim(p);
+
+	p = strim(data);
+	while ((d = strsep(&p, "\n")) != NULL) {
+		int len = strlen(d);
+		struct dm_verity_loadpin_trusted_root_digest *trd;
+
+		if (len % 2) {
+			rc = -EPROTO;
+			goto err;
+		}
+
+		len /= 2;
+
+		trd = kzalloc(struct_size(trd, data, len), GFP_KERNEL);
+		if (!trd) {
+			rc = -ENOMEM;
+			goto err;
+		}
+
+		if (hex2bin(trd->data, d, len)) {
+			kfree(trd);
+			rc = -EPROTO;
+			goto err;
+		}
+
+		trd->len = len;
+
+		list_add_tail(&trd->node, &dm_verity_loadpin_trusted_root_digests);
+	}
+
+	if (list_empty(&dm_verity_loadpin_trusted_root_digests)) {
+		rc = -EPROTO;
+		goto err;
+	}
+
+	kfree(data);
+	fdput(f);
+
+	return 0;
+
+err:
+	kfree(data);
+
+	/* any failure in loading/parsing invalidates the entire list */
+	{
+		struct dm_verity_loadpin_trusted_root_digest *trd, *tmp;
+
+		list_for_each_entry_safe(trd, tmp, &dm_verity_loadpin_trusted_root_digests, node) {
+			list_del(&trd->node);
+			kfree(trd);
+		}
+	}
+
+	/* disallow further attempts after reading a corrupt/invalid file */
+	deny_reading_verity_digests = true;
+
+	fdput(f);
+
+	return rc;
+}
+
+/******************************** securityfs ********************************/
+
+static long dm_verity_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
+{
+	void __user *uarg = (void __user *)arg;
+	unsigned int fd;
+	int rc;
+
+	switch (cmd) {
+	case LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS:
+		rc = copy_from_user(&fd, uarg, sizeof(fd));
+		if (rc)
+			return rc;
+
+		return read_trusted_verity_root_digests(fd);
+
+	default:
+		return -EINVAL;
+	}
+}
+
+static const struct file_operations loadpin_dm_verity_ops = {
+	.unlocked_ioctl = dm_verity_ioctl,
+	.compat_ioctl = compat_ptr_ioctl,
+};
+
+/**
+ * init_loadpin_securityfs - create the securityfs directory for LoadPin
+ *
+ * We can not put this method normally under the loadpin_init() code path since
+ * the security subsystem gets initialized before the vfs caches.
+ *
+ * Returns 0 if the securityfs directory creation was successful.
+ */
+static int __init init_loadpin_securityfs(void)
+{
+	struct dentry *loadpin_dir, *dentry;
+
+	loadpin_dir = securityfs_create_dir("loadpin", NULL);
+	if (IS_ERR(loadpin_dir)) {
+		pr_err("LoadPin: could not create securityfs dir: %ld\n",
+		       PTR_ERR(loadpin_dir));
+		return PTR_ERR(loadpin_dir);
+	}
+
+	dentry = securityfs_create_file("dm-verity", 0600, loadpin_dir,
+					(void *)LOADPIN_DM_VERITY, &loadpin_dm_verity_ops);
+	if (IS_ERR(dentry)) {
+		pr_err("LoadPin: could not create securityfs entry 'dm-verity': %ld\n",
+		       PTR_ERR(dentry));
+		return PTR_ERR(dentry);
+	}
+
+	return 0;
+}
+
+fs_initcall(init_loadpin_securityfs);
+
+#endif /* CONFIG_SECURITY_LOADPIN_VERITY */
+
 /* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
 module_param(enforce, int, 0);
 MODULE_PARM_DESC(enforce, "Enforce module/firmware pinning");