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. Userspace can use the new systl file
'loadpin/trusted_verity_root_digests' to provide LoadPin
with a list of root digests from dm-verity devices that
LoadPin should consider as trusted. 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.

The list of trusted root digests can only be written once
(typically at boot time), to limit the possiblity of
attackers setting up rogue verity devices and marking them
as trusted.

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.md

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

security/loadpin/Kconfig

@@ -18,3 +18,14 @@ 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 DM_VERITY=y
+	help
+	  If selected LoadPin can allow reading files from filesystems
+	  that use dm-verity. A verity filesystem can be configured as
+	  being trusted by LoadPin by writing its root digest to the
+	  sysctl file 'trusted_verity_root_digests'. The sysctl file
+	  can only be written once (typically at boot) and accepts a
+	  list of comma separated 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/device-mapper.h>
+#include <linux/dm-verity-loadpin.h>
 
 static void report_load(const char *origin, struct file *file, char *operation)
 {
@@ -43,6 +45,10 @@ 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 verity_digests_set;
+static LIST_HEAD(trusted_verity_root_digests);
+#endif
 
 #ifdef CONFIG_SYSCTL
 
@@ -65,6 +71,144 @@ static struct ctl_table loadpin_sysctl_table[] = {
 	{ }
 };
 
+#ifdef CONFIG_SECURITY_LOADPIN_VERITY
+
+static int proc_verity_root_digests(struct ctl_table *table, int write,
+				    void *buffer, size_t *lenp, loff_t *ppos)
+{
+	struct trusted_root_digest *trd;
+	char *buf;
+	int rc;
+
+	if (write) {
+		struct ctl_table tbl = *table;
+		char *p, *d;
+
+		if (*ppos)
+			return -EINVAL;
+
+		if (verity_digests_set)
+			return -EPERM;
+
+		buf = kzalloc(tbl.maxlen, GFP_KERNEL);
+		if (!buf)
+			return -ENOMEM;
+
+		tbl.data = buf;
+
+		rc = proc_dostring(&tbl, write, buffer, lenp, ppos);
+		if (rc) {
+			kfree(buf);
+			return rc;
+		}
+
+		if (strlen(buf) == 0) {
+			verity_digests_set = true;
+			return false;
+		}
+
+		p = buf;
+		while ((d = strsep(&p, ",")) != NULL) {
+			int len = strlen(d);
+
+			if (len % 2) {
+				rc = -EINVAL;
+				goto err;
+			}
+
+			len /= 2;
+
+			trd = kzalloc(sizeof(*trd), GFP_KERNEL);
+			if (!trd) {
+				rc = -ENOMEM;
+				goto err;
+			}
+
+			list_add_tail(&trd->node, &trusted_verity_root_digests);
+
+			trd->data = kzalloc(len, GFP_KERNEL);
+			if (!trd->data) {
+				rc = -ENOMEM;
+				goto err;
+			}
+
+			if (hex2bin(trd->data, d, len)) {
+				rc = -EINVAL;
+				goto err;
+			}
+
+			trd->len = len;
+		}
+
+		kfree(buf);
+
+		if (!list_empty(&trusted_verity_root_digests)) {
+			dm_verity_loadpin_set_trusted_root_digests(&trusted_verity_root_digests);
+			verity_digests_set = true;
+		}
+	} else {
+		struct ctl_table tbl = { .maxlen = 0 };
+		size_t off = 0;
+
+		list_for_each_entry(trd, &trusted_verity_root_digests, node) {
+			/* two ASCII characters per byte and a comma / '\0' */
+			tbl.maxlen += (trd->len * 2) + 1;
+		}
+
+		tbl.data = kzalloc(tbl.maxlen, GFP_KERNEL);
+
+		list_for_each_entry(trd, &trusted_verity_root_digests, node) {
+			int i;
+
+			if (off > 0) {
+				snprintf(tbl.data + off, tbl.maxlen - off, ",");
+				off++;
+			}
+
+			for (i = 0; i < trd->len; i++) {
+				snprintf(tbl.data + off, tbl.maxlen - off, "%02x", trd->data[i]);
+				off += 2;
+			}
+		}
+
+		proc_dostring(&tbl, write, buffer, lenp, ppos);
+		kfree(tbl.data);
+	}
+
+	return 0;
+
+err:
+	kfree(buf);
+
+	{
+		struct trusted_root_digest *tmp;
+
+		list_for_each_entry_safe(trd, tmp, &trusted_verity_root_digests, node) {
+			kfree(trd->data);
+			list_del(&trd->node);
+			kfree(trd);
+		}
+	}
+
+	return rc;
+}
+
+static struct ctl_table loadpin_sysctl_table_verity[] = {
+	{
+		.procname       = "trusted_verity_root_digests",
+		.maxlen         = SZ_4K,
+		.mode           = 0644,
+		.proc_handler   = proc_verity_root_digests,
+	},
+
+	{ }
+};
+#else
+
+static struct ctl_table loadpin_sysctl_table_verity[] = {};
+
+#endif /* CONFIG_SECURITY_LOADPIN_VERITY */
+
 /*
  * This must be called after early kernel init, since then the rootdev
  * is available.
@@ -118,6 +262,20 @@ static void loadpin_sb_free_security(struct super_block *mnt_sb)
 	}
 }
 
+static bool loadpin_is_fs_trusted(struct super_block *sb)
+{
+	struct mapped_device *md = dm_get_md(sb->s_bdev->bd_dev);
+	bool trusted;
+
+	if (!md)
+		return false;
+
+	trusted = dm_verity_loadpin_is_md_trusted(md);
+	dm_put(md);
+
+	return trusted;
+}
+
 static int loadpin_read_file(struct file *file, enum kernel_read_file_id id,
 			     bool contents)
 {
@@ -174,7 +332,8 @@ 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) && !loadpin_is_fs_trusted(load_root))) {
 		if (unlikely(!enforce)) {
 			report_load(origin, file, "pinning-ignored");
 			return 0;
@@ -240,6 +399,13 @@ static int __init loadpin_init(void)
 		enforce ? "" : "not ");
 	parse_exclude();
 	security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
+
+	if (IS_ENABLED(CONFIG_SECURITY_LOADPIN_VERITY)) {
+		if (!register_sysctl_paths(loadpin_sysctl_path,
+					   loadpin_sysctl_table_verity))
+			pr_notice("sysctl registration failed!\n");
+	}
+
 	return 0;
 }