diff mbox series

[2/3] LoadPin: Enable loading from trusted dm-verity devices

Message ID 20220418141545.2.I01c67af41d2f6525c6d023101671d7339a9bc8b5@changeid (mailing list archive)
State New
Headers show
Series LoadPin: Enable loading from trusted dm-verity devices | expand

Commit Message

Matthias Kaehlcke April 18, 2022, 9:15 p.m. UTC
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   |  11 +++
 security/loadpin/loadpin.c | 168 ++++++++++++++++++++++++++++++++++++-
 2 files changed, 178 insertions(+), 1 deletion(-)

Comments

kernel test robot April 19, 2022, 4:32 a.m. UTC | #1
Hi Matthias,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on device-mapper-dm/for-next]
[also build test ERROR on song-md/md-next kees/for-next/pstore v5.18-rc3 next-20220414]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/intel-lab-lkp/linux/commits/Matthias-Kaehlcke/LoadPin-Enable-loading-from-trusted-dm-verity-devices/20220419-051810
base:   https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git for-next
config: powerpc-buildonly-randconfig-r002-20220417 (https://download.01.org/0day-ci/archive/20220419/202204191222.n0HWtNe0-lkp@intel.com/config)
compiler: powerpc-linux-gcc (GCC) 11.2.0
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://github.com/intel-lab-lkp/linux/commit/5af892969b227a2602732e4be44a0c4a41967400
        git remote add linux-review https://github.com/intel-lab-lkp/linux
        git fetch --no-tags linux-review Matthias-Kaehlcke/LoadPin-Enable-loading-from-trusted-dm-verity-devices/20220419-051810
        git checkout 5af892969b227a2602732e4be44a0c4a41967400
        # save the config file to linux build tree
        mkdir build_dir
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-11.2.0 make.cross O=build_dir ARCH=powerpc SHELL=/bin/bash security/loadpin/

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   security/loadpin/loadpin.c: In function 'loadpin_init':
>> security/loadpin/loadpin.c:404:44: error: 'loadpin_sysctl_path' undeclared (first use in this function)
     404 |                 if (!register_sysctl_paths(loadpin_sysctl_path,
         |                                            ^~~~~~~~~~~~~~~~~~~
   security/loadpin/loadpin.c:404:44: note: each undeclared identifier is reported only once for each function it appears in
>> security/loadpin/loadpin.c:405:44: error: 'loadpin_sysctl_table_verity' undeclared (first use in this function)
     405 |                                            loadpin_sysctl_table_verity))
         |                                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~


vim +/loadpin_sysctl_path +404 security/loadpin/loadpin.c

   395	
   396	static int __init loadpin_init(void)
   397	{
   398		pr_info("ready to pin (currently %senforcing)\n",
   399			enforce ? "" : "not ");
   400		parse_exclude();
   401		security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks), "loadpin");
   402	
   403		if (IS_ENABLED(CONFIG_SECURITY_LOADPIN_VERITY)) {
 > 404			if (!register_sysctl_paths(loadpin_sysctl_path,
 > 405						   loadpin_sysctl_table_verity))
   406				pr_notice("sysctl registration failed!\n");
   407		}
   408	
   409		return 0;
   410	}
   411
diff mbox series

Patch

diff --git a/security/loadpin/Kconfig b/security/loadpin/Kconfig
index 91be65dec2ab..cf3e6431e02d 100644
--- a/security/loadpin/Kconfig
+++ b/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.
diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
index b12f7d986b1e..b9c174f8687d 100644
--- a/security/loadpin/loadpin.c
+++ b/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;
 }