dm-verity: expose root hash digest and signature data to LSMs

dm-verity provides a strong guarantee of a block device's integrity. As
a generic way to check the integrity of a block device, it provides
those integrity guarantees to its higher layers, including the filesystem
level.

However, critical security metadata like the dm-verity roothash and its
signing information are not easily accessible to the LSMs.
To address this limitation, this patch introduces a mechanism to store
and manage these essential security details within a newly added LSM blob
in the block_device structure.

This addition allows LSMs to make access control decisions on the integrity
data stored within the block_device, enabling more flexible security
policies. For instance, LSMs can now revoke access to dm-verity devices
based on their roothashes, ensuring that only authorized and verified
content is accessible. Additionally, LSMs can enforce policies to only
allow files from dm-verity devices that have a valid digital signature to
execute, effectively blocking any unsigned files from execution, thus
enhancing security against unauthorized modifications.

The patch includes new hook calls, `security_bdev_setintegrity()`, in
dm-verity to expose the dm-verity roothash and the roothash signature to
LSMs via preresume() callback. By using the preresume() callback, it
ensures that the security metadata is consistently in sync with the
metadata of the dm-verity target in the current active mapping table.
The hook calls are depended on CONFIG_SECURITY.
Signed-off-by: Deven Bowers <deven.desai@linux.microsoft.com>
Signed-off-by: Fan Wu <wufan@linux.microsoft.com>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
[PM: moved sig_size field as discussed]
Signed-off-by: Paul Moore <paul@paul-moore.com>

drivers/md/dm-verity-target.c

@@ -22,6 +22,7 @@
 #include <linux/scatterlist.h>
 #include <linux/string.h>
 #include <linux/jump_label.h>
+#include <linux/security.h>
 
 #define DM_MSG_PREFIX			"verity"
 
@@ -930,6 +931,41 @@ static void verity_io_hints(struct dm_target *ti, struct queue_limits *limits)
 	limits->dma_alignment = limits->logical_block_size - 1;
 }
 
+#ifdef CONFIG_SECURITY
+
+static int verity_init_sig(struct dm_verity *v, const void *sig,
+			   size_t sig_size)
+{
+	v->sig_size = sig_size;
+
+	if (sig) {
+		v->root_digest_sig = kmemdup(sig, v->sig_size, GFP_KERNEL);
+		if (!v->root_digest_sig)
+			return -ENOMEM;
+	}
+
+	return 0;
+}
+
+static void verity_free_sig(struct dm_verity *v)
+{
+	kfree(v->root_digest_sig);
+}
+
+#else
+
+static inline int verity_init_sig(struct dm_verity *v, const void *sig,
+				  size_t sig_size)
+{
+	return 0;
+}
+
+static inline void verity_free_sig(struct dm_verity *v)
+{
+}
+
+#endif /* CONFIG_SECURITY */
+
 static void verity_dtr(struct dm_target *ti)
 {
 	struct dm_verity *v = ti->private;
@@ -949,6 +985,7 @@ static void verity_dtr(struct dm_target *ti)
 	kfree(v->initial_hashstate);
 	kfree(v->root_digest);
 	kfree(v->zero_digest);
+	verity_free_sig(v);
 
 	if (v->ahash_tfm) {
 		static_branch_dec(&ahash_enabled);
@@ -1418,6 +1455,13 @@ static int verity_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 		ti->error = "Root hash verification failed";
 		goto bad;
 	}
+
+	r = verity_init_sig(v, verify_args.sig, verify_args.sig_size);
+	if (r < 0) {
+		ti->error = "Cannot allocate root digest signature";
+		goto bad;
+	}
+
 	v->hash_per_block_bits =
 		__fls((1 << v->hash_dev_block_bits) / v->digest_size);
 
@@ -1559,8 +1603,79 @@ int dm_verity_get_root_digest(struct dm_target *ti, u8 **root_digest, unsigned i
 	return 0;
 }
 
+#ifdef CONFIG_SECURITY
+
+#ifdef CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG
+
+static int verity_security_set_signature(struct block_device *bdev,
+					 struct dm_verity *v)
+{
+	/*
+	 * if the dm-verity target is unsigned, v->root_digest_sig will
+	 * be NULL, and the hook call is still required to let LSMs mark
+	 * the device as unsigned. This information is crucial for LSMs to
+	 * block operations such as execution on unsigned files
+	 */
+	return security_bdev_setintegrity(bdev,
+					  LSM_INT_DMVERITY_SIG_VALID,
+					  v->root_digest_sig,
+					  v->sig_size);
+}
+
+#else
+
+static inline int verity_security_set_signature(struct block_device *bdev,
+						struct dm_verity *v)
+{
+	return 0;
+}
+
+#endif /* CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG */
+
+/*
+ * Expose verity target's root hash and signature data to LSMs before resume.
+ *
+ * Returns 0 on success, or -ENOMEM if the system is out of memory.
+ */
+static int verity_preresume(struct dm_target *ti)
+{
+	struct block_device *bdev;
+	struct dm_verity_digest root_digest;
+	struct dm_verity *v;
+	int r;
+
+	v = ti->private;
+	bdev = dm_disk(dm_table_get_md(ti->table))->part0;
+	root_digest.digest = v->root_digest;
+	root_digest.digest_len = v->digest_size;
+	if (static_branch_unlikely(&ahash_enabled) && !v->shash_tfm)
+		root_digest.alg = crypto_ahash_alg_name(v->ahash_tfm);
+	else
+		root_digest.alg = crypto_shash_alg_name(v->shash_tfm);
+
+	r = security_bdev_setintegrity(bdev, LSM_INT_DMVERITY_ROOTHASH, &root_digest,
+				       sizeof(root_digest));
+	if (r)
+		return r;
+
+	r =  verity_security_set_signature(bdev, v);
+	if (r)
+		goto bad;
+
+	return 0;
+
+bad:
+
+	security_bdev_setintegrity(bdev, LSM_INT_DMVERITY_ROOTHASH, NULL, 0);
+
+	return r;
+}
+
+#endif /* CONFIG_SECURITY */
+
 static struct target_type verity_target = {
 	.name		= "verity",
+/* Note: the LSMs depend on the singleton and immutable features */
 	.features	= DM_TARGET_SINGLETON | DM_TARGET_IMMUTABLE,
 	.version	= {1, 10, 0},
 	.module		= THIS_MODULE,
@@ -1571,6 +1686,9 @@ static struct target_type verity_target = {
 	.prepare_ioctl	= verity_prepare_ioctl,
 	.iterate_devices = verity_iterate_devices,
 	.io_hints	= verity_io_hints,
+#ifdef CONFIG_SECURITY
+	.preresume	= verity_preresume,
+#endif /* CONFIG_SECURITY */
 };
 module_dm(verity);
 

drivers/md/dm-verity.h

@@ -45,6 +45,10 @@ struct dm_verity {
 	u8 *salt;		/* salt: its size is salt_size */
 	u8 *initial_hashstate;	/* salted initial state, if shash_tfm is set */
 	u8 *zero_digest;	/* digest for a zero block */
+#ifdef CONFIG_SECURITY
+	u8 *root_digest_sig;	/* signature of the root digest */
+	unsigned int sig_size;	/* root digest signature size */
+#endif /* CONFIG_SECURITY */
 	unsigned int salt_size;
 	sector_t data_start;	/* data offset in 512-byte sectors */
 	sector_t hash_start;	/* hash start in blocks */

include/linux/security.h

@@ -83,8 +83,15 @@ enum lsm_event {
 	LSM_POLICY_CHANGE,
 };
 
+struct dm_verity_digest {
+	const char *alg;
+	const u8 *digest;
+	size_t digest_len;
+};
+
 enum lsm_integrity_type {
-	__LSM_INT_MAX
+	LSM_INT_DMVERITY_SIG_VALID,
+	LSM_INT_DMVERITY_ROOTHASH,
 };
 
 /*