[PATCH v2 5/6] ima: support fs-verity file digest based signatures

From: Mimi Zohar <zohar@linux.ibm.com>
To: linux-integrity@vger.kernel.org
Cc: Mimi Zohar <zohar@linux.ibm.com>,
	Eric Biggers <ebiggers@kernel.org>,
	linux-fscrypt@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH v2 5/6] ima: support fs-verity file digest based signatures
Date: Sun,  9 Jan 2022 13:55:16 -0500	[thread overview]
Message-ID: <20220109185517.312280-6-zohar@linux.ibm.com> (raw)
In-Reply-To: <20220109185517.312280-1-zohar@linux.ibm.com>

Instead of calculating a file hash and verifying the signature stored
in the security.ima xattr against the calculated file hash, verify the
signature based on a hash of fs-verity's file digest and the digest's
metadata.

To differentiate between a regular file hash and an fs-verity file digest
based signature stored as security.ima xattr, define a new signature type
named IMA_VERITY_DIGSIG.

The hash format of fs-verity's file digest and the digest's metadata to
be signed is defined as a structure named "ima_tbs_hash".

Update the 'ima-sig' template field to display the new fs-verity signature
type as well.

For example:
  appraise func=BPRM_CHECK digest_type=hash|verity

Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
---
 Documentation/ABI/testing/ima_policy      | 10 +++
 Documentation/security/IMA-templates.rst  |  4 +-
 include/uapi/linux/ima.h                  | 26 ++++++++
 security/integrity/ima/ima_appraise.c     | 81 +++++++++++++++++++++++
 security/integrity/ima/ima_template_lib.c |  3 +-
 security/integrity/integrity.h            |  1 +
 6 files changed, 122 insertions(+), 3 deletions(-)
 create mode 100644 include/uapi/linux/ima.h

diff --git a/Documentation/ABI/testing/ima_policy b/Documentation/ABI/testing/ima_policy
index 444bb7ccbe03..fadf90dde289 100644
--- a/Documentation/ABI/testing/ima_policy
+++ b/Documentation/ABI/testing/ima_policy
@@ -151,6 +151,16 @@ Description:
 
 			appraise func=SETXATTR_CHECK appraise_algos=sha256,sha384,sha512
 
+		Example of measure and appraise rules allowing fs-verity
+		signed digests on a particular filesystem identified by
+		it's fsuuid:
+
+			measure func=BPRM_CHECK digest_type=hash|verity \
+				fsuuid=... template=ima-sig
+			appraise func=BPRM_CHECK digest_type=hash|verity \
+				fsuuid=...
+
+
 		Example of measure rule allowing fs-verity's digests on a
 		particular filesystem with indication of type of digest.
 
diff --git a/Documentation/security/IMA-templates.rst b/Documentation/security/IMA-templates.rst
index 5e31513e8ec4..390936810ebc 100644
--- a/Documentation/security/IMA-templates.rst
+++ b/Documentation/security/IMA-templates.rst
@@ -71,8 +71,8 @@ descriptors by adding their identifier to the format string
  - 'd-modsig': the digest of the event without the appended modsig;
  - 'd-type': the type of file digest (e.g. hash, verity[1]);
  - 'n-ng': the name of the event, without size limitations;
- - 'sig': the file signature, or the EVM portable signature if the file
-   signature is not found;
+ - 'sig': the file signature, based on either the file's/fsverity's digest[1],
+   or the EVM portable signature if the file signature is not found;
  - 'modsig' the appended file signature;
  - 'buf': the buffer data that was used to generate the hash without size limitations;
  - 'evmsig': the EVM portable signature;
diff --git a/include/uapi/linux/ima.h b/include/uapi/linux/ima.h
new file mode 100644
index 000000000000..6a2a68fc0fad
--- /dev/null
+++ b/include/uapi/linux/ima.h
@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0-only WITH Linux-syscall-note */
+/*
+ * IMA user API
+ *
+ */
+#ifndef _UAPI_LINUX_IMA_H
+#define _UAPI_LINUX_IMA_H
+
+#include <linux/types.h>
+
+/*
+ * The hash format of fs-verity's file digest and other file metadata
+ * to be signed.  The resulting signature is stored as a security.ima
+ * xattr.
+ *
+ * "type" is defined as IMA_VERITY_DIGSIG
+ * "algo" is the hash_algo enum of fs-verity's file digest
+ * (e.g. HASH_ALGO_SHA256, HASH_ALGO_SHA512).
+ */
+struct ima_tbs_hash {
+	__u8 type;        /* xattr type [enum evm_ima_xattr_type] */
+	__u8 algo;        /* Digest algorithm [enum hash_algo] */
+	__u8 digest[];    /* fs-verity digest */
+};
+
+#endif /* _UAPI_LINUX_IMA_H */
diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index dbba51583e7c..4e092c189ed0 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -13,7 +13,10 @@
 #include <linux/magic.h>
 #include <linux/ima.h>
 #include <linux/evm.h>
+#include <linux/fsverity.h>
 #include <keys/system_keyring.h>
+#include <uapi/linux/fsverity.h>
+#include <uapi/linux/ima.h>
 
 #include "ima.h"
 
@@ -183,6 +186,8 @@ enum hash_algo ima_get_hash_algo(const struct evm_ima_xattr_data *xattr_value,
 		return ima_hash_algo;
 
 	switch (xattr_value->type) {
+	case IMA_VERITY_DIGSIG:
+		fallthrough;
 	case EVM_IMA_XATTR_DIGSIG:
 		sig = (typeof(sig))xattr_value;
 		if (sig->version != 2 || xattr_len <= sizeof(*sig)
@@ -225,6 +230,47 @@ int ima_read_xattr(struct dentry *dentry,
 	return ret;
 }
 
+/*
+ * calc_tbs_hash - calculate hash of a digest and digest metadata
+ * @type: signature type [IMA_VERITY_DIGSIG]
+ * @algo: hash algorithm [enum hash_algo]
+ * @digest: pointer to the digest to be hashed
+ * @hash: (out) pointer to the hash
+ *
+ * The IMA signature is a signature over the hash of fs-verity's file digest
+ * with other digest metadata, not just fs-verity's file digest. Calculate
+ * the to be signed hash.
+ *
+ * Return 0 on success, error code otherwise.
+ */
+static int calc_tbs_hash(enum evm_ima_xattr_type xattr_type,
+			 enum hash_algo algo, const u8 *digest,
+			 struct ima_digest_data *hash)
+{
+	struct ima_tbs_hash *tbs_h;
+	int rc = 0;
+
+	if (xattr_type != IMA_VERITY_DIGSIG)
+		return -EINVAL;
+
+	tbs_h = kzalloc(sizeof(*tbs_h) + hash_digest_size[algo], GFP_KERNEL);
+	if (!tbs_h)
+		return -ENOMEM;
+
+	tbs_h->type = xattr_type;
+	tbs_h->algo = algo;
+	memcpy(tbs_h->digest, digest, hash_digest_size[algo]);
+
+	hash->algo = algo;
+	hash->length = hash_digest_size[algo];
+
+	rc = ima_calc_buffer_hash(tbs_h,
+				  sizeof(*tbs_h) + hash_digest_size[algo],
+				  hash);
+	kfree(tbs_h);
+	return rc;
+}
+
 /*
  * xattr_verify - verify xattr digest or signature
  *
@@ -236,7 +282,9 @@ static int xattr_verify(enum ima_hooks func, struct integrity_iint_cache *iint,
 			struct evm_ima_xattr_data *xattr_value, int xattr_len,
 			enum integrity_status *status, const char **cause)
 {
+	struct ima_digest_data *hash = NULL;
 	int rc = -EINVAL, hash_start = 0;
+	u8 algo;  /* Digest algorithm [enum hash_algo] */
 
 	switch (xattr_value->type) {
 	case IMA_XATTR_DIGEST_NG:
@@ -271,6 +319,38 @@ static int xattr_verify(enum ima_hooks func, struct integrity_iint_cache *iint,
 			break;
 		}
 		*status = INTEGRITY_PASS;
+		break;
+	case IMA_VERITY_DIGSIG:
+		set_bit(IMA_DIGSIG, &iint->atomic_flags);
+
+		algo = iint->ima_hash->algo;
+		hash = kzalloc(sizeof(*hash) + hash_digest_size[algo],
+			       GFP_KERNEL);
+		if (!hash) {
+			*cause = "verity-hashing-error";
+			*status = INTEGRITY_FAIL;
+			break;
+		}
+
+		rc = calc_tbs_hash(IMA_VERITY_DIGSIG, iint->ima_hash->algo,
+				   iint->ima_hash->digest, hash);
+		if (rc) {
+			*cause = "verity-hashing-error";
+			*status = INTEGRITY_FAIL;
+			break;
+		}
+
+		rc = integrity_digsig_verify(INTEGRITY_KEYRING_IMA,
+					     (const char *)xattr_value,
+					     xattr_len, hash->digest,
+					     hash->length);
+		if (rc) {
+			*cause = "invalid-verity-signature";
+			*status = INTEGRITY_FAIL;
+		} else {
+			*status = INTEGRITY_PASS;
+		}
+
 		break;
 	case EVM_IMA_XATTR_DIGSIG:
 		set_bit(IMA_DIGSIG, &iint->atomic_flags);
@@ -303,6 +383,7 @@ static int xattr_verify(enum ima_hooks func, struct integrity_iint_cache *iint,
 		break;
 	}
 
+	kfree(hash);
 	return rc;
 }
 
diff --git a/security/integrity/ima/ima_template_lib.c b/security/integrity/ima/ima_template_lib.c
index 1c0cea2b805f..31a14943e459 100644
--- a/security/integrity/ima/ima_template_lib.c
+++ b/security/integrity/ima/ima_template_lib.c
@@ -498,7 +498,8 @@ int ima_eventsig_init(struct ima_event_data *event_data,
 {
 	struct evm_ima_xattr_data *xattr_value = event_data->xattr_value;
 
-	if ((!xattr_value) || (xattr_value->type != EVM_IMA_XATTR_DIGSIG))
+	if ((!xattr_value) ||
+	     !(xattr_value->type & (EVM_IMA_XATTR_DIGSIG | IMA_VERITY_DIGSIG)))
 		return ima_eventevmsig_init(event_data, field_data);
 
 	return ima_write_template_field_data(xattr_value, event_data->xattr_len,
diff --git a/security/integrity/integrity.h b/security/integrity/integrity.h
index e7ac1086d1d9..51124708c072 100644
--- a/security/integrity/integrity.h
+++ b/security/integrity/integrity.h
@@ -79,6 +79,7 @@ enum evm_ima_xattr_type {
 	EVM_IMA_XATTR_DIGSIG,
 	IMA_XATTR_DIGEST_NG,
 	EVM_XATTR_PORTABLE_DIGSIG,
+	IMA_VERITY_DIGSIG,
 	IMA_XATTR_LAST
 };
 
-- 
2.27.0

