Re: [PATCH v3 4/4] IMA: introduce a new policy option func=SETXATTR_CHECK

From: THOBY Simon <Simon.THOBY@viveris.fr>
To: "zohar@linux.ibm.com" <zohar@linux.ibm.com>,
	"dmitry.kasatkin@gmail.com" <dmitry.kasatkin@gmail.com>,
	"linux-integrity@vger.kernel.org"
	<linux-integrity@vger.kernel.org>,
	BARVAUX Didier <Didier.BARVAUX@viveris.fr>
Subject: Re: [PATCH v3 4/4] IMA: introduce a new policy option func=SETXATTR_CHECK
Date: Tue, 27 Jul 2021 11:24:08 +0000	[thread overview]
Message-ID: <4248e017-0e83-bba4-6694-a321a7d38640@viveris.fr> (raw)
In-Reply-To: <20210727102307.552052-5-simon.thoby@viveris.fr>

Sorry, I forgot to run checkpatch on this one prior to sending :/
There was a few style issues in this patch, so here is the fixed version:

This patch defines a new value for the ima policy option 'func'.
That value restricts the hash algorithms accepted when writing the
security.ima xattr.

When a policy contains a rule of the form
	appraise func=SETXATTR_CHECK appraise_hash=sha256,sha384,sha512
only values corresponding to one of these three digest algorithms
will be accepted for writing the security.ima xattr.
Attempting to write the attribute using another algorithm (or "free-form"
data) will be denied with an audit log message.
In the absence of such a policy rule, the default is still to only
accept hash algorithms built in the kernel (with all the limitations
that entails).

On policy update, the latest SETXATTR_CHECK rule is the only one
that apply, and other SETXATTR_CHECK rules are deleted.

Signed-off-by: Simon Thoby <simon.thoby@viveris.fr>
---
 Documentation/ABI/testing/ima_policy  |  9 +++-
 security/integrity/ima/ima.h          |  4 ++
 security/integrity/ima/ima_appraise.c | 28 +++++++++---
 security/integrity/ima/ima_policy.c   | 64 +++++++++++++++++++++++++++
 4 files changed, 99 insertions(+), 6 deletions(-)

diff --git a/Documentation/ABI/testing/ima_policy b/Documentation/ABI/testing/ima_policy
index 365e4c91719e..c05a21007272 100644
--- a/Documentation/ABI/testing/ima_policy
+++ b/Documentation/ABI/testing/ima_policy
@@ -30,9 +30,10 @@ Description:
 				[appraise_flag=] [keyrings=] [appraise_hash=]
 		  base:
 			func:= [BPRM_CHECK][MMAP_CHECK][CREDS_CHECK][FILE_CHECK][MODULE_CHECK]
-			        [FIRMWARE_CHECK]
+				[FIRMWARE_CHECK]
 				[KEXEC_KERNEL_CHECK] [KEXEC_INITRAMFS_CHECK]
 				[KEXEC_CMDLINE] [KEY_CHECK] [CRITICAL_DATA]
+				[SETXATTR_CHECK]
 			mask:= [[^]MAY_READ] [[^]MAY_WRITE] [[^]MAY_APPEND]
 			       [[^]MAY_EXEC]
 			fsmagic:= hex value
@@ -138,3 +139,9 @@ Description:
 		keys added to .builtin_trusted_keys or .ima keyring:
 
 			measure func=KEY_CHECK keyrings=.builtin_trusted_keys|.ima
+
+		Example of the special SETXATTR_CHECK appraise rule, that
+		restricts the hash algorithms allowed when writing to the
+		security.ima xattr of a file:
+
+			appraise func=SETXATTR_CHECK appraise_hash=sha256,sha384,sha512
diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index 7ef1b214d358..aeb3bf30c0f9 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -46,6 +46,9 @@ enum tpm_pcrs { TPM_PCR0 = 0, TPM_PCR8 = 8, TPM_PCR10 = 10 };
 /* current content of the policy */
 extern int ima_policy_flag;
 
+/* bitset of digests algorithms allowed in the setxattr hook */
+extern atomic_t ima_setxattr_allowed_hash_algorithms;
+
 /* set during initialization */
 extern int ima_hash_algo __ro_after_init;
 extern int ima_sha1_idx __ro_after_init;
@@ -198,6 +201,7 @@ static inline unsigned int ima_hash_key(u8 *digest)
 	hook(KEXEC_CMDLINE, kexec_cmdline)		\
 	hook(KEY_CHECK, key)				\
 	hook(CRITICAL_DATA, critical_data)		\
+	hook(SETXATTR_CHECK, setxattr_check)		\
 	hook(MAX_CHECK, none)
 
 #define __ima_hook_enumify(ENUM, str)	ENUM,
diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index 6d121819ae9e..f3d52bbfdf0f 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -590,14 +590,32 @@ int ima_setxattr_validate_hash_alg(struct dentry *dentry,
 {
 	int res = -EACCES;
 	char *path = NULL, *pathbuf = NULL;
+	const char *errmsg = "unavailable-hash-algorithm";
 	enum hash_algo hash_alg =
 		ima_get_hash_algo((struct evm_ima_xattr_data *)xattr_value,
 				  xattr_value_len);
+	unsigned int allowed_hashes = atomic_read(
+			&ima_setxattr_allowed_hash_algorithms);
 
-	/* disallow xattr writes with algorithms not built in the kernel */
-	if (likely(hash_alg == ima_hash_algo
-	    || crypto_has_alg(hash_algo_name[hash_alg], 0, 0)))
-		return 0;
+	if (allowed_hashes) {
+		/* success if the algorithm is whitelisted in the ima policy */
+		if (allowed_hashes & (1U << hash_alg))
+			return 0;
+
+		/*
+		 * We use a different audit message when the hash algorithm
+		 * is denied by a policy rule, instead of not being built
+		 * in the kernel image
+		 */
+		errmsg = "denied-hash-algorithm";
+	} else {
+		if (likely(hash_alg == ima_hash_algo))
+			return 0;
+
+		/* allow any xattr using an algorithm built in the kernel */
+		if (crypto_has_alg(hash_algo_name[hash_alg], 0, 0))
+			return 0;
+	}
 
 	pathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
 	/* no memory available ? no file path for you */
@@ -605,7 +623,7 @@ int ima_setxattr_validate_hash_alg(struct dentry *dentry,
 		path = dentry_path(dentry, pathbuf, PATH_MAX);
 
 	integrity_audit_msg(AUDIT_INTEGRITY_DATA, d_inode(dentry),
-		path, "collect_data", "unavailable-hash-algorithm", res, 0);
+		path, "collect_data", errmsg, res, 0);
 
 	kfree(pathbuf);
 
diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index a7f110cbbff0..cfebf8b01cc0 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -53,6 +53,8 @@ int ima_policy_flag;
 static int temp_ima_appraise;
 static int build_ima_appraise __ro_after_init;
 
+atomic_t ima_setxattr_allowed_hash_algorithms;
+
 #define MAX_LSM_RULES 6
 enum lsm_rule_types { LSM_OBJ_USER, LSM_OBJ_ROLE, LSM_OBJ_TYPE,
 	LSM_SUBJ_USER, LSM_SUBJ_ROLE, LSM_SUBJ_TYPE
@@ -915,6 +917,50 @@ int ima_check_policy(void)
 	return 0;
 }
 
+/** ima_update_setxattr_allowed_hash_algorithms - cleanup SETXATTR_CHECK rules
+ * in the new ruleset
+ *
+ * Called when updating the IMA policy. Delete non-applicable rules with
+ * 'func' set to SETXATTR_CHECK and update the atomic variable to hold
+ * the list of allowed hash algorithms for the security.ima xattr.
+ *
+ * SETXATTR_CHECK rules do not implement a full policy check because of
+ * the performance impact performing rules checking on setxattr() would
+ * have. The consequence is that only one SETXATTR_CHECK can be active at
+ * a time. To prevent confusion, on policy updates, if a new SETXATTR_CHECK
+ * is defined, other SETXATTR_CHECK rules are remove from the ruleset.
+ */
+void ima_update_setxattr_allowed_hash_algorithms(struct list_head *policy)
+{
+	struct ima_rule_entry *entry, *tmp;
+	bool setxattr_check_already_defined = false;
+
+	list_for_each_entry_safe_reverse(entry, tmp, policy, list) {
+		if (entry->func != SETXATTR_CHECK)
+			continue;
+
+		if (setxattr_check_already_defined) {
+			/*
+			 * delete old SETXATTR_CHECK entries when a newer
+			 * one already exists
+			 */
+			list_del(&entry->list);
+			ima_free_rule(entry);
+		} else {
+			/*
+			 * only the last entry with the SETXATTR_CHECK func
+			 * apply: this allows runtime upgrades of the
+			 * digest algorithm policy, unlike the other IMA
+			 * rules
+			 */
+			atomic_xchg(&ima_setxattr_allowed_hash_algorithms,
+				    entry->allowed_hashes);
+			setxattr_check_already_defined = true;
+		}
+	}
+
+}
+
 /**
  * ima_update_policy - update default_rules with new measure rules
  *
@@ -932,9 +978,12 @@ void ima_update_policy(void)
 
 	list_splice_tail_init_rcu(&ima_temp_rules, policy, synchronize_rcu);
 
+	ima_update_setxattr_allowed_hash_algorithms(policy);
+
 	if (ima_rules != policy) {
 		ima_policy_flag = 0;
 		ima_rules = policy;
+		atomic_xchg(&ima_setxattr_allowed_hash_algorithms, 0);
 
 		/*
 		 * IMA architecture specific policy rules are specified
@@ -1176,6 +1225,19 @@ static bool ima_validate_rule(struct ima_rule_entry *entry)
 		if (ima_rule_contains_lsm_cond(entry))
 			return false;
 
+		break;
+	case SETXATTR_CHECK:
+		/* any action other than APPRAISE is unsupported */
+		if (entry->action != APPRAISE)
+			return false;
+
+		/*
+		 * full policies are not supported, they would have too
+		 * much of a performance impact
+		 */
+		if (entry->flags & ~(IMA_FUNC | IMA_VALIDATE_HASH))
+			return false;
+
 		break;
 	default:
 		return false;
@@ -1332,6 +1394,8 @@ static int ima_parse_rule(char *rule, struct ima_rule_entry *entry)
 				entry->func = KEY_CHECK;
 			else if (strcmp(args[0].from, "CRITICAL_DATA") == 0)
 				entry->func = CRITICAL_DATA;
+			else if (strcmp(args[0].from, "SETXATTR_CHECK") == 0)
+				entry->func = SETXATTR_CHECK;
 			else
 				result = -EINVAL;
 			if (!result)
-- 
2.31.1
