[RFC][PATCH 1/2] ima: preserve the integrity of appraised files

[Roberto Sassu <roberto.sassu@huawei.com>]

The existing 'ima_appraise_tcb' policy aims at protecting the integrity
of files owned by root against offline attacks, while LSMs should decide
if those files can be modified, and new files can be created. However,
LSMs cannot take this decision independently, if IMA appraises only
a subset of files that a process is allowed to access. A process can
become compromised due to reading files not appraised by IMA.

To avoid this issue, the IMA policy should contain as criteria process
credentials rather than files attributes. Then, when a process matches
those criteria, files will be always appraised by IMA, regardless of
file attributes.

The IMA policy with process credentials will define which process belongs
to the TCB and which not. With this information, IMA would be be able
to preserve the integrity of appraised files, without an LSM, for example
by denying writes by processes outside the TCB (Biba strict policy).

This patch adds support for enforcing the Biba strict policy. More
policies will be introduced later. Enforcement will be enabled by
adding 'ima_integrity_policy=biba-strict' to the kernel command line.

To enforce this policy, it is necessary to upload to IMA a new policy
which defines the subjects part of the TCB. For example, the rule
'appraise fowner=0' could be replaced with two rules: 'appraise uid=0'
and 'appraise euid=0'.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  4 ++
 security/integrity/ima/ima.h                    | 23 ++++++++++
 security/integrity/ima/ima_appraise.c           | 61 +++++++++++++++++++++++++
 security/integrity/ima/ima_main.c               | 25 ++++++----
 4 files changed, 104 insertions(+), 9 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 05496622b4ef..37810c6a3b28 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1532,6 +1532,10 @@
 	                [IMA] Define a custom template format.
 			Format: { "field1|...|fieldN" }
 
+	ima_integrity_policy=
+			[IMA] Select an integrity policy to enforce.
+			Policies: { "biba-strict" }
+
 	ima.ahash_minsize= [IMA] Minimum file size for asynchronous hash usage
 			Format: <min_file_size>
 			Set the minimal file size for using asynchronous hash.
diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index d52b487ad259..377e1d8c2afb 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -36,6 +36,8 @@ enum ima_show_type { IMA_SHOW_BINARY, IMA_SHOW_BINARY_NO_FIELD_LEN,
 		     IMA_SHOW_BINARY_OLD_STRING_FMT, IMA_SHOW_ASCII };
 enum tpm_pcrs { TPM_PCR0 = 0, TPM_PCR8 = 8 };
 
+enum integrity_policies { BIBA_STRICT = 1, INTEGRITY_POLICY__LAST };
+
 /* digest size for IMA, fits SHA1 or MD5 */
 #define IMA_DIGEST_SIZE		SHA1_DIGEST_SIZE
 #define IMA_EVENT_NAME_LEN_MAX	255
@@ -57,6 +59,7 @@ extern int ima_initialized;
 extern int ima_used_chip;
 extern int ima_hash_algo;
 extern int ima_appraise;
+extern int ima_integrity_policy;
 
 /* IMA event related data */
 struct ima_event_data {
@@ -247,6 +250,11 @@ enum hash_algo ima_get_hash_algo(struct evm_ima_xattr_data *xattr_value,
 				 int xattr_len);
 int ima_read_xattr(struct dentry *dentry,
 		   struct evm_ima_xattr_data **xattr_value);
+bool ima_inode_is_appraised(struct dentry *dentry, struct inode *inode);
+bool ima_appraise_biba_check(struct file *file,
+			     struct integrity_iint_cache *iint,
+			     int must_appraise, char **pathbuf,
+			     const char **pathname, char *namebuf);
 
 #else
 static inline int ima_appraise_measurement(enum ima_hooks func,
@@ -289,6 +297,21 @@ static inline int ima_read_xattr(struct dentry *dentry,
 	return 0;
 }
 
+static inline bool ima_inode_is_appraised(struct dentry *dentry,
+					  struct inode *inode);
+{
+	return false;
+}
+
+static inline bool ima_appraise_biba_check(struct file *file,
+					   struct integrity_iint_cache *iint,
+					   int must_appraise, char **pathbuf,
+					   const char **pathname,
+					   char *namebuf)
+{
+	return false;
+}
+
 #endif /* CONFIG_IMA_APPRAISE */
 
 /* LSM based policy rules require audit */
diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index 809ba70fbbbf..c24824ef64c4 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -18,6 +18,10 @@
 
 #include "ima.h"
 
+static char *ima_integrity_policies_str[INTEGRITY_POLICY__LAST] = {
+	[BIBA_STRICT] = "biba-strict",
+};
+
 static int __init default_appraise_setup(char *str)
 {
 #ifdef CONFIG_IMA_APPRAISE_BOOTPARAM
@@ -33,6 +37,15 @@ static int __init default_appraise_setup(char *str)
 
 __setup("ima_appraise=", default_appraise_setup);
 
+static int __init integrity_policy_setup(char *str)
+{
+	if (strcmp(str, ima_integrity_policies_str[BIBA_STRICT]) == 0)
+		ima_integrity_policy = BIBA_STRICT;
+
+	return 1;
+}
+__setup("ima_integrity_policy=", integrity_policy_setup);
+
 /*
  * is_ima_appraise_enabled - return appraise status
  *
@@ -189,6 +202,54 @@ int ima_read_xattr(struct dentry *dentry,
 	return ret;
 }
 
+bool ima_inode_is_appraised(struct dentry *dentry, struct inode *inode)
+{
+	ssize_t len;
+
+	len =  __vfs_getxattr(dentry, inode, XATTR_NAME_IMA, NULL, 0);
+
+	return (len > 0 && len >= sizeof(struct evm_ima_xattr_data));
+}
+
+/*
+ * ima_appraise_biba_check - detect violations of a Biba policy
+ *
+ * The appraisal policy identifies which subjects belong to the TCB. Files
+ * with a valid digital signature or HMAC are also part of the TCB. This
+ * function detects attempts to write appraised files by subjects outside
+ * the TCB. The Biba strict policy denies this operation.
+ *
+ * Return: true if current operation violates a Biba policy, false otherwise
+ */
+bool ima_appraise_biba_check(struct file *file,
+			     struct integrity_iint_cache *iint,
+			     int must_appraise, char **pathbuf,
+			     const char **pathname, char *namebuf)
+{
+	struct inode *inode = file_inode(file);
+	fmode_t mode = file->f_mode;
+	char *cause = "write_down";
+
+	/* check write up, ima_appraise_measurement() checks read down */
+	if (!must_appraise && (mode & FMODE_WRITE)) {
+		if (IS_IMA(inode)) {
+			if (!iint)
+				iint = integrity_iint_find(inode);
+			if (iint->flags & IMA_APPRAISE)
+				goto out_violation;
+		} else if (ima_inode_is_appraised(file_dentry(file), inode)) {
+			goto out_violation;
+		}
+	}
+	return false;
+out_violation:
+	*pathname = ima_d_path(&file->f_path, pathbuf, namebuf);
+	integrity_audit_msg(AUDIT_INTEGRITY_DATA, inode, *pathname,
+			    ima_integrity_policies_str[ima_integrity_policy],
+			    cause, 0, 0);
+	return true;
+}
+
 /*
  * ima_appraise_measurement - appraise file measurement
  *
diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index bb7e36e90c79..6e85ea8e2373 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -31,6 +31,7 @@ int ima_initialized;
 
 #ifdef CONFIG_IMA_APPRAISE
 int ima_appraise = IMA_APPRAISE_ENFORCE;
+int ima_integrity_policy;
 #else
 int ima_appraise;
 #endif
@@ -103,7 +104,8 @@ static void ima_rdwr_violation_check(struct file *file,
 	if (!send_tomtou && !send_writers)
 		return;
 
-	*pathname = ima_d_path(&file->f_path, pathbuf, filename);
+	if (!*pathname)
+		*pathname = ima_d_path(&file->f_path, pathbuf, filename);
 
 	if (send_tomtou)
 		ima_add_violation(file, *pathname, iint,
@@ -168,7 +170,7 @@ static int process_measurement(struct file *file, char *buf, loff_t size,
 	int pcr = CONFIG_IMA_MEASURE_PCR_IDX;
 	struct evm_ima_xattr_data *xattr_value = NULL;
 	int xattr_len = 0;
-	bool violation_check;
+	bool violation_check, policy_violation = false;
 	enum hash_algo hash_algo;
 
 	if (!ima_policy_flag || !S_ISREG(inode->i_mode))
@@ -181,7 +183,7 @@ static int process_measurement(struct file *file, char *buf, loff_t size,
 	action = ima_get_action(inode, mask, func, &pcr);
 	violation_check = ((func == FILE_CHECK || func == MMAP_CHECK) &&
 			   (ima_policy_flag & IMA_MEASURE));
-	if (!action && !violation_check)
+	if (!action && !violation_check && !ima_integrity_policy)
 		return 0;
 
 	must_appraise = action & IMA_APPRAISE;
@@ -198,13 +200,16 @@ static int process_measurement(struct file *file, char *buf, loff_t size,
 			goto out;
 	}
 
-	if (violation_check) {
+	if (ima_integrity_policy)
+		policy_violation = ima_appraise_biba_check(file, iint,
+						must_appraise, &pathbuf,
+						&pathname, filename);
+	if (violation_check)
 		ima_rdwr_violation_check(file, iint, action & IMA_MEASURE,
 					 &pathbuf, &pathname);
-		if (!action) {
-			rc = 0;
-			goto out_free;
-		}
+	if (!action) {
+		rc = 0;
+		goto out_free;
 	}
 
 	/* Determine if already appraised/measured based on bitmask
@@ -260,7 +265,9 @@ static int process_measurement(struct file *file, char *buf, loff_t size,
 		__putname(pathbuf);
 out:
 	inode_unlock(inode);
-	if ((rc && must_appraise) && (ima_appraise & IMA_APPRAISE_ENFORCE))
+	if (((rc && must_appraise) ||
+	    (ima_integrity_policy && policy_violation)) &&
+	    (ima_appraise & IMA_APPRAISE_ENFORCE))
 		return -EACCES;
 	return 0;
 }
-- 
2.11.0