[RFC PATCH] ima: instantiate the bprm_creds_for_exec() hook

[Mimi Zohar <zohar@linux.ibm.com>]

Like direct file execution (e.g. ./script.sh), indirect file exection
(e.g. sh script.sh) need to be measured and appraised.  Instantiate
the new security_bprm_creds_for_exec() hook to measure and verify the
indirect file's integrity.  Unlike direct file execution, indirect file
execution is optionally enforced by the interpreter.

Define two new audit messages:
- Userspace-enforcing-IMA-signature-required
- Userspace-not-enforcing-IMA-signature-required

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
---
 security/integrity/ima/ima_appraise.c | 24 +++++++++++++++++++++++-
 security/integrity/ima/ima_main.c     | 22 ++++++++++++++++++++++
 2 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index 656c709b974f..5a3b5cdecb51 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -8,6 +8,7 @@
 #include <linux/module.h>
 #include <linux/init.h>
 #include <linux/file.h>
+#include <linux/binfmts.h>
 #include <linux/fs.h>
 #include <linux/xattr.h>
 #include <linux/magic.h>
@@ -16,6 +17,7 @@
 #include <linux/fsverity.h>
 #include <keys/system_keyring.h>
 #include <uapi/linux/fsverity.h>
+#include <linux/securebits.h>
 
 #include "ima.h"
 
@@ -469,6 +471,26 @@ int ima_check_blacklist(struct ima_iint_cache *iint,
 	return rc;
 }
 
+static int is_bprm_creds_for_exec(enum ima_hooks func, struct file *file,
+				  const char **cause)
+{
+	const struct cred *cred = current_cred();
+	struct linux_binprm *bprm = NULL;
+
+	if (func == BPRM_CHECK) {
+		bprm = container_of(&file, struct linux_binprm, file);
+		if (!bprm->is_check)
+			return 0;
+
+		if (cred->securebits & SECBIT_EXEC_RESTRICT_FILE)
+			*cause = "Userspace-enforcing-IMA-signature-required";
+		else
+			*cause = "Userspace-not-enforcing-IMA-signature-required";
+		return 1;
+	}
+	return 0;
+}
+
 /*
  * ima_appraise_measurement - appraise file measurement
  *
@@ -502,7 +524,7 @@ int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
 		if (iint->flags & IMA_DIGSIG_REQUIRED) {
 			if (iint->flags & IMA_VERITY_REQUIRED)
 				cause = "verity-signature-required";
-			else
+			else if (!is_bprm_creds_for_exec(func, file, &cause))
 				cause = "IMA-signature-required";
 		} else {
 			cause = "missing-hash";
diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index 06132cf47016..2b5d6bae77a4 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -554,6 +554,27 @@ static int ima_bprm_check(struct linux_binprm *bprm)
 				   MAY_EXEC, CREDS_CHECK);
 }
 
+/**
+ * ima_bprm_creds_for_exec - based on policy, collect/store/appraise measurement.
+ * @bprm: contains the linux_binprm structure
+ *
+ * Based on the IMA policy and the execvat(2) AT_CHECK flag, measure and
+ * appraise the integrity of a file to be executed by script interpreters.
+ * Unlike any of the other LSM hooks where the kernel enforces file integrity,
+ * enforcing file integrity is left up to the discretion of the script
+ * interpreter (userspace).
+ *
+ * On success return 0.  On integrity appraisal error, assuming the file
+ * is in policy and IMA-appraisal is in enforcing mode, return -EACCES.
+ */
+static int ima_bprm_creds_for_exec(struct linux_binprm *bprm)
+{
+	if (!bprm->is_check)
+		return 0;
+
+	return ima_bprm_check(bprm);
+}
+
 /**
  * ima_file_check - based on policy, collect/store measurement.
  * @file: pointer to the file to be measured
@@ -1177,6 +1198,7 @@ static int __init init_ima(void)
 
 static struct security_hook_list ima_hooks[] __ro_after_init = {
 	LSM_HOOK_INIT(bprm_check_security, ima_bprm_check),
+	LSM_HOOK_INIT(bprm_creds_for_exec, ima_bprm_creds_for_exec),
 	LSM_HOOK_INIT(file_post_open, ima_file_check),
 	LSM_HOOK_INIT(inode_post_create_tmpfile, ima_post_create_tmpfile),
 	LSM_HOOK_INIT(file_release, ima_file_free),
-- 
2.47.0