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@xxxxxxxxxx>
Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
---
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
