linux-kernel - [PATCH v2] security: Introduce LSM_ORDER_LAST and set it for the integrity LSM

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Message-Id: <20230308171119.1784326-1-roberto.sassu@huaweicloud.com>
Date:   Wed,  8 Mar 2023 18:11:18 +0100
From:   Roberto Sassu <roberto.sassu@...weicloud.com>
To:     zohar@...ux.ibm.com, dmitry.kasatkin@...il.com,
        paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com
Cc:     linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, keescook@...omium.org,
        Roberto Sassu <roberto.sassu@...wei.com>
Subject: [PATCH v2] security: Introduce LSM_ORDER_LAST and set it for the integrity LSM

From: Roberto Sassu <roberto.sassu@...wei.com>

Introduce LSM_ORDER_LAST, to satisfy the requirement of LSMs willing to be
the last, e.g. the 'integrity' LSM, without changing the kernel command
line or configuration.

Also, set this order for the 'integrity' LSM. While not enforced, this is
the only LSM expected to use it.

Similarly to LSM_ORDER_FIRST, LSMs with LSM_ORDER_LAST are always enabled
and put at the end of the LSM list.

Finally, for LSM_ORDER_MUTABLE LSMs, set the found variable to true if an
LSM is found, regardless of its order. In this way, the kernel would not
wrongly report that the LSM is not built-in in the kernel if its order is
LSM_ORDER_LAST.

Fixes: 79f7865d844c ("LSM: Introduce "lsm=" for boottime LSM selection")
Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
---

Changelog

v1:
- Add comment for LSM_ORDER_LAST definition (suggested by Mimi)
- Add Fixes tag (suggested by Mimi)
- Do minor corrections in the commit messages (suggested by Mimi and Stefan)

 include/linux/lsm_hooks.h |  1 +
 security/integrity/iint.c |  1 +
 security/security.c       | 12 +++++++++---
 3 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 6e156d2acff..c55761d93a2 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -1716,6 +1716,7 @@ extern void security_add_hooks(struct security_hook_list *hooks, int count,
 enum lsm_order {
 	LSM_ORDER_FIRST = -1,	/* This is only for capabilities. */
 	LSM_ORDER_MUTABLE = 0,
+	LSM_ORDER_LAST = 1,	/* This is only for integrity. */
 };
 
 struct lsm_info {
diff --git a/security/integrity/iint.c b/security/integrity/iint.c
index 8638976f799..b97eb59e0e3 100644
--- a/security/integrity/iint.c
+++ b/security/integrity/iint.c
@@ -182,6 +182,7 @@ static int __init integrity_iintcache_init(void)
 DEFINE_LSM(integrity) = {
 	.name = "integrity",
 	.init = integrity_iintcache_init,
+	.order = LSM_ORDER_LAST,
 };
 
 
diff --git a/security/security.c b/security/security.c
index cf6cc576736..2f36229d5b6 100644
--- a/security/security.c
+++ b/security/security.c
@@ -284,9 +284,9 @@ static void __init ordered_lsm_parse(const char *order, const char *origin)
 		bool found = false;
 
 		for (lsm = __start_lsm_info; lsm < __end_lsm_info; lsm++) {
-			if (lsm->order == LSM_ORDER_MUTABLE &&
-			    strcmp(lsm->name, name) == 0) {
-				append_ordered_lsm(lsm, origin);
+			if (strcmp(lsm->name, name) == 0) {
+				if (lsm->order == LSM_ORDER_MUTABLE)
+					append_ordered_lsm(lsm, origin);
 				found = true;
 			}
 		}
@@ -306,6 +306,12 @@ static void __init ordered_lsm_parse(const char *order, const char *origin)
 		}
 	}
 
+	/* LSM_ORDER_LAST is always last. */
+	for (lsm = __start_lsm_info; lsm < __end_lsm_info; lsm++) {
+		if (lsm->order == LSM_ORDER_LAST)
+			append_ordered_lsm(lsm, "   last");
+	}
+
 	/* Disable all LSMs not in the ordered list. */
 	for (lsm = __start_lsm_info; lsm < __end_lsm_info; lsm++) {
 		if (exists_ordered_lsm(lsm))
-- 
2.25.1