[PATCH 11/12] IMA: turn ima_policy_flags into __wr_after_init

Igor Stoppa
2018-12-21T18:14:22Z

The policy flags could be targeted by an attacker aiming at disabling IMA,
so that there would be no trace of a file system modification in the
measurement list.

Since the flags can be altered at runtime, it is not possible to make
them become fully read-only, for example with __ro_after_init.

__wr_after_init can still provide some protection, at least against
simple memory overwrite attacks

Signed-off-by: Igor Stoppa <igor.stoppa@huawei.com>

CC: Andy Lutomirski <luto@amacapital.net>
CC: Nadav Amit <nadav.amit@gmail.com>
CC: Matthew Wilcox <willy@infradead.org>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Kees Cook <keescook@chromium.org>
CC: Dave Hansen <dave.hansen@linux.intel.com>
CC: Mimi Zohar <zohar@linux.vnet.ibm.com>
CC: Thiago Jung Bauermann <bauerman@linux.ibm.com>
CC: Ahmed Soliman <ahmedsoliman@mena.vt.edu>
CC: linux-integrity@vger.kernel.org
CC: kernel-hardening@lists.openwall.com
CC: linux-mm@kvack.org
CC: linux-kernel@vger.kernel.org
---
security/integrity/ima/ima.h | 3 ++-
security/integrity/ima/ima_policy.c | 9 +++++----
2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index cc12f3449a72..297c25f5122e 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -24,6 +24,7 @@
#include <linux/hash.h>
#include <linux/tpm.h>
#include <linux/audit.h>
+#include <linux/prmem.h>
#include <crypto/hash_info.h>

#include "../integrity.h"
@@ -50,7 +51,7 @@ enum tpm_pcrs { TPM_PCR0 = 0, TPM_PCR8 = 8 };
#define IMA_TEMPLATE_IMA_FMT "d|n"

/* current content of the policy */
-extern int ima_policy_flag;
+extern int ima_policy_flag __wr_after_init;

/* set during initialization */
extern int ima_hash_algo;
diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index 7489cb7de6dc..2004de818d92 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -47,7 +47,7 @@
#define INVALID_PCR(a) (((a) < 0) || \
(a) >= (FIELD_SIZEOF(struct integrity_iint_cache, measured_pcrs) * 8))

-int ima_policy_flag;
+int ima_policy_flag __wr_after_init;
static int temp_ima_appraise;
static int build_ima_appraise __ro_after_init;

@@ -452,12 +452,13 @@ void ima_update_policy_flag(void)

list_for_each_entry(entry, ima_rules, list) {
if (entry->action & IMA_DO_MASK)
- ima_policy_flag |= entry->action;
+ wr_assign(ima_policy_flag,
+ ima_policy_flag | entry->action);
}

ima_appraise |= (build_ima_appraise | temp_ima_appraise);
if (!ima_appraise)
- ima_policy_flag &= ~IMA_APPRAISE;
+ wr_assign(ima_policy_flag, ima_policy_flag & ~IMA_APPRAISE);
}

static int ima_appraise_flag(enum ima_hooks func)
@@ -574,7 +575,7 @@ void ima_update_policy(void)
list_splice_tail_init_rcu(&ima_temp_rules, policy, synchronize_rcu);

if (ima_rules != policy) {
- ima_policy_flag = 0;
+ wr_assign(ima_policy_flag, 0);
ima_rules = policy;
}
ima_update_policy_flag();
--
2.19.1