linux-kernel - [PATCH 06/12] ima: Fail rule parsing when the KEXEC_CMDLINE hook is combined with an invalid cond

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Message-Id: <20200623003236.830149-7-tyhicks@linux.microsoft.com>
Date:   Mon, 22 Jun 2020 19:32:30 -0500
From:   Tyler Hicks <tyhicks@...ux.microsoft.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>
Cc:     James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        Prakhar Srivastava <prsriva02@...il.com>,
        linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: [PATCH 06/12] ima: Fail rule parsing when the KEXEC_CMDLINE hook is combined with an invalid cond

The KEXEC_CMDLINE hook function only supports the pcr conditional. Make
this clear at policy load so that IMA policy authors don't assume that
other conditionals are supported.

Since KEXEC_CMDLINE's inception, ima_match_rules() has always returned
true on any loaded KEXEC_CMDLINE rule without any consideration for
other conditionals present in the rule. Make it clear that pcr is the
only supported KEXEC_CMDLINE conditional by returning an error during
policy load.

An example of why this is a problem can be explained with the following
rule:

 dont_measure func=KEXEC_CMDLINE obj_type=foo_t

An IMA policy author would have assumed that rule is valid because the
parser accepted it but the result was that measurements for all
KEXEC_CMDLINE operations would be disabled.

Fixes: b0935123a183 ("IMA: Define a new hook to measure the kexec boot command line arguments")
Signed-off-by: Tyler Hicks <tyhicks@...ux.microsoft.com>
---
 security/integrity/ima/ima_policy.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index ecc234b956a2..83975ad22907 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -349,6 +349,17 @@ static int ima_lsm_update_rule(struct ima_rule_entry *entry)
 	return 0;
 }
 
+static bool ima_rule_contains_lsm_cond(struct ima_rule_entry *entry)
+{
+	int i;
+
+	for (i = 0; i < MAX_LSM_RULES; i++)
+		if (entry->lsm[i].args_p)
+			return true;
+
+	return false;
+}
+
 /*
  * The LSM policy can be reloaded, leaving the IMA LSM based rules referring
  * to the old, stale LSM policy.  Update the IMA LSM based rules to reflect
@@ -999,6 +1010,16 @@ static bool ima_validate_rule(struct ima_rule_entry *entry)
 		case POLICY_CHECK:
 			break;
 		case KEXEC_CMDLINE:
+			if (entry->action & ~(MEASURE | DONT_MEASURE))
+				return false;
+
+			if (entry->flags & ~(IMA_FUNC | IMA_PCR))
+				return false;
+
+			if (ima_rule_contains_lsm_cond(entry))
+				return false;
+
+			break;
 		case KEY_CHECK:
 			if (entry->action & ~(MEASURE | DONT_MEASURE))
 				return false;
-- 
2.25.1