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From:   Casey Schaufler <casey@schaufler-ca.com>
To:     casey.schaufler@intel.com, jmorris@namei.org,
        linux-security-module@vger.kernel.org, selinux@vger.kernel.org
Cc:     casey@schaufler-ca.com, linux-audit@redhat.com,
        keescook@chromium.org, john.johansen@canonical.com,
        penguin-kernel@i-love.sakura.ne.jp, paul@paul-moore.com,
        sds@tycho.nsa.gov, linux-kernel@vger.kernel.org
Subject: [PATCH v23 18/23] LSM: Verify LSM display sanity in binder
Date:   Fri, 20 Nov 2020 12:15:02 -0800
Message-Id: <20201120201507.11993-19-casey@schaufler-ca.com>
In-Reply-To: <20201120201507.11993-1-casey@schaufler-ca.com>
References: <20201120201507.11993-1-casey@schaufler-ca.com>
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Verify that the tasks on the ends of a binder transaction
use the same "display" security module. This prevents confusion
of security "contexts".

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: John Johansen <john.johansen@canonical.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
---
 security/security.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/security/security.c b/security/security.c
index 31c983a252b9..8803347424d4 100644
--- a/security/security.c
+++ b/security/security.c
@@ -791,9 +791,38 @@ int security_binder_set_context_mgr(struct task_struct *mgr)
 	return call_int_hook(binder_set_context_mgr, 0, mgr);
 }
 
+/**
+ * security_binder_transaction - Binder driver transaction check
+ * @from: source of the transaction
+ * @to: destination of the transaction
+ *
+ * Verify that the tasks have the same LSM "display", then
+ * call the security module hooks.
+ *
+ * Returns -EINVAL if the displays don't match, or the
+ * result of the security module checks.
+ */
 int security_binder_transaction(struct task_struct *from,
 				struct task_struct *to)
 {
+	int from_ilsm = lsm_task_ilsm(from);
+	int to_ilsm = lsm_task_ilsm(to);
+
+	/*
+	 * If the ilsm is LSMBLOB_INVALID the first module that has
+	 * an entry is used. This will be in the 0 slot.
+	 *
+	 * This is currently only required if the server has requested
+	 * peer contexts, but it would be unwieldly to have too much of
+	 * the binder driver detail here.
+	 */
+	if (from_ilsm == LSMBLOB_INVALID)
+		from_ilsm = 0;
+	if (to_ilsm == LSMBLOB_INVALID)
+		to_ilsm = 0;
+	if (from_ilsm != to_ilsm)
+		return -EINVAL;
+
 	return call_int_hook(binder_transaction, 0, from, to);
 }
 
-- 
2.24.1