tee: amdtee: fix race condition in amdtee_open_session

There is a potential race condition in amdtee_open_session that may
lead to use-after-free. For instance, in amdtee_open_session() after
sess->sess_mask is set, and before setting:

    sess->session_info[i] = session_info;

if amdtee_close_session() closes this same session, then 'sess' data
structure will be released, causing kernel panic when 'sess' is
accessed within amdtee_open_session().

The solution is to set the bit sess->sess_mask as the last step in
amdtee_open_session().

Fixes: 757cc3e ("tee: add AMD-TEE driver")
Cc: stable@vger.kernel.org
Signed-off-by: Rijo Thomas <Rijo-john.Thomas@amd.com>
Acked-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

Author: Rijo Thomas

drivers/tee/amdtee/core.c

@@ -267,35 +267,34 @@ int amdtee_open_session(struct tee_context *ctx,
 		goto out;
 	}
 
+	/* Open session with loaded TA */
+	handle_open_session(arg, &session_info, param);
+	if (arg->ret != TEEC_SUCCESS) {
+		pr_err("open_session failed %d\n", arg->ret);
+		handle_unload_ta(ta_handle);
+		kref_put(&sess->refcount, destroy_session);
+		goto out;
+	}
+
 	/* Find an empty session index for the given TA */
 	spin_lock(&sess->lock);
 	i = find_first_zero_bit(sess->sess_mask, TEE_NUM_SESSIONS);
-	if (i < TEE_NUM_SESSIONS)
+	if (i < TEE_NUM_SESSIONS) {
+		sess->session_info[i] = session_info;
+		set_session_id(ta_handle, i, &arg->session);
 		set_bit(i, sess->sess_mask);
+	}
 	spin_unlock(&sess->lock);
 
 	if (i >= TEE_NUM_SESSIONS) {
 		pr_err("reached maximum session count %d\n", TEE_NUM_SESSIONS);
+		handle_close_session(ta_handle, session_info);
 		handle_unload_ta(ta_handle);
 		kref_put(&sess->refcount, destroy_session);
 		rc = -ENOMEM;
 		goto out;
 	}
 
-	/* Open session with loaded TA */
-	handle_open_session(arg, &session_info, param);
-	if (arg->ret != TEEC_SUCCESS) {
-		pr_err("open_session failed %d\n", arg->ret);
-		spin_lock(&sess->lock);
-		clear_bit(i, sess->sess_mask);
-		spin_unlock(&sess->lock);
-		handle_unload_ta(ta_handle);
-		kref_put(&sess->refcount, destroy_session);
-		goto out;
-	}
-
-	sess->session_info[i] = session_info;
-	set_session_id(ta_handle, i, &arg->session);
 out:
 	free_pages((u64)ta, get_order(ta_size));
 	return rc;