tee: amdtee: fix use-after-free vulnerability in amdtee_close_session

There is a potential race condition in amdtee_close_session that may
cause use-after-free in amdtee_open_session. For instance, if a session
has refcount == 1, and one thread tries to free this session via:

    kref_put(&sess->refcount, destroy_session);

the reference count will get decremented, and the next step would be to
call destroy_session(). However, if in another thread,
amdtee_open_session() is called before destroy_session() has completed
execution, alloc_session() may return 'sess' that will be freed up
later in destroy_session() leading to use-after-free in
amdtee_open_session.

To fix this issue, treat decrement of sess->refcount and removal of
'sess' from session list in destroy_session() as a critical section, so
that it is executed atomically.

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

Author: Rijo Thomas

drivers/tee/amdtee/core.c

@@ -217,12 +217,12 @@ static int copy_ta_binary(struct tee_context *ctx, void *ptr, void **ta,
 	return rc;
 }
 
+/* mutex must be held by caller */
 static void destroy_session(struct kref *ref)
 {
 	struct amdtee_session *sess = container_of(ref, struct amdtee_session,
 						   refcount);
 
-	mutex_lock(&session_list_mutex);
 	list_del(&sess->list_node);
 	mutex_unlock(&session_list_mutex);
 	kfree(sess);
@@ -272,7 +272,8 @@ int amdtee_open_session(struct tee_context *ctx,
 	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);
+		kref_put_mutex(&sess->refcount, destroy_session,
+			       &session_list_mutex);
 		goto out;
 	}
 
@@ -290,7 +291,8 @@ int amdtee_open_session(struct tee_context *ctx,
 		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);
+		kref_put_mutex(&sess->refcount, destroy_session,
+			       &session_list_mutex);
 		rc = -ENOMEM;
 		goto out;
 	}
@@ -331,7 +333,7 @@ int amdtee_close_session(struct tee_context *ctx, u32 session)
 	handle_close_session(ta_handle, session_info);
 	handle_unload_ta(ta_handle);
 
-	kref_put(&sess->refcount, destroy_session);
+	kref_put_mutex(&sess->refcount, destroy_session, &session_list_mutex);
 
 	return 0;
 }