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: 757cc3e9ff1d ("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>

diff --git a/drivers/tee/amdtee/core.c b/drivers/tee/amdtee/core.c
index 372d647..3c15f6a 100644 (file)
--- a/drivers/tee/amdtee/core.c
+++ b/drivers/tee/amdtee/core.c
@@ -217,12 +217,12 @@ unlock:
        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;
 }