From: Thadeu Lima de Souza Cascardo cascardo@canonical.com

mainline inclusion from mainline-v5.14-rc1 commit 3cfdf8fcaafa62a4123f92eb0f4a72650da3a479 category: bugfix bugzilla: NA CVE: CVE-2021-34981

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When cmtp_attach_device fails, cmtp_add_connection returns the error value which leads to the caller to doing fput through sockfd_put. But cmtp_session kthread, which is stopped in this path will also call fput, leading to a potential refcount underflow or a use-after-free.

Add a refcount before we signal the kthread to stop. The kthread will try to grab the cmtp_session_sem mutex before doing the fput, which is held when get_file is called, so there should be no races there.

Reported-by: Ryota Shiga Signed-off-by: Thadeu Lima de Souza Cascardo cascardo@canonical.com Signed-off-by: Marcel Holtmann marcel@holtmann.org Signed-off-by: Yang Yingliang yangyingliang@huawei.com Reviewed-by: weiyang wang wangweiyang2@huawei.com Signed-off-by: Yang Yingliang yangyingliang@huawei.com --- net/bluetooth/cmtp/core.c | 5 +++++ 1 file changed, 5 insertions(+)

diff --git a/net/bluetooth/cmtp/core.c b/net/bluetooth/cmtp/core.c index 7f26a5a19ff6d..9873684a9d8ff 100644 --- a/net/bluetooth/cmtp/core.c +++ b/net/bluetooth/cmtp/core.c @@ -391,6 +391,11 @@ int cmtp_add_connection(struct cmtp_connadd_req *req, struct socket *sock) if (!(session->flags & BIT(CMTP_LOOPBACK))) { err = cmtp_attach_device(session); if (err < 0) { + /* Caller will call fput in case of failure, and so + * will cmtp_session kthread. + */ + get_file(session->sock->file); + atomic_inc(&session->terminate); wake_up_interruptible(sk_sleep(session->sock->sk)); up_write(&cmtp_session_sem);