linux-kernel - Re: [PATCH] mailbox: fix a UAF bug in msg

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Message-ID: <CABb+yY337=BVjHygv7wurj9Hh8-dj2aMyMnh54WL0CovyMfi_w@mail.gmail.com>
Date:   Tue, 17 Aug 2021 18:33:38 -0500
From:   Jassi Brar <jassisinghbrar@...il.com>
To:     Xianting TIan <xianting.tian@...ux.alibaba.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        guoren@...nel.org
Subject: Re: [PATCH] mailbox: fix a UAF bug in msg_submit()

On Tue, Aug 17, 2021 at 3:01 AM Xianting TIan
<xianting.tian@...ux.alibaba.com> wrote:
>
>
> 在 2021/8/17 下午1:58, Xianting TIan 写道:
> >
> > 在 2021/8/17 下午12:29, Jassi Brar 写道:
> >> On Fri, Aug 6, 2021 at 7:15 AM Xianting Tian
> >> <xianting.tian@...ux.alibaba.com> wrote:
> >>> We met a UAF issue during our mailbox testing.
> >>>
> >>> In synchronous mailbox, we use mbox_send_message() to send a message
> >>> and wait for completion. mbox_send_message() calls msg_submit() to
> >>> send the message for the first time, if timeout, it will send the
> >>> message in tx_tick() for the second time.
> >>>
> >> Seems like your controller's  .send_data() returns error. Can you
> >> please explain why it does so? Because
> >> send_data() only _accepts_ data for further transmission... which
> >> should seldom be a problem.
> >
> > Thanks for the comments,
> >
> > We developed virtio-mailbox for heterogeneous virtualization system.
> >
> > virtio-mailbox is based on the mialbox framework.
> >
> > In virtio framework, its send func 'virtqueue_add_outbuf()' may return
> > error, which caused .send_data() return error.  And also contains
> > other csenarios.
> >
> > But I think mailbox framework shouldn't depend on .send_data() always
> > return OK,  as .send_data() is implemented by mailbox hardware
> > manufacturer, which is not controlled by mailbox framework itself.
> >
As I said, send_data() is basically "accepted for transfer",  and not
"transferred fine".

> > You said 'seldom',  but it still possible we can meet such issue. sucn
> > as flexrm_send_data() of drivers/mailbox/bcm-flexrm-mailbox.c.
> >
The api is used not just in synchronous mode.
And the flexrm driver relies on ACK method, not the synchronous one.

> > I think mailbox framework should be work normaly no matter
> > .send_data() returns ok or not ok.  Do you think so? thanks
>
Normal is your controller driver should be ready after .startup() and
accepts the first message submitted to it.
If it does that, everything would work.

> Another solution is to ignore the return value of .send_data() in
> msg_submit(),
>
> change
>
>          err = chan->mbox->ops->send_data(chan, data);
>          if (!err) {
>                  chan->active_req = data;
>                  chan->msg_count--;
>          }
>
> to
>
>          chan->mbox->ops->send_data(chan, data);
>          chan->active_req = data;
>          chan->msg_count--;
>
Yes, I also have something like this in mind. Definitely not the hack
in your original post.

> But the side effect of the solution is obvious, as if send failed in the
> first time, it will have no chance to sent it in tx_tick() for the
> second time. That is to say, no retry.
>
The api doesn't retry.  The  .last_tx_done() is supposed to tell the
api when is it ok to send a message.

The following should work for you (though I believe your code needs
fixing), which anyway, should have been there.

diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
index 3e7d4b20ab34..9824a51b82fa 100644
--- a/drivers/mailbox/mailbox.c
+++ b/drivers/mailbox/mailbox.c
@@ -75,10 +75,12 @@ static void msg_submit(struct mbox_chan *chan)
                chan->cl->tx_prepare(chan->cl, data);
        /* Try to submit a message to the MBOX controller */
        err = chan->mbox->ops->send_data(chan, data);
-       if (!err) {
+       if (!err)
                chan->active_req = data;
-               chan->msg_count--;
-       }
+
+       /* done with another message */
+       chan->msg_count--;
+
 exit:
        spin_unlock_irqrestore(&chan->lock, flags);