Re: [PATCH] mailbox: fix a UAF bug in msg_submit()

[Guo Ren]

On Fri, Aug 6, 2021 at 8:15 PM Xianting Tian
<xianting.tian@xxxxxxxxxxxxxxxxx> 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.
Yes, in chan->cl->tx_block will give a second chance to transmit the
msg in tx_tick. If chan->mbox->ops->send_data returned error again,
then mbox_send_message would return to the user's driver, and the user
driver would free the mssg which is still in chan->msg_data &
msg_count. Then it caused "Used After Free" problem.

The current second chance for chan->cl->tx_block is tricky and it's
hard for mailbox driver and user driver implementation in the way.
I recommend deleting the second chance mechanism totally to make the
code understandable. (If first failed, 2th & 3th & 4th would be still
failed.)


>
> We assume message sending failed for both two times,  then the message
> will be still in the internal buffer of a chan(chan->msg_data[idx]).
> It will be send again in the same way when mbox_send_message() is
> called next time. But, at this time this message (chan->msg_data[idx])
> may be a UAF pointer, as the message is passed to mailbox core by user.
> User may free it after last calling of mbox_send_message() returned
> or not. Who knows!!!
>
> In this patch, if the first time sending timeout, we pass timeout
> info(-ETIME) to msg_submit() when do the second time sending by
> tx_tick(). If it still send failed (chan->mbox->ops->send_data()
> returned non-zero value) in the second time, we will give up this
> message(chan->msg_count--) sending. It doesn't matter, user can chose
> to send it again.
>
> Actually, the issue I described above doesn't exist if
> 'chan->mbox->ops->send_data()' always return 0. Because if it always
> returns 0, we will always do 'chan->msg_count—' regardless of message
> sending success or failure. We have such mailbox driver, for example,
> hi6220_mbox_send_data() always return 0. But we still have mailbox
> drivers, which don't always return 0, for example, flexrm_send_data()
> of drivers/mailbox/bcm-flexrm-mailbox.c.
>
> Signed-off-by: Xianting Tian <xianting.tian@xxxxxxxxxxxxxxxxx>
> ---
>  drivers/mailbox/mailbox.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
> index 3e7d4b20a..3e010aafa 100644
> --- a/drivers/mailbox/mailbox.c
> +++ b/drivers/mailbox/mailbox.c
> @@ -50,7 +50,7 @@ static int add_to_rbuf(struct mbox_chan *chan, void *mssg)
>         return idx;
>  }
>
> -static void msg_submit(struct mbox_chan *chan)
> +static void msg_submit(struct mbox_chan *chan, int last_submit)
>  {
>         unsigned count, idx;
>         unsigned long flags;
> @@ -75,7 +75,7 @@ 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 || last_submit == -ETIME) {
>                 chan->active_req = data;
>                 chan->msg_count--;
>         }
> @@ -101,7 +101,7 @@ static void tx_tick(struct mbox_chan *chan, int r)
>         spin_unlock_irqrestore(&chan->lock, flags);
>
>         /* Submit next message */
> -       msg_submit(chan);
> +       msg_submit(chan, r);
>
>         if (!mssg)
>                 return;
> @@ -260,7 +260,7 @@ int mbox_send_message(struct mbox_chan *chan, void *mssg)
>                 return t;
>         }
>
> -       msg_submit(chan);
> +       msg_submit(chan, 0);
>
>         if (chan->cl->tx_block) {
>                 unsigned long wait;
> --
> 2.17.1
>


-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/