linux-kernel - Re: How to reproduce the BUG general protection fault in hci_uart_tty

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Date:   Wed, 5 Jul 2023 17:36:36 +0800
From:   joeyli <jlee@...e.com>
To:     Weiteng Chen <wchen130@....edu>, Yu Hao <yhao016@....edu>
Cc:     linux-bluetooth@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: How to reproduce the BUG general protection fault in
 hci_uart_tty_ioctl?

On Wed, Jul 05, 2023 at 04:41:48PM +0800, joeyli wrote:
> Hi Weiteng Chen, Yu Hao, 
> 
> On Mon, Jul 03, 2023 at 09:07:38PM -0700, Weiteng Chen wrote:
> > Hi Joey,
> > 
> > Sorry for my late response.
> > 
> > https://elixir.bootlin.com/linux/v6.3-rc7/source/drivers/bluetooth/hci_ldisc.c#L764
> > 
> > switch (cmd) {
> >         case HCIUARTSETPROTO:
> >                 if (!test_and_set_bit(HCI_UART_PROTO_SET, &hu->flags)) {
> >                         printk(“test_and_set_bit…”) // insert a prink to make the race easy to trigger
> >                         err = hci_uart_set_proto(hu, arg);
> >                         if (err)
> >                                 clear_bit(HCI_UART_PROTO_SET, &hu->flags);
> >                 } else
> >                         err = -EBUSY;
> >                 break;
> > 
> >         case HCIUARTGETPROTO:
> >                 if (test_bit(HCI_UART_PROTO_SET, &hu->flags))
> >                         err = hu->proto->id;  ←- null pointer deference
> >                 else
> >                         err = -EUNATCH;
> >                 break;
> > 
> > This is a race condition between HCIUARTSETPROTO and HCIUARTGETPROTO. HCI_UART_PROTO_SET is set before hu->proto is set and thus it may dereference a null pointer.
> > 
> > To easily trigger this bug, I inserted a prink in the source code so that the C producer can easily trigger the bug. Please let me know if you have any questions.
> > 
> 
> Thanks! I can reproduce the issue now. 
> 
> Weiteng, Yu Hao, do you have plan for sending patch to fix this problem?
> 
> Joey Lee

Looks that check HCI_UART_PROTO_READY is enough to avoid problem:

--- linux.orig/drivers/bluetooth/hci_ldisc.c
+++ linux/drivers/bluetooth/hci_ldisc.c
@@ -771,7 +771,7 @@ static int hci_uart_tty_ioctl(struct tty
                break;
 
        case HCIUARTGETPROTO:
-               if (test_bit(HCI_UART_PROTO_SET, &hu->flags))
+               if (test_bit(HCI_UART_PROTO_READY, &hu->flags))
                        err = hu->proto->id;
                else
                        err = -EUNATCH;

If you do not have plan to send patch, then I will send the above change.

Thanks
Joey Lee

> 
> 
> > Best,
> > Weiteng Chen
> > 
> > > On Jul 3, 2023, at 8:01 PM, joeyli <jlee@...e.com> wrote:
> > > 
> > > Hi,
> > > 
> > > On Wed, Jun 28, 2023 at 06:57:47PM -0700, Yu Hao wrote:
> > >> Hi Weiteng,
> > >> 
> > >> Could you give more info about the bug, e.g., kernel configuration,
> > >> qemu arguments.
> > >> 
> > > 
> > > Base on kernel code, looks that the HCIUARTSETPROTO and HCIUARTGETPROTO
> > > blocks in hci_uart_tty_ioctl() should use hci_uart->proto_lock. 
> > > 
> > > I have run the C reproducer a couple of days in qemu, but it did not
> > > reproduce issue until now.
> > > 
> > > Does anyone know how to reproduce this issue easily?
> > > 
> > > Thanks
> > > Joey Lee
> > >> 
> > >> On Wed, Jun 28, 2023 at 8:02 AM joeyli <jlee@...e.com> wrote:
> > >>> 
> > >>> Hi Yu Hao,
> > >>> 
> > >>> I am looking at your "BUG: general protection fault in hci_uart_tty_ioctl":
> > >>> 
> > >>> https://lore.kernel.org/all/CA+UBctC3p49aTgzbVgkSZ2+TQcqq4fPDO7yZitFT5uBPDeCO2g@mail.gmail.com/
> > >>> 
> > >>> I am trying the C reproducer in your URL, but it is not success yet:
> > >>> https://gist.github.com/ZHYfeng/a3e3ff2bdfea5ed5de5475f0b54d55cb
> > >>> 
> > >>> I am using v6.2 mainline kernel to run the C reproducer.
> > >>> 
> > >>> Could you please provide suggestions for how to reproduce this issue?
> > >>> And what is your qemu environment for reproducing issue?
> > >>> 
> > >>> Thanks a lot!
> > >>> Joey Lee