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

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Date:   Mon, 3 Jul 2023 21:07:38 -0700
From:   Weiteng Chen <wchen130@....edu>
To:     joeyli <jlee@...e.com>
Cc:     Yu Hao <yhao016@....edu>, 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?

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.

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