Re: Potential data race in uart_ioctl

[Dmitry Vyukov]

On Tue, Aug 25, 2015 at 9:03 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
> On 08/25/2015 02:38 PM, Dmitry Vyukov wrote:
>> On Tue, Aug 25, 2015 at 8:32 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>>> On Tue, Aug 25, 2015 at 8:26 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
>>>> On 08/25/2015 08:17 AM, Andrey Konovalov wrote:
>>>>> Hi!
>>>>>
>>>>> We are working on a dynamic data race detector for the Linux kernel
>>>>> called KernelThreadSanitizer (ktsan)
>>>>> (https://github.com/google/ktsan/wiki).
>>>>>
>>>>> While booting the kernel (upstream revision 21bdb584af8c) we got a report:
>>>>>
>>>>> ==================================================================
>>>>> ThreadSanitizer: data-race in uart_ioctl
>>>>>
>>>>> Read of size 8 by thread T424 (K971):
>>>>>  [<ffffffff81673c56>] uart_ioctl+0x36/0x11e0
>>>>> drivers/tty/serial/serial_core.c:1216
>>>>>  [<ffffffff81643802>] tty_ioctl+0x4f2/0x11d0 drivers/tty/tty_io.c:2924
>>>>>  [<     inlined    >] do_vfs_ioctl+0x44a/0x750 vfs_ioctl fs/ioctl.c:43
>>>>>  [<ffffffff8127b0ca>] do_vfs_ioctl+0x44a/0x750 fs/ioctl.c:607
>>>>>  [<     inlined    >] SyS_ioctl+0x79/0xa0 SYSC_ioctl fs/ioctl.c:622
>>>>>  [<ffffffff8127b449>] SyS_ioctl+0x79/0xa0 fs/ioctl.c:613
>>>>>  [<ffffffff81eae0ae>] entry_SYSCALL_64_fastpath+0x12/0x71
>>>>> arch/x86/entry/entry_64.S:186
>>>>> DBG: cpu = ffff88063fc1fe68
>>>>> DBG: cpu id = 0
>>>>>
>>>>> Previous write of size 8 by thread T422 (K970):
>>>>>  [<ffffffff816737ef>] uart_open+0x12f/0x220
>>>>> drivers/tty/serial/serial_core.c:1629
>>>>>  [<ffffffff81645be2>] tty_open+0x192/0x8f0 drivers/tty/tty_io.c:2105
>>>>>  [<ffffffff812628fc>] chrdev_open+0x13c/0x290 fs/char_dev.c:388
>>>>>  [<ffffffff812582fc>] do_dentry_open+0x3ac/0x550 fs/open.c:736
>>>>>  [<ffffffff81259d68>] vfs_open+0xb8/0xe0 fs/open.c:853
>>>>>  [<     inlined    >] path_openat+0x81c/0x2440 do_last fs/namei.c:3163
>>>>>  [<ffffffff81272f1c>] path_openat+0x81c/0x2440 fs/namei.c:3295
>>>>>  [<ffffffff8127656a>] do_filp_open+0xfa/0x170 fs/namei.c:3330
>>>>>  [<ffffffff8125a243>] do_sys_open+0x183/0x2b0 fs/open.c:1025
>>>>>  [<     inlined    >] SyS_open+0x35/0x50 SYSC_open fs/open.c:1043
>>>>>  [<ffffffff8125a3a5>] SyS_open+0x35/0x50 fs/open.c:1038
>>>>>  [<ffffffff81eae0ae>] entry_SYSCALL_64_fastpath+0x12/0x71
>>>>> arch/x86/entry/entry_64.S:186
>>>>> DBG: cpu = ffff88063fd1fe68
>>>>>
>>>>> DBG: addr: ffff8801d2a0ce88
>>>>> DBG: first offset: 0, second offset: 0
>>>>> DBG: T424 clock: {T424: 211057, T422: 275728}
>>>>> DBG: T422 clock: {T422: 275819}
>>>>> ==================================================================
>>>>>
>>>>> It seems that one thread reads and uses tty->driver_data while it's
>>>>> being initialized in another one. The second thread holds port->mutex,
>>>>> but the first one does a few accesses to tty->driver_data before
>>>>> locking it.
>>>>>
>>>>> Could you confirm if this is a real race?
>>>>
>>>> Although I don't understand what triggers ktsan to signal a race
>>>> condition, this doesn't appear to be an actual race.
>>>>
>>>> For an ioctl() syscall to act on any given tty requires a successful
>>>> open() syscall to have nearly completed (do_sys_open() => fd_install()
>>>> initializes the file descriptor; ioctl() => fdget() derefs the descriptor).
>>>>
>>>> Perhaps what's tripping the race detection is that 2nd and subsequent
>>>> opens also (redundantly) write the same values as from the first open?
>>>
>>> Since we use a fuzzer, yes, it is possible that open is called twice.
>>
>> Oh, no, sorry, this happens during booting.
>> The race is on tty_struct, which is probably shared between several
>> file descriptors.
>
> Yep, but there is 1:1 correspondence between tty_struct and uart_state;
> so once the first open() initializes tty->driver_data, subsequent opens
> are just re-writing the same value for tty->driver_data.
>
> Is ktsan just triggering on the fact there is a memory write, without
> checking the value has changed?

Yes, that is correct. Ktsan is based on C memory model which says that
any write racing with other memory access leads to undefined behavior.

As far as I see the setup is quite complex and not just writes to
tty->driver_data. For example, it also calls uart_startup which does
set_bit(TTY_IO_ERROR, &tty->flags) on the tty. Can't that lead to some
issues? E.g. we set TTY_IO_ERROR, but since tty is already used by
other threads, operations on the tty in other threads will spuriously
fail.

195 static int uart_startup(struct tty_struct *tty, struct uart_state *state,
196                 int init_hw)
197 {
198         struct tty_port *port = &state->port;
199         int retval;
200
201         if (port->flags & ASYNC_INITIALIZED)
202                 return 0;
203
204         /*
205          * Set the TTY IO error marker - we will only clear this
206          * once we have successfully opened the port.
207          */
208         set_bit(TTY_IO_ERROR, &tty->flags);
209
210         retval = uart_port_startup(tty, state, init_hw);
211         if (!retval) {
212                 set_bit(ASYNCB_INITIALIZED, &port->flags);
213                 clear_bit(TTY_IO_ERROR, &tty->flags);
214         } else if (retval > 0)
215                 retval = 0;
216
217         return retval;
218 }
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