Re: Potential data race in uart_ioctl

[Dmitry Vyukov]

On Tue, Aug 25, 2015 at 8:26 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> wrote:
> Hi Andrey,
>
> 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.
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