Re: [PATCH] USB: core: Make usb_start_wait_urb() interruptible

[Johan Hovold <johan@kernel.org>]

On Sat, Aug 28, 2021 at 09:58:25PM -0400, Alan Stern wrote:
> usb_start_wait_urb() can be called from user processes by means of the
> USBDEVFS_BULK and USBDEVFS_CONTROL ioctls in usbfs.  Consequently it
> should not contain an uninterruptible wait of arbitrarily long length
> (the timeout value is specified here by the user, so it can be
> practically anything).  Doing so leads the kernel to complain about
> "Task X blocked for more than N seconds", as found in testing by
> syzbot:
> 
> INFO: task syz-executor.0:8700 blocked for more than 143 seconds.
>       Not tainted 5.14.0-rc7-syzkaller #0
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:syz-executor.0  state:D stack:23192 pid: 8700 ppid:  8455 flags:0x00004004
> Call Trace:
>  context_switch kernel/sched/core.c:4681 [inline]
>  __schedule+0xc07/0x11f0 kernel/sched/core.c:5938
>  schedule+0x14b/0x210 kernel/sched/core.c:6017
>  schedule_timeout+0x98/0x2f0 kernel/time/timer.c:1857
>  do_wait_for_common+0x2da/0x480 kernel/sched/completion.c:85
>  __wait_for_common kernel/sched/completion.c:106 [inline]
>  wait_for_common kernel/sched/completion.c:117 [inline]
>  wait_for_completion_timeout+0x46/0x60 kernel/sched/completion.c:157
>  usb_start_wait_urb+0x167/0x550 drivers/usb/core/message.c:63
>  do_proc_bulk+0x978/0x1080 drivers/usb/core/devio.c:1236
>  proc_bulk drivers/usb/core/devio.c:1273 [inline]
>  usbdev_do_ioctl drivers/usb/core/devio.c:2547 [inline]
>  usbdev_ioctl+0x3441/0x6b10 drivers/usb/core/devio.c:2713
> ...
> 
> This patch fixes the problem by converting the uninterruptible wait to
> an interruptible one.  For the most part this won't affect calls to
> usb_start_wait_urb(), because they are made by kernel threads and so
> can't receive most signals.
> 
> But in some cases such calls may occur in user threads in contexts
> other than usbfs ioctls.  A signal in these circumstances could cause
> a USB transfer to fail when otherwise it wouldn't.  The outcome
> wouldn't be too dreadful, since USB transfers can fail at any time and
> the system is prepared to handle these failures gracefully.  In
> theory, for example, a signal might cause a driver's probe routine to
> fail; in practice if the user doesn't want a probe to fail then he
> shouldn't send interrupt signals to the probing process.

While probe() triggered through sysfs or by module loading is one
example, the USB msg helpers are also called in a lot of other
user-thread contexts such as open() calls etc. It might even be that the
majority of these calls can be done from user threads (post
enumeration).

> Overall, then, making these delays interruptible seems to be an
> acceptable risk.

Possibly, but changing the API like this to fix the usbfs ioctls seems
like using a bit of a too big hammer to me, especially when backporting
to stable.

> Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> Reported-and-tested-by: syzbot+ada0f7d3d9fd2016d927@syzkaller.appspotmail.com
> CC: stable@vger.kernel.org

Johan