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@xxxxxxxxxxxxxxxxxxx> > Reported-and-tested-by: syzbot+ada0f7d3d9fd2016d927@xxxxxxxxxxxxxxxxxxxxxxxxx > CC: stable@xxxxxxxxxxxxxxx Johan
