On October 4, 2018 12:28:56 PM PDT, Eric Dumazet <edumazet@xxxxxxxxxx> wrote: >On Thu, Oct 4, 2018 at 11:59 AM Dmitry Torokhov ><dmitry.torokhov@xxxxxxxxx> wrote: >> >> Hi Eric, >> >> On Thu, Oct 04, 2018 at 08:47:49AM -0700, Eric Dumazet wrote: >> > syzbot was able to trigger rcu stalls by calling write() >> > with large number of bytes. >> > >> > Add a cond_resched() in the loop to avoid this. >> >> I think this simply masks a deeper issue. The code fetches characters >> from userspace in a loop, takes a lock, quickly places response in an >> output buffer, and releases interrupt. I do not see why this should >> cause stalls as we do not hold spinlock/interrupts off for extended >> period of time. >> >> Adding Paul so he can straighten me out... >> > >Well... > >write(fd, buffer, 0x7FFF0000); > >Takes between 20 seconds and 2 minutes depending on CONFIG options .... That's fine even if it takes a couple of years. We are not holding spinlock for the entirety of this time, so we should get bumped off CPU at some point. > >So either apply my patch, or add a limit on the max count, and >possibly break legitimate user space ? Legitimate users write a single character at a time and read response, so exciting after, let's say, 32 bytes would be fine. But I still want to understand why we have to do that. > >I dunno... > >> > >> > Link: https://lkml.org/lkml/2018/8/23/1106 >> > Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx> >> > Reported-by: syzbot+9436b02171ac0894d33e@xxxxxxxxxxxxxxxxxxxxxxxxx >> > Cc: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx> >> > Cc: linux-input@xxxxxxxxxxxxxxx >> > --- >> > drivers/input/mousedev.c | 1 + >> > 1 file changed, 1 insertion(+) >> > >> > diff --git a/drivers/input/mousedev.c b/drivers/input/mousedev.c >> > index >e08228061bcdd2f97aaadece31d6c83eb7539ae5..412fa71245afe26a7a8ad75705566f83633ba347 >100644 >> > --- a/drivers/input/mousedev.c >> > +++ b/drivers/input/mousedev.c >> > @@ -707,6 +707,7 @@ static ssize_t mousedev_write(struct file >*file, const char __user *buffer, >> > mousedev_generate_response(client, c); >> > >> > spin_unlock_irq(&client->packet_lock); >> > + cond_resched(); >> > } >> > >> > kill_fasync(&client->fasync, SIGIO, POLL_IN); >> > -- >> > 2.19.0.605.g01d371f741-goog >> > >> >> Thanks. >> >> -- >> Dmitry Thanks. -- Dmitry
